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47.1E: Present-Time Extinctions - Biology

47.1E: Present-Time Extinctions - Biology



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Human activities probably caused the Holocene mass extinctions; many methods have been employed to estimate these extinction rates.

Learning Objectives

  • Describe the biodiversity loss during the Holocene extinction

Key Points

  • The dodo was one of the first-known examples of a species that went extinct (in the 1600s) during the Holocene period.
  • Steller’s sea cows, passenger pigeons, Carolina parakeets, Japanese sea lions, and Caribbean monk seals are examples of species that went extinct from the 1700s-1900s.
  • Major reasons for species extinctions during the Holocene period are due to overhunting, overfishing, and other human-related activities.
  • Extinction rates can be estimated by comparing extinction events since the 1500s, but this method may underestimate the actual extinction rate value.
  • Extinction rates can be estimated by observing species-area relationships and correlating species loss with habitat loss; however, this method may lead to overestimation.

Key Terms

  • species-area relationship: relationship between area surveyed and number of species encountered; typically measured by incrementally increasing the area of a survey and determining the cumulative numbers of species
  • Holocene: of a geologic epoch within the Neogene period from about 11,000 years ago to the present
  • extinction rate: number of species becoming extinct over time, sometimes defined as extinctions per million species–years to make numbers manageable (E/MSY)

Present-Time Extinctions

The sixth, or Holocene, mass extinction appears to have begun earlier than previously believed and is mostly due to the activities of Homo sapiens. Since the beginning of the Holocene period, there have been numerous recent extinctions of individual species that are recorded in human writings. Most of these coincide with the expansion of the European colonies in the 1500s.

One of the earlier and popularly-known examples of extinction in this period is the dodo bird. The dodo bird lived in the forests of Mauritius, an island in the Indian Ocean, but became extinct around 1662. It was hunted for its meat by sailors as it was easy prey because the dodo, which did not evolve with humans, would approach people without fear. Introduced pigs, rats, and dogs, brought to the island by European ships, also killed dodo young and eggs.

Another example, Steller’s sea cows, became extinct in 1768.The sea cow, first discovered by Europeans in 1741, was hunted for meat and oil. The last of the species was killed in 1768, which amounts to 27 years between the species’ first contact with Europeans and its extinction. In addition, the last living passenger pigeon died in a zoo in Cincinnati, Ohio in 1914. This species was hunted and suffered from habitat loss through the clearing of forests for farmland. Furthermore, in 1918, the last living Carolina parakeet died in captivity. This species, once common in the eastern United States, was a victim of habitat loss and hunting as well. Adding to the extinction list, the Japanese sea lion, which inhabited a broad area around Japan and the coast of Korea, became extinct in the 1950s due to overfishing. The Caribbean monk seal, found in the Caribbean Sea, was driven to extinction through hunting by 1952.

These are only a few of the recorded extinctions in the past 500 years. The International Union for Conservation of Nature (IUCN) keeps a list of extinct and endangered species called the Red List. The list is not complete, but it describes 380 extinct species of vertebrates after 1500 AD, 86 of which were made extinct by over-hunting or overfishing.

Estimates of Present-Time Extinction Rates

Estimates of extinction rates are hampered by the fact that most extinctions are probably happening without observation since there are many organisms that are of less interest to humans and many that are undescribed.

The background extinction rate is estimated to be about one per million species per year (E/MSY). For example, assuming there are about ten million species in existence, the expectation is that ten species would become extinct each year.

One contemporary extinction rate estimate uses the extinctions in the written record since the year 1500. For birds alone, this method yields an estimate of 26 E/MSY. However, this value may be underestimated for three reasons. First, many species would not have been described until much later in the time period, so their loss would have gone unnoticed. Secondly, the number of recently-extinct species is increasing because extinct species now are being described from skeletal remains. Lastly, some species are probably already extinct even though conservationists are reluctant to name them as such. Taking these factors into account raises the estimated extinction rate closer to 100 E/MSY. The predicted rate by the end of the century is 1500 E/MSY.

A second approach to estimating present-day extinction rates is to correlate species loss with habitat loss by measuring forest-area loss and understanding species-area relationships. The species-area relationship is the rate at which new species are seen when the area surveyed is increased. Studies have shown that the number of species present increases as the size of the island increases. This phenomenon has also been shown to hold true in other habitats as well. Turning this relationship around, if the habitat area is reduced, the number of species living there will also decline. Estimates of extinction rates based on habitat loss and species-area relationships have suggested that with about 90 percent habitat loss an expected 50 percent of species would become extinct. Species-area estimates have led to species extinction rate calculations of about 1000 E/MSY and higher. In general, actual observations do not show this amount of loss, suggesting that there is a delay in extinction. Recent work has also called into question the applicability of the species-area relationship when estimating the loss of species. This work argues that the species-area relationship leads to an overestimate of extinction rates. A better relationship to use may be the endemics-area relationship. Using this method would bring estimates down to around 500 E/MSY in the coming century. Note that this value is still 500 times the background rate.


How To Shake Up Gender Norms

W hat determines your destiny? That&rsquos a big question with what should be a complicated answer. But for many, the answer can be reduced to one word: anatomy. Freud&rsquos assertion in 1924 that biology is the key determinant of gender identity, for instance, was for years a hegemonic idea in both law and culture.

Ever since Freud made this notion famous, critics have been objecting to body parts as central predictors of one&rsquos professional and personal path. Many now believe that identity isn&rsquot solely the domain of nature or nurture, but some combination of the two. Still, Freud&rsquos theory isn&rsquot yet dead enduring gender norms show us that the bodies we&rsquore born into still govern lives of women and men around the world.

But according to some recent research, its influence may be fading. In one new study, a majority of millennials surveyed argued that gender shouldn&rsquot define us the way it has historically, and individuals shouldn&rsquot feel pressure to conform to traditional gender roles or behaviors. Enforcing norms can even have health risks, according to another study. Some women&rsquos colleges are now reportedly rethinking their admissions policies to account for gender non-conforming students. And even President Obama is getting in on the norm-questioning trend: While sorting holiday gifts for kids at a Toys for Tots in December, the president decided to place sporting equipment in the box for girls. &ldquoI&rsquom just trying to break down these gender stereotypes,&rdquo he said in a viral video.

But will continuing to challenge gender norms and document their harmful impacts lead to their extinction? To answer that question, we need to first consider another: What&rsquos so bad about traditional gender norms and the way we currently categorize men and women?

For one thing, the way we categorize gender is far too facile, explained Alice Dreger, a leading historian of science and medicine, in a 2010 TED Talk. &ldquoWe now know that sex is complicated enough that we have to admit nature doesn&rsquot draw the line for us between male and female&hellip we actually draw that line on nature,&rdquo she told the audience. &ldquoWhat we have is a sort of situation where the farther our science goes, the more we have to admit to ourselves that these categories that we thought of as stable anatomical categories that mapped very simply to stable identity categories are a lot more fuzzy than we thought.&rdquo

Fuzzy &ndash and maybe not entirely real in the first place.

&ldquoIf there&rsquos a leading edge that is the future of gender, it&rsquos going to be one that understands that gender is relative to context,&rdquo said author and gender theorist Kate Bornstein at a recent New America event, noting that geography, religion, and family attitudes are all contextual factors that can alter one&rsquos perception of gender as a determinant of identity. As long as we hold onto the notion that gender is a constant, &ldquowe&rsquoll keep doing things to keep the lie in place,&rdquo she said. But the fact is that &ldquoit doesn&rsquot stand on its own, and is always relative to something.&rdquo Bornstein argues that the trick to stripping these norms of their harmful power is to mock and expose them for both their flimsiness and stringency.

Which is what photographer Sophia Wallace attempts with her work. Girls Will Be Bois, for example, is a documentary of female masculinity, featuring women who have traditionally &ldquoun-feminine&rdquo occupations &ndash bus driver, boxer, basketball player &ndash and a sartorial masculinity (baggy pants, and bare-chested). In Modern Dandy, Wallace switches up the way women and men are directed to look at the camera (or not) in photographs &ndash whether to appear submissive (traditionally feminine) or dominant (traditionally masculine). Cliteracy, Wallace&rsquos most recent work, uses imagery of the clitoris and text about female sexuality to illuminate a paradox: we&rsquore obsessed with sexualizing female bodies, and yet the world is &ldquoilliterate when it comes to female sexuality.&rdquo

But it&rsquos not as bad as it once was. Wallace thinks that photography is evolving &ndash that some gender-focused imagery is less tinged with ignorance today. &ldquoThere&rsquos so much that I&rsquove seen that has been hopeful,&rdquo she said. &ldquoThere are actually images of female masculinity, trans-men and trans-women now that didn&rsquot exist when I was in my teens and early 20s. In other ways we have so far to go.&rdquo

Part of the struggle of relinquishing gender norms comes from an uncomfortable truth. &ldquoMen have everything to gain when we overthrow patriarchy&hellipbut they also have something to lose from giving up their traditional masculinity,&rdquo said Tavia Nyong&rsquoo, an associate professor of performance studies at NYU, emphasizing that male rights vary widely across race and class divisions and that white men have even more to lose than men of color. What do they lose, exactly? Privileges (the ability to open carry a gun and not be worried that they&rsquoll be shot by the police, Nyong&rsquoo argued). Control &ndash over political, economic and cultural domains. Access &ndash to networks, jobs and economic opportunities. Put simply, they lose power.

&ldquoYou walk out the door in the morning with a penis and your income is 20 percent higher on average for nothing that you did,&rdquo said Gary Barker, the international director of Promundo, an organization that engages men and boys around the world on issues of gender equality.

When asked whether the future of gender was evolution and extinction, Barker, Nyong&rsquoo, Wallace and Bornstein all said they hoped for extinction. But at the same time, each acknowledged how difficult that goal would be to achieve. Beyond the power dynamics, there&rsquos a level of comfort in well-worn identities. &ldquoIt&rsquos easy to sit in these old roles that we&rsquove watched and to feel a certain comfort in their stability in a world that feels kind of hard to understand,&rdquo Barker said.

But change is not impossible. Barker advises demonstrating how our traditional version of masculinity may not actually be worth the fight. &ldquoMen who have more rigid views of what it means to be men are more likely to suicidal thoughts, more likely to be depressed, less likely to report they&rsquore happy with life overall, less likely to take care of their health, more likely to own guns, the list goes on,&rdquo he said. &ldquoThere is something toxic about this version of masculinity out there.&rdquo

Detoxing society requires ripping off a mask of sorts. &ldquoIt&rsquos about getting as many people as possible to have that Matrix moment, Barker said, when they realize, &ldquowait &ndash [masculinity] isn&rsquot real. It&rsquos all illusory, it&rsquos all performance.&rdquo

Elizabeth Weingarten is the associate director of New America&rsquos Global Gender Parity Initiative. This piece was originally published in New America&rsquos digital magazine, The Weekly Wonk. Sign up to get it delivered to your inbox each Thursday here, and follow @New America on Twitter.


47.1 The Biodiversity Crisis

By the end of this section, you will be able to do the following:

  • Define biodiversity in terms of species diversity and abundance
  • Describe biodiversity as the equilibrium of naturally fluctuating rates of extinction and speciation
  • Identify historical causes of high extinction rates in Earth’s history

Traditionally, ecologists have measured biodiversity , a general term for the number of species present in the biosphere, by taking into account both the number of species and their relative abundance to each other. Biodiversity can be estimated at a number of levels of organization of living organisms. These estimation indices, which came from information theory, are most useful as a first step in quantifying biodiversity between and within ecosystems they are less useful when the main concern among conservation biologists is simply the loss of biodiversity. However, biologists recognize that measures of biodiversity, in terms of species diversity, may help focus efforts to preserve the biologically or technologically important elements of biodiversity.

The Lake Victoria cichlids provide an example with which we can begin to understand biodiversity. The biologists studying cichlids in the 1980s discovered hundreds of cichlid species representing a variety of specializations to specialized habitat types and specific feeding strategies: such as eating plankton floating in the water, scraping/eating algae from rocks, eating insect larvae from the lake bottom, and eating the eggs of other species of cichlid. The cichlids of Lake Victoria are the product of an complex adaptive radiation. An adaptive radiation is a rapid (less than three million years in the case of the Lake Victoria cichlids) branching through speciation of a phylogenetic clade into many closely related species. Typically, the species “radiate” into different habitats and niches. The Galápagos Island finches are an example of a modest adaptive radiation with 15 species. The cichlids of Lake Victoria are an example of a spectacular adaptive radiation that formerly included about 500 species.

At the time biologists were making this discovery, some species began to quickly disappear. A culprit in these declines was the Nile perch, a species of large predatory fish that was introduced to Lake Victoria by fisheries to feed the people living around the lake. The Nile perch was introduced in 1963, but its populations did not begin to surge until the 1980s. The perch population grew by consuming cichlids, driving species after species to the point of extinction (the disappearance of a species). In fact, there were several factors that played a role in the extinction of perhaps 200 cichlid species in Lake Victoria: the Nile perch, declining lake water quality due to agriculture and land clearing on the shores of Lake Victoria, and increased fishing pressure. Scientists had not even catalogued all of the species present—so many were lost that were never named. The diversity is now a shadow of what it once was.

The cichlids of Lake Victoria are a thumbnail sketch of contemporary rapid species loss that occurs all over Earth that is caused primarily by human activity. Extinction is a natural process of macroevolution that occurs at the rate of about one out of 1 million species becoming extinct per year. The fossil record reveals that there have been five periods of mass extinction in history with much higher rates of species loss, and the rate of species loss today is comparable to those periods of mass extinction. However, there is a major difference between the previous mass extinctions and the current extinction we are experiencing: human activity. Specifically, three human activities have a major impact: 1) destruction of habitat, 2) introduction of exotic species, and 3) over-harvesting. Predictions of species loss within the next century, a tiny amount of time on geological timescales, range from 10 percent to 50 percent. Extinctions on this scale have only happened five other times in the history of the planet, and these extinctions were caused by cataclysmic events that changed the course of the history of life in each instance.

Types of Biodiversity

Scientists generally accept that the term biodiversity describes the number and kinds of species and their abundance in a given location or on the planet. Species can be difficult to define, but most biologists still feel comfortable with the concept and are able to identify and count eukaryotic species in most contexts. Biologists have also identified alternate measures of biodiversity, some of which are important for planning how to preserve biodiversity.

Genetic diversity is one of those alternate concepts. Genetic diversity , or genetic variation defines the raw material for evolution and adaptation in a species. A species’ future potential for adaptation depends on the genetic diversity held in the genomes of the individuals in populations that make up the species. The same is true for higher taxonomic categories. A genus with very different types of species will have more genetic diversity than a genus with species that are genetically similar and have similar ecologies. If there were a choice between one of these genera of species being preserved, the one with the greatest potential for subsequent evolution is the most genetically diverse one.

Many genes code for proteins, which in turn carry out the metabolic processes that keep organisms alive and reproducing. Genetic diversity can be measured as chemical diversity in that different species produce a variety of chemicals in their cells, both the proteins as well as the products and byproducts of metabolism. This chemical diversity has potential benefit for humans as a source of pharmaceuticals, so it provides one way to measure diversity that is important to human health and welfare.

Humans have generated diversity in domestic animals, plants, and fungi, among many other organisms. This diversity is also suffering losses because of migration, market forces, and increasing globalism in agriculture, especially in densely populated regions such as China, India, and Japan. The human population directly depends on this diversity as a stable food source, and its decline is troubling biologists and agricultural scientists.

It is also useful to define ecosystem diversity , meaning the number of different ecosystems on the planet or within a given geographic area (Figure 47.2). Whole ecosystems can disappear even if some of the species might survive by adapting to other ecosystems. The loss of an ecosystem means the loss of interactions between species, the loss of unique features of coadaptation, and the loss of biological productivity that an ecosystem is able to create. An example of a largely extinct ecosystem in North America is the prairie ecosystem. Prairies once spanned central North America from the boreal forest in northern Canada down into Mexico. They are now all but gone, replaced by crop fields, pasture lands, and suburban sprawl. Many of the species survive elsewhere, but the hugely productive ecosystem that was responsible for creating the most productive agricultural soils in the United States is now gone. As a consequence, native soils are disappearing or must be maintained and enhanced at great expense.

Current Species Diversity

Despite considerable effort, knowledge of the species that inhabit the planet is limited and always will be because of a continuing lack of financial resources and political willpower. A recent estimate suggests that the eukaryote species for which science has names, about 1.5 million species, account for less than 20 percent of the total number of eukaryote species present on the planet (8.7 million species, by one estimate). Estimates of numbers of prokaryotic species are largely guesses, but biologists agree that science has only begun to catalog their diversity. Even with what is known, there is no central repository of names or samples of the described species therefore, there is no way to be sure that the 1.5 million descriptions is an accurate accounting. It is a best guess based on the opinions of experts in different taxonomic groups. Given that Earth is losing species at an accelerating pace, science is very much in the place it was with the Lake Victoria cichlids: knowing little about what is being lost. Table 47.1 presents recent estimates of biodiversity in different groups.

Mora et al. 2011 1 Chapman 2009 2 Groombridge & Jenkins 2002 3
Described Predicted Described Predicted Described Predicted
Animalia 1,124,516 9,920,000 1,424,153 6,836,330 1,225,500 10,820,000
Chromista 17,892 34,900 25,044 200,500
Fungi 44,368 616,320 98,998 1,500,000 72,000 1,500,000
Plantae 224,244 314,600 310,129 390,800 270,000 320,000
Protozoa 16,236 72,800 28,871 1,000,000 80,000 600,000
Prokaryotes 10,307 1,000,000 10,175
Total 1,438,769 10,960,000 1,897,502 10,897,630 1,657,675 13,240,000

There are various initiatives to catalog described species in accessible ways, and the internet is facilitating that effort. Nevertheless, it has been pointed out that at the current rate of new species descriptions (which according to the State of Observed Species Report is 17,000 to 20,000 new species per year), it will take close to 500 years to finish describing life on this planet. 4 Over time, the task becomes both increasingly difficult and increasingly easier as extinction removes species from the planet.

Naming and counting species may seem like an unimportant pursuit given the other needs of humanity, but determining biodiversity it is not simply an accounting of species. Describing a species is a complex process through which biologists determine an organism’s unique characteristics and whether or not that organism belongs to any other described species or genus. It allows biologists to find and recognize the species after the initial discovery, and allows them to follow up on questions about its biology. In addition, the unique characteristics of each species make it potentially valuable to humans or other species on which humans depend.

Patterns of Biodiversity

Biodiversity is not evenly distributed on Earth. Lake Victoria contained almost 500 species of cichlids alone, ignoring the other fish families present in the lake. All of these species were found only in Lake Victoria therefore, the 500 species of cichlids were endemic. Endemic species are found in only one location. Endemics with highly restricted distributions are particularly vulnerable to extinction. Higher taxonomic levels, such as genera and families, can also be endemic. Lake Michigan contains about 79 species of fish, many of which are found in other lakes in North America. What accounts for the difference in fish diversity in these two lakes? Lake Victoria is an ancient tropical lake, while Lake Michigan is a recently formed temperate lake. Lake Michigan in its present form is only about 7,000 years old, while Lake Victoria in its present form is about 15,000 years old, although its basin is about 400,000 years in age. Biogeographers have suggested these two factors, latitude and age, are two of several hypotheses to explain biodiversity patterns on the planet.

Career Connection

Biogeographer

Biogeography is the study of the distribution of the world’s species—both in the past and in the present. The work of biogeographers is critical to understanding our physical environment, how the environment affects species, and how environmental changes impact the distribution of a species it has also been critical to developing modern evolutionary theory. Biogeographers need to understand both biology and ecology. They also need to be well-versed in evolutionary studies, soil science, and climatology.

There are three main fields of study under the heading of biogeography: ecological biogeography, historical biogeography (called paleobiogeography), and conservation biogeography. Ecological biogeography studies the current factors affecting the distribution of plants and animals. Historical biogeography , as the name implies, studies the past distribution of species. Conservation biogeography , on the other hand, is focused on the protection and restoration of species based upon known historical and current ecological information. Each of these fields considers both zoogeography and phytogeography—the past and present distribution of animals and plants.

One of the oldest observed patterns in ecology is that species biodiversity in almost every taxonomic group increases as latitude declines. In other words, biodiversity increases closer to the equator (Figure 47.3).

It is not yet clear why biodiversity increases closer to the equator, but scientists have several hypotheses. One factor may be the greater age of the ecosystems in the tropics versus those in temperate regions the temperate regions were largely devoid of life or were drastically reduced during the last glaciation. The idea is that greater age provides more time for speciation. Another possible explanation is the increased direct energy the tropics receive from the sun versus the decreased intensity of the solar energy that temperate and polar regions receive. Tropical ecosystem complexity may promote speciation by increasing the heterogeneity , or number of ecological niches, in the tropics relative to higher latitudes. The greater heterogeneity provides more opportunities for coevolution, specialization, and perhaps greater selection pressures leading to population differentiation. However, this hypothesis suffers from some circularity—ecosystems with more species encourage speciation, but how did they get more species to begin with?

The tropics have been perceived as being more stable than temperate regions, which have a pronounced climate and day-length seasonality. The tropics have their own forms of seasonality, such as rainfall, but they are generally assumed to be more stable environments and this stability might promote speciation into highly specialized niches.

Regardless of the mechanisms, it is certainly true that all levels of biodiversity are greatest in the tropics. Additionally, the rate of endemism is highest, and there are more biodiversity “hotspots.” However, this richness of diversity also means that knowledge of species is unfortunately very low, and there is a high potential for biodiversity loss.

Conservation of Biodiversity

In 1988, British environmentalist Norman Myers developed a conservation concept to identify areas rich in species and at significant risk for species loss: biodiversity hotspots. Biodiversity hotspots are geographical areas that contain high numbers of endemic species. The purpose of the concept was to identify important locations on the planet for conservation efforts, a kind of conservation triage. By protecting hotspots, governments are able to protect a larger number of species. The original criteria for a hotspot included the presence of 1500 or more endemic plant species and 70 percent of the area disturbed by human activity. There are now 34 biodiversity hotspots (Figure 47.4) containing large numbers of endemic species, which include half of Earth’s endemic plants.

Biodiversity Change through Geological Time

The number of species on the planet, or in any geographical area, is the result of an equilibrium of two evolutionary processes that are continuously ongoing: speciation and extinction. Both are natural “birth” and “death” processes of macroevolution. When speciation rates begin to outstrip extinction rates, the number of species will increase likewise, the number of species will decrease when extinction rates begin to overtake speciation rates. Throughout Earth’s history, these two processes have fluctuated—sometimes leading to dramatic changes in the number of species on Earth as reflected in the fossil record (Figure 47.5).

Paleontologists have identified five strata in the fossil record that appear to show sudden and dramatic (greater than half of all extant species disappearing from the fossil record) losses in biodiversity. These are called mass extinctions . There are many lesser, yet still dramatic, extinction events, but the five mass extinctions have attracted the most research. An argument can be made that the five mass extinctions are only the five most extreme events in a continuous series of large extinction events throughout the Phanerozoic (since 542 million years ago). In most cases, the hypothesized causes are still controversial however, the most recent mass extinction event seems clear.

The Five Mass Extinctions

The fossil record of the mass extinctions was the basis for defining periods of geological history, so they typically occur at the transition point between geological periods. The transition in fossils from one period to another reflects the dramatic loss of species and the gradual origin of new species. These transitions can be seen in the rock strata. Table 47.2 provides data on the five mass extinctions.

Geological Period Mass Extinction Name Time (millions of years ago)
Ordovician–Silurianend-Ordovician O–S450–440
Late Devonianend-Devonian375–360
Permian–Triassicend-Permian251
Triassic–Jurassicend-Triassic205
Cretaceous–Paleogeneend-Cretaceous K–Pg (K–T)65.5

The Ordovician-Silurian extinction event is the first recorded mass extinction and the second largest. During this period, about 85 percent of marine species (few species lived outside the oceans) became extinct. The main hypothesis for its cause is a period of glaciation and then warming. The extinction event actually consists of two extinction events separated by about 1 million years. The first event was caused by cooling, and the second event was due to the subsequent warming. The climate changes affected temperatures and sea levels. Some researchers have suggested that a gamma-ray burst, caused by a nearby supernova, was a possible cause of the Ordovician-Silurian extinction. The gamma-ray burst would have stripped away the Earth’s protective ozone layer, allowing intense ultraviolet radiation from the sun to reach the surface of the earth—and may account for climate changes observed at the time. The hypothesis is very speculative, and extraterrestrial influences on Earth’s history are an active line of research. Recovery of biodiversity after the mass extinction took from 5 to 20 million years, depending on the location.

The late Devonian extinction may have occurred over a relatively long period of time. It appears to have mostly affected marine species and not so much the plants or animals inhabiting terrestrial habitats. The causes of this extinction are poorly understood.

The end-Permian extinction was the largest in the history of life. Indeed, an argument could be made that Earth became nearly devoid of life during this extinction event. Estimates are that 96 percent of all marine species and 70 percent of all terrestrial species were lost. It was at this time, for example, that the trilobites, a group that survived the Ordovician–Silurian extinction, became extinct. The causes for this mass extinction are not clear, but the leading suspect is extended and widespread volcanic activity that led to a runaway global-warming event. The oceans became largely anoxic, suffocating marine life. Terrestrial tetrapod diversity took 30 million years to recover after the end-Permian extinction. The Permian extinction dramatically altered Earth’s biodiversity makeup and the course of evolution.

The causes of the Triassic–Jurassic extinction event are not clear, and researchers argue hypotheses including climate change, asteroid impact, and volcanic eruptions. The extinction event occurred just before the breakup of the supercontinent Pangaea, although recent scholarship suggests that the extinctions may have occurred more gradually throughout the Triassic.

The causes of the end-Cretaceous extinction event are the ones that are best understood. It was during this extinction event about 65 million years ago that the majority of the dinosaurs, the dominant vertebrate group for millions of years, disappeared from the planet (with the exception of a theropod clade that gave rise to birds).

The cause of this extinction is now understood to be the result of a cataclysmic impact of a large meteorite, or asteroid, off the coast of what is now the Yucatán Peninsula. This hypothesis, proposed first in 1980, was a radical explanation based on a sharp spike in the levels of iridium (which enters our atmosphere from meteors at a fairly constant rate but is otherwise absent on Earth’s surface) in the rock stratum that marks the boundary between the Cretaceous and Paleogene periods (Figure 47.6). This boundary marked the disappearance of the dinosaurs in fossils as well as many other taxa. The researchers who discovered the iridium spike interpreted it as a rapid influx of iridium from space to the atmosphere (in the form of a large asteroid) rather than a slowing in the deposition of sediments during that period. It was a radical explanation, but the report of an appropriately aged and sized impact crater in 1991 made the hypothesis more believable. Now an abundance of geological evidence supports the theory. Recovery times for biodiversity after the end-Cretaceous extinction are shorter, in geological time, than for the end-Permian extinction, on the order of 10 million years.

Another possibility, perhaps coincidental with the impact of the Yucatan asteroid, was extensive volcanism that began forming about 66 million years ago, about the same time as the Yucatan asteroid impact, at the end of the Cretaceous. The lava flows covered over 50 percent of what is now India. The release of volcanic gases, particularly sulphur dioxide, during the formation of the traps contributed to climate change, which may have induced the mass extinction.

Visual Connection

Scientists measured the relative abundance of fern spores above and below the K–Pg boundary in this rock sample. Which of the following statements most likely represents their findings?


CAREER CONNECTION

Biogeographer

Biogeography is the study of the distribution of the world’s species—both in the past and in the present. The work of biogeographers is critical to understanding our physical environment, how the environment affects species, and how environmental changes impact the distribution of a species it has also been critical to developing modern evolutionary theory. Biogeographers need to understand both biology and ecology. They also need to be well-versed in evolutionary studies, soil science, and climatology.

There are three main fields of study under the heading of biogeography: ecological biogeography, historical biogeography (called paleobiogeography), and conservation biogeography. Ecological biogeography studies the current factors affecting the distribution of plants and animals. Historical biogeography , as the name implies, studies the past distribution of species. Conservation biogeography , on the other hand, is focused on the protection and restoration of species based upon known historical and current ecological information. Each of these fields considers both zoogeography and phytogeography—the past and present distribution of animals and plants.

One of the oldest observed patterns in ecology is that species biodiversity in almost every taxonomic group increases as latitude declines. In other words, biodiversity increases closer to the equator (Figure 2).

Figure 2: This map illustrates the number of amphibian species across the globe and shows the trend toward higher biodiversity at lower latitudes. A similar pattern is observed for most taxonomic groups. The white areas indicate a lack of data in this particular study.

It is not yet clear why biodiversity increases closer to the equator, but scientists have several hypotheses. One factor may be the greater age of the ecosystems in the tropics versus those in temperate regions the temperate regions were largely devoid of life or were drastically reduced during the last glaciation. The idea is that greater age provides more time for speciation. Another possible explanation is the increased direct energy the tropics receive from the sun versus the decreased intensity of the solar energy that temperate and polar regions receive. Tropical ecosystem complexity may promote speciation by increasing the heterogeneity , or number of ecological niches, in the tropics relative to higher latitudes. The greater heterogeneity provides more opportunities for coevolution, specialization, and perhaps greater selection pressures leading to population differentiation. However, this hypothesis suffers from some circularity—ecosystems with more species encourage speciation, but how did they get more species to begin with?

The tropics have been perceived as being more stable than temperate regions, which have a pronounced climate and day-length seasonality. The tropics have their own forms of seasonality, such as rainfall, but they are generally assumed to be more stable environments and this stability might promote speciation into highly specialized niches.

Regardless of the mechanisms, it is certainly true that all levels of biodiversity are greatest in the tropics. Additionally, the rate of endemism is highest, and there are more biodiversity “hotspots.” However, this richness of diversity also means that knowledge of species is unfortunately very low, and there is a high potential for biodiversity loss.

Conservation of Biodiversity

In 1988, British environmentalist Norman Myers developed a conservation concept to identify areas rich in species and at significant risk for species loss: biodiversity hotspots. Biodiversity hotspots are geographical areas that contain high numbers of endemic species. The purpose of the concept was to identify important locations on the planet for conservation efforts, a kind of conservation triage. By protecting hotspots, governments are able to protect a larger number of species. The original criteria for a hotspot included the presence of 1500 or more endemic plant species and 70 percent of the area disturbed by human activity. There are now 34 biodiversity hotspots (Figure 3) containing large numbers of endemic species, which include half of Earth’s endemic plants.

Figure 3: Conservation International has identified 34 biodiversity hotspots, which cover only 2.3 percent of the Earth’s surface but have endemic to them 42 percent of the terrestrial vertebrate species and 50 percent of the world’s plants.

Mass extinctions

There have been at least five mass extinctions in the history of life, and four in the last 3.5 billion years in which many species have disappeared in a relatively short period of geological time. These are covered in more detail in the article on extinction events. The most recent of these, the K-T extinction 65 million years ago at the end of the Cretaceous period, is best known for having wiped out the non- avian dinosaurs, among many other species.

According to a 1998 survey of 400 biologists conducted by New York's American Museum of Natural History, nearly 70 percent of biologists believe that we are currently in the early stages of a human-caused mass extinction, known as the Holocene extinction event. In that survey, the same proportion of respondents agreed with the prediction that up to 20 percent of all living species could become extinct within 30 years (by 2028). Biologist E.O. Wilson estimated in 2002 that if current rates of human destruction of the biosphere continue, one-half of all species of life on earth will be extinct in 100 years.


Contents

A tabular overview of the taxonomic ranking of Homo sapiens (with age estimates for each rank) is shown below.

Rank Name Common name Millions of years ago (commencement)
Life 4,200
Archaea 3,700
Domain Eukaryota Eukaryotes 2,100
Podiata Excludes Plants and their relatives 1,540
Amorphea
Obazoa Excludes Amoebozoa (Amoebas)
Opisthokonts Holozoa + Holomycota (Cristidicoidea and Fungi) 1,300
Holozoa Excludes Holomycota 1,100
Filozoa Choanozoa + Filasterea
Choanozoa Choanoflagellates + Animals 900
Kingdom Animalia Animals 610
Subkingdom Eumetazoa Excludes Porifera (Sponges)
Parahoxozoa Excludes Ctenophora (Comb Jellies)
Bilateria Triploblasts / Worms 560
Nephrozoa
Deuterostomes Division from Protostomes
Phylum Chordata Chordates (Vertebrates and closely related invertebrates) 530
Olfactores Excludes cephalochordates (Lancelets)
Subphylum Vertebrata Fish / Vertebrates 505
Infraphylum Gnathostomata Jawed fish 460
Teleostomi Bony fish 420
Sarcopterygii Lobe finned fish
Superclass Tetrapoda Tetrapods (animals with four limbs) 395
Amniota Amniotes (fully terrestrial tetrapods whose eggs are "equipped with an amnion") 340
Synapsida Proto-Mammals 308
Therapsid Limbs beneath the body and other mammalian traits 280
Class Mammalia Mammals 220
Subclass Theria Mammals that give birth to live young (i.e., non-egg-laying) 160
Infraclass Eutheria Placental mammals (i.e., non-marsupials) 125
Magnorder Boreoeutheria Supraprimates, (most) hoofed mammals, (most) carnivorous mammals, whales, and bats 124–101
Superorder Euarchontoglires Supraprimates: primates, colugos, tree shrews, rodents, and rabbits 100
Grandorder Euarchonta Primates, colugos, and tree shrews 99–80
Mirorder Primatomorpha Primates and colugos 79.6
Order Primates Primates / Plesiadapiformes 66
Suborder Haplorrhini "Dry-nosed" (literally, "simple-nosed") primates: tarsiers and monkeys (incl. apes) 63
Infraorder Simiiformes monkeys (incl. apes) 40
Parvorder Catarrhini "Downward-nosed" primates: apes and old-world monkeys 30
Superfamily Hominoidea Apes: great apes and lesser apes (gibbons) 22-20
Family Hominidae Great apes: humans, chimpanzees, gorillas and orangutans—the hominids 20–15
Subfamily Homininae Humans, chimpanzees, and gorillas (the African apes) [1] 14–12
Tribe Hominini Includes both Homo, Pan (chimpanzees), but not Gorilla. 10–8
Subtribe Hominina Genus Homo and close human relatives and ancestors after splitting from Pan—the hominins 8–4 [2]
(Genus) Ardipithecus s.l. 6-4
(Genus) Australopithecus 3
Genus Homo (H. habilis) Humans 2.5
(Species) H. erectus s.l.
(Species) H. heidelbergensis s.l.
Species Homo sapiens s.s. Anatomically modern humans 0.8–0.3 [3]

Unicellular life Edit

The choanoflagellates may look similar to the ancestors of the entire animal kingdom, and in particular they may be the ancestors of sponges. [5] [6]

Proterospongia (members of the Choanoflagellata) are the best living examples of what the ancestor of all animals may have looked like. They live in colonies, and show a primitive level of cellular specialization for different tasks.

Animals or Animalia Edit

Urmetazoan: The first fossils that might represent animals appear in the 665-million-year-old rocks of the Trezona Formation of South Australia. These fossils are interpreted as being early sponges. [7] Separation from the Porifera (sponges) lineage. Eumetazoa/Diploblast: separation from the Ctenophora ("comb jellies") lineage. Planulozoa/ParaHoxozoa: separation from the Placozoa and Cnidaria lineages. Almost all cnidarians possess nerves and muscles. Because they are the simplest animals to possess them, their ancestors were very probably the first animals to use nerves and muscles together. Cnidarians are also the first animals with an actual body of definite form and shape. They have radial symmetry. The first eyes evolved at this time.

Urbilaterian: Bilateria/Triploblasts, Nephrozoa (555 Ma), last common ancestor of protostomes (including the arthropod [insect, crustacean] and platyzoan [flatworms] lineages) and the deuterostomes (including the vertebrate [human] lineage). Earliest development of the brain, and of bilateral symmetry. Archaic representatives of this stage are flatworms, the simplest animals with organs that form from three germ layers.

Most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion. Deuterostomes, last common ancestor of the chordate [human] lineage, the Echinodermata (starfish, sea urchins, sea cucumbers, etc.) and Hemichordata (acorn worms and graptolites).

An archaic survivor from this stage is the acorn worm, sporting a circulatory system with a heart that also functions as a kidney. Acorn worms have a gill-like structure used for breathing, a structure similar to that of primitive fish. Acorn worms have a plexus concentrated into both dorsal and ventral nerve cords. The dorsal cord reaches into the proboscis, and is partially separated from the epidermis in that region. This part of the dorsal nerve cord is often hollow, and may well be homologous with the brain of vertebrates. [8]

Chordates Edit

The lancelet, still living today, retains some characteristics of the primitive chordates. It resembles Pikaia.

The first vertebrates appear: the ostracoderms, jawless fish related to present-day lampreys and hagfishes. Haikouichthys and Myllokunmingia are examples of these jawless fish, or Agnatha. (See also prehistoric fish). They were jawless and their internal skeletons were cartilaginous. They lacked the paired (pectoral and pelvic) fins of more advanced fish. They were precursors to the Osteichthyes (bony fish). [13]

The Placodermi were prehistoric fishes. Placoderms were some of the first jawed fishes (Gnathostomata), their jaws evolving from the first gill arch. [14] A placoderm's head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked. However, the fossil record indicates that they left no descendants after the end of the Devonian and are less closely related to living bony fishes than sharks are. [ citation needed ]

Tetrapods Edit

Some fresh water lobe-finned fish (Sarcopterygii) develop legs and give rise to the Tetrapoda.

The first tetrapods evolved in shallow and swampy freshwater habitats.

Primitive tetrapods developed from a lobe-finned fish (an "osteolepid Sarcopterygian"), with a two-lobed brain in a flattened skull, a wide mouth and a short snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed adaptations of fins with fleshy bases and bones. (The "living fossil" coelacanth is a related lobe-finned fish without these shallow-water adaptations.) Tetrapod fishes used their fins as paddles in shallow-water habitats choked with plants and detritus. The universal tetrapod characteristics of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water. [16]

Panderichthys is a 90–130 cm (35–50 in) long fish from the Late Devonian period (380 Mya). It has a large tetrapod-like head. Panderichthys exhibits features transitional between lobe-finned fishes and early tetrapods.

Trackway impressions made by something that resembles Ichthyostega's limbs were formed 390 Ma in Polish marine tidal sediments. This suggests tetrapod evolution is older than the dated fossils of Panderichthys through to Ichthyostega.

Lungfishes retain some characteristics of the early Tetrapoda. One example is the Queensland lungfish.

Tiktaalik is a genus of sarcopterygian (lobe-finned) fishes from the late Devonian with many tetrapod-like features. It shows a clear link between Panderichthys and Acanthostega.

Acanthostega is an extinct amphibian, among the first animals to have recognizable limbs. It is a candidate for being one of the first vertebrates to be capable of coming onto land. It lacked wrists, and was generally poorly adapted for life on land. The limbs could not support the animal's weight. Acanthostega had both lungs and gills, also indicating it was a link between lobe-finned fish and terrestrial vertebrates.

Ichthyostega is an early tetrapod. Being one of the first animals with legs, arms, and finger bones, Ichthyostega is seen as a hybrid between a fish and an amphibian. Ichthyostega had legs but its limbs probably were not used for walking. They may have spent very brief periods out of water and would have used their legs to paw their way through the mud. [17]

Amphibia were the first four-legged animals to develop lungs which may have evolved from Hynerpeton 360 Mya.

Amphibians living today still retain many characteristics of the early tetrapods.

From amphibians came the first reptiles: Hylonomus is the earliest known reptile. It was 20 cm (8 in) long (including the tail) and probably would have looked rather similar to modern lizards. It had small sharp teeth and probably ate millipedes and early insects. It is a precursor of later Amniotes and mammal-like reptiles. Αlpha keratin first evolves here. It is used in the claws of modern lizards and birds, and hair in mammals. [18]

Evolution of the amniotic egg gives rise to the Amniota, reptiles that can reproduce on land and lay eggs on dry land. They did not need to return to water for reproduction. This adaptation gave them the capability to inhabit the uplands for the first time.

Reptiles have advanced nervous systems, compared to amphibians, with twelve pairs of cranial nerves.

Mammals Edit

The earliest mammal-like reptiles are the pelycosaurs. The pelycosaurs were the first animals to have temporal fenestrae. Pelycosaurs are not therapsids but soon they gave rise to them. The Therapsida were the ancestor of mammals.

The therapsids have temporal fenestrae larger and more mammal-like than pelycosaurs, their teeth show more serial differentiation, and later forms had evolved a secondary palate. A secondary palate enables the animal to eat and breathe at the same time and is a sign of a more active, perhaps warm-blooded, way of life. [19]

One subgroup of therapsids, the cynodonts, evolved more mammal-like characteristics.

The jaws of cynodonts resemble modern mammal jaws. This group of animals likely contains a species which is the ancestor of all modern mammals. [20]

From Eucynodontia (cynodonts) came the first mammals. Most early mammals were small shrew-like animals that fed on insects. Although there is no evidence in the fossil record, it is likely that these animals had a constant body temperature and milk glands for their young. The neocortex region of the brain first evolved in mammals and thus is unique to them.

Monotremes are an egg-laying group of mammals represented amongst modern animals by the platypus and echidna. Recent genome sequencing of the platypus indicates that its sex genes are closer to those of birds than to those of the therian (live birthing) mammals. Comparing this to other mammals, it can be inferred that the first mammals to gain sexual differentiation through the existence or lack of SRY gene (found in the y-Chromosome) evolved after the monotreme lineage split off.

Juramaia sinensis [21] is the earliest known eutherian mammal fossil.

Primates Edit

A group of small, nocturnal, arboreal, insect-eating mammals called Euarchonta begins a speciation that will lead to the orders of primates, treeshrews and flying lemurs. Primatomorpha is a subdivision of Euarchonta including primates and their ancestral stem-primates Plesiadapiformes. An early stem-primate, Plesiadapis, still had claws and eyes on the side of the head, making it faster on the ground than in the trees, but it began to spend long times on lower branches, feeding on fruits and leaves.

The Plesiadapiformes very likely contain the ancestor species of all primates. [22] They first appeared in the fossil record around 66 million years ago, soon after the Cretaceous–Paleogene extinction event that eliminated about three-quarters of plant and animal species on Earth, including most dinosaurs. [23] [24]

One of the last Plesiadapiformes is Carpolestes simpsoni, having grasping digits but not forward-facing eyes.

Haplorrhini splits into infraorders Platyrrhini and Catarrhini. Platyrrhines, New World monkeys, have prehensile tails and males are color blind. The individuals whose descendants would become Platyrrhini are conjectured to have migrated to South America either on a raft of vegetation or via a land bridge (the hypothesis now favored [25] ). Catarrhines mostly stayed in Africa as the two continents drifted apart. Possible early ancestors of catarrhines include Aegyptopithecus and Saadanius.

Catarrhini splits into 2 superfamilies, Old World monkeys (Cercopithecoidea) and apes (Hominoidea). Human trichromatic color vision had its genetic origins in this period.

Proconsul was an early genus of catarrhine primates. They had a mixture of Old World monkey and ape characteristics. Proconsul's monkey-like features include thin tooth enamel, a light build with a narrow chest and short forelimbs, and an arboreal quadrupedal lifestyle. Its ape-like features are its lack of a tail, ape-like elbows, and a slightly larger brain relative to body size.

Proconsul africanus is a possible ancestor of both great and lesser apes, including humans.

Hominidae Edit

Date Event
18 Ma Hominidae (great ape ancestors) speciate from the ancestors of the gibbon (lesser apes) between c. 20 to 16 Ma. [26]
16 Ma Homininae ancestors speciate from the ancestors of the orangutan between c. 18 to 14 Ma. [27]

Pierolapithecus catalaunicus is thought to be a common ancestor of humans and the other great apes, or at least a species that brings us closer to a common ancestor than any previous fossil discovery. It had the special adaptations for tree climbing as do present-day humans and other great apes: a wide, flat rib cage, a stiff lower spine, flexible wrists, and shoulder blades that lie along its back.

Hominini: The latest common ancestor of humans and chimpanzees is estimated to have lived between roughly 10 to 5 million years ago. Both chimpanzees and humans have a larynx that repositions during the first two years of life to a spot between the pharynx and the lungs, indicating that the common ancestors have this feature, a precondition for vocalized speech in humans. Speciation may have begun shortly after 10 Ma, but late admixture between the lineages may have taken place until after 5 Ma. Candidates of Hominina or Homininae species which lived in this time period include Ouranopithecus (c. 8 Ma), Graecopithecus (c. 7 Ma), Sahelanthropus tchadensis (c. 7 Ma), Orrorin tugenensis (c. 6 Ma).

Ardipithecus was arboreal, meaning it lived largely in the forest where it competed with other forest animals for food, no doubt including the contemporary ancestor of the chimpanzees. Ardipithecus was probably bipedal as evidenced by its bowl shaped pelvis, the angle of its foramen magnum and its thinner wrist bones, though its feet were still adapted for grasping rather than walking for long distances.

A member of the Australopithecus afarensis left human-like footprints on volcanic ash in Laetoli, northern Tanzania, providing strong evidence of full-time bipedalism. Australopithecus afarensis lived between 3.9 and 2.9 million years ago, and is considered one of the earliest hominins—those species that developed and comprised the lineage of Homo and Homo ' s closest relatives after the split from the line of the chimpanzees.

It is thought that A. afarensis was ancestral to both the genus Australopithecus and the genus Homo. Compared to the modern and extinct great apes, A. afarensis had reduced canines and molars, although they were still relatively larger than in modern humans. A. afarensis also has a relatively small brain size (380–430 cm³) and a prognathic (anterior-projecting) face.

Australopithecines have been found in savannah environments they probably developed their diet to include scavenged meat. Analyses of Australopithecus africanus lower vertebrae suggests that these bones changed in females to support bipedalism even during pregnancy.

Homo homo Edit

Early Homo appears in East Africa, speciating from australopithecine ancestors. Sophisticated stone tools mark the beginning of the Lower Paleolithic. Australopithecus garhi was using stone tools at about 2.5 Ma. Homo habilis is the oldest species given the designation Homo, by Leakey et al. (1964). H. habilis is intermediate between Australopithecus afarensis and H. erectus, and there have been suggestions to re-classify it within genus Australopithecus, as Australopithecus habilis.

Stone tools found at the Shangchen site in China and dated to 2.12 million years ago are considered the earliest known evidence of hominins outside Africa, surpassing Dmanisi in Georgia by 300,000 years. [34]

Homo erectus derives from early Homo or late Australopithecus.

Homo habilis, although significantly different of anatomy and physiology, is thought to be the ancestor of Homo ergaster, or African Homo erectus but it is also known to have coexisted with H. erectus for almost half a million years (until about 1.5 Ma). From its earliest appearance at about 1.9 Ma, H. erectus is distributed in East Africa and Southwest Asia (Homo georgicus). H. erectus is the first known species to develop control of fire, by about 1.5 Ma.

H. erectus later migrates throughout Eurasia, reaching Southeast Asia by 0.7 Ma. It is described in a number of subspecies. [35]

Homo antecessor may be a common ancestor of humans and Neanderthals. [37] [38] At present estimate, humans have approximately 20,000–25,000 genes and share 99% of their DNA with the now extinct Neanderthal [39] and 95–99% of their DNA with their closest living evolutionary relative, the chimpanzees. [40] [41] The human variant of the FOXP2 gene (linked to the control of speech) has been found to be identical in Neanderthals. [42]

Divergence of Neanderthal and Denisovan lineages from a common ancestor. [43] Homo heidelbergensis (in Africa also known as Homo rhodesiensis) had long been thought to be a likely candidate for the last common ancestor of the Neanderthal and modern human lineages. However, genetic evidence from the Sima de los Huesos fossils published in 2016 seems to suggest that H. heidelbergensis in its entirety should be included in the Neanderthal lineage, as "pre-Neanderthal" or "early Neanderthal", while the divergence time between the Neanderthal and modern lineages has been pushed back to before the emergence of H. heidelbergensis, to about 600,000 to 800,000 years ago, the approximate age of Homo antecessor. [44] [45]

Solidified footprints dated to about 350 ka and associated with H. heidelbergensis were found in southern Italy in 2003. [46]

Homo sapiens Edit

Fossils attributed to H. sapiens, along with stone tools, dated to approximately 300,000 years ago, found at Jebel Irhoud, Morocco [47] yield the earliest fossil evidence for anatomically modern Homo sapiens. Modern human presence in East Africa (Gademotta), at 276 kya. [48] A 177,000-year-old jawbone fossil discovered in Israel in 2017 is the oldest human remains found outside Africa. [49] However, in July 2019, anthropologists reported the discovery of 210,000 year old remains of a H. sapiens and 170,000 year old remains of a H. neanderthalensis in Apidima Cave, Peloponnese, Greece, more than 150,000 years older than previous H. sapiens finds in Europe. [50] [51] [52]

Neanderthals emerge from the Homo heidelbergensis lineage at about the same time (300 ka).

Patrilineal and matrilineal most recent common ancestors (MRCAs) of living humans roughly between 200 and 100 ka [53] [54] with some estimates on the patrilineal MRCA somewhat higher, ranging up to 250 to 500 kya. [55]

160,000 years ago, Homo sapiens idaltu in the Awash River Valley (near present-day Herto village, Ethiopia) practiced excarnation. [56]

Modern human presence in Southern Africa and West Africa. [57] Appearance of mitochondrial haplogroup (mt-haplogroup) L2.

Early evidence for behavioral modernity. [58] Appearance of mt-haplogroups M and N. Southern Dispersal migration out of Africa, Proto-Australoid peopling of Oceania. [59] Archaic admixture from Neanderthals in Eurasia, [60] [61] from Denisovans in Oceania with trace amounts in Eastern Eurasia, [62] and from an unspecified African lineage of archaic humans in Sub-Saharan Africa as well as an interbred species of Neanderthals and Denisovans in Asia and Oceania. [63] [64] [65] [66]

Behavioral modernity develops, according to the "great leap forward" theory. [67] Extinction of Homo floresiensis. [68] M168 mutation (carried by all non-African males). Appearance of mt-haplogroups U and K. Peopling of Europe, peopling of the North Asian Mammoth steppe. Paleolithic art. Extinction of Neanderthals and other archaic human variants (with possible survival of hybrid populations in Asia and Africa.) Appearance of Y-Haplogroup R2 mt-haplogroups J and X.


21.3 Preserving Biodiversity

Preserving biodiversity is an extraordinary challenge that must be met by greater understanding of biodiversity itself, changes in human behavior and beliefs, and various preservation strategies.

Change in Biodiversity through Time

The number of species on the planet, or in any geographical area, is the result of an equilibrium of two evolutionary processes that are ongoing: speciation and extinction. Both are natural “birth” and “death” processes of macroevolution. When speciation rates begin to outstrip extinction rates, the number of species will increase likewise, the reverse is true when extinction rates begin to overtake speciation rates. Throughout the history of life on Earth, as reflected in the fossil record, these two processes have fluctuated to a greater or lesser extent, sometimes leading to dramatic changes in the number of species on the planet as reflected in the fossil record (Figure 21.13).

Paleontologists have identified five strata in the fossil record that appear to show sudden and dramatic (greater than half of all extant species disappearing from the fossil record) losses in biodiversity. These are called mass extinctions. There are many lesser, yet still dramatic, extinction events, but the five mass extinctions have attracted the most research into their causes. An argument can be made that the five mass extinctions are only the five most extreme events in a continuous series of large extinction events throughout the fossil record (since 542 million years ago). In most cases, the hypothesized causes are still controversial in one, the most recent, the cause seems clear. The most recent extinction in geological time, about 65 million years ago, saw the disappearance of the dinosaurs and many other species. Most scientists now agree the cause of this extinction was the impact of a large asteroid in the present-day Yucatán Peninsula and the subsequent energy release and global climate changes caused by dust ejected into the atmosphere.

Recent and Current Extinction Rates

A sixth, or Holocene, mass extinction has mostly to do with the activities of Homo sapiens. There are numerous recent extinctions of individual species that are recorded in human writings. Most of these are coincident with the expansion of the European colonies since the 1500s.

One of the earlier and popularly known examples is the dodo bird. The dodo bird lived in the forests of Mauritius, an island in the Indian Ocean. The dodo bird became extinct around 1662. It was hunted for its meat by sailors and was easy prey because the dodo, which did not evolve with humans, would approach people without fear. Introduced pigs, rats, and dogs brought to the island by European ships also killed dodo young and eggs (Figure 21.14).

Steller’s sea cow became extinct in 1768 it was related to the manatee and probably once lived along the northwest coast of North America. Steller’s sea cow was discovered by Europeans in 1741, and it was hunted for meat and oil. A total of 27 years elapsed between the sea cow’s first contact with Europeans and extinction of the species. The last Steller’s sea cow was killed in 1768. In another example, the last living passenger pigeon died in a zoo in Cincinnati, Ohio, in 1914. This species had once migrated in the millions but declined in numbers because of overhunting and loss of habitat through the clearing of forests for farmland.

These are only a few of the recorded extinctions in the past 500 years. The International Union for Conservation of Nature (IUCN) keeps a list of extinct and endangered species called the Red List. The list is not complete, but it describes 380 vertebrates that became extinct after 1500 AD, 86 of which were driven extinct by overhunting or overfishing.

Estimates of Present-day Extinction Rates

Estimates of extinction rates are hampered by the fact that most extinctions are probably happening without being observed. The extinction of a bird or mammal is often noticed by humans, especially if it has been hunted or used in some other way. But there are many organisms that are less noticeable to humans (not necessarily of less value) and many that are undescribed.

The background extinction rate is estimated to be about 1 per million species years (E/MSY). One “species year” is one species in existence for one year. One million species years could be one species persisting for one million years, or a million species persisting for one year. If it is the latter, then one extinction per million species years would be one of those million species becoming extinct in that year. For example, if there are 10 million species in existence, then we would expect 10 of those species to become extinct in a year. This is the background rate.

One contemporary extinction-rate estimate uses the extinctions in the written record since the year 1500. For birds alone, this method yields an estimate of 26 E/MSY, almost three times the background rate. However, this value may be underestimated for three reasons. First, many existing species would not have been described until much later in the time period and so their loss would have gone unnoticed. Second, we know the number is higher than the written record suggests because now extinct species are being described from skeletal remains that were never mentioned in written history. And third, some species are probably already extinct even though conservationists are reluctant to name them as such. Taking these factors into account raises the estimated extinction rate to nearer 100 E/MSY. The predicted rate by the end of the century is 1500 E/MSY.

A second approach to estimating present-time extinction rates is to correlate species loss with habitat loss, and it is based on measuring forest-area loss and understanding species–area relationships. The species-area relationship is the rate at which new species are seen when the area surveyed is increased (Figure 21.15). Likewise, if the habitat area is reduced, the number of species seen will also decline. This kind of relationship is also seen in the relationship between an island’s area and the number of species present on the island: as one increases, so does the other, though not in a straight line. Estimates of extinction rates based on habitat loss and species–area relationships have suggested that with about 90 percent of habitat loss an expected 50 percent of species would become extinct. Figure 21.15 shows that reducing forest area from 100 km 2 to 10 km 2 , a decline of 90 percent, reduces the number of species by about 50 percent. Species–area estimates have led to estimates of present-day species extinction rates of about 1000 E/MSY and higher. In general, actual observations do not show this amount of loss and one explanation put forward is that there is a delay in extinction. According to this explanation, it takes some time for species to fully suffer the effects of habitat loss and they linger on for some time after their habitat is destroyed, but eventually they will become extinct. Recent work has also called into question the applicability of the species-area relationship when estimating the loss of species. This work argues that the species–area relationship leads to an overestimate of extinction rates. Using an alternate method would bring estimates down to around 500 E/MSY in the coming century. Note that this value is still 500 times the background rate.

Concepts in Action

Go to this website for an interactive exploration of endangered and extinct species, their ecosystems, and the causes of their endangerment or extinction.

Conservation of Biodiversity

The threats to biodiversity at the genetic, species, and ecosystem levels have been recognized for some time. In the United States, the first national park with land set aside to remain in a wilderness state was Yellowstone Park in 1890. However, attempts to preserve nature for various reasons have occurred for centuries. Today, the main efforts to preserve biodiversity involve legislative approaches to regulate human and corporate behavior, setting aside protected areas, and habitat restoration.

Changing Human Behavior

Legislation has been enacted to protect species throughout the world. The legislation includes international treaties as well as national and state laws. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) treaty came into force in 1975. The treaty, and the national legislation that supports it, provides a legal framework for preventing “listed” species from being transported across nations’ borders, thus protecting them from being caught or killed in the first place when the purpose involves international trade. The listed species that are protected to one degree or another by the treaty number some 33,000. The treaty is limited in its reach because it only deals with international movement of organisms or their parts. It is also limited by various countries’ ability or willingness to enforce the treaty and supporting legislation. The illegal trade in organisms and their parts is probably a market in the hundreds of millions of dollars.

Within many countries there are laws that protect endangered species and that regulate hunting and fishing. In the United States, the Endangered Species Act was enacted in 1973. When an at-risk species is listed by the Act, the U.S. Fish & Wildlife Service is required by law to develop a management plan to protect the species and bring it back to sustainable numbers. The Act, and others like it in other countries, is a useful tool, but it suffers because it is often difficult to get a species listed, or to get an effective management plan in place once a species is listed. Additionally, species may be controversially taken off the list without necessarily having had a change in their situation. More fundamentally, the approach to protecting individual species rather than entire ecosystems (although the management plans commonly involve protection of the individual species’ habitat) is both inefficient and focuses efforts on a few highly visible and often charismatic species, perhaps at the expense of other species that go unprotected.

The Migratory Bird Treaty Act (MBTA) is an agreement between the United States and Canada that was signed into law in 1918 in response to declines in North American bird species caused by hunting. The Act now lists over 800 protected species. It makes it illegal to disturb or kill the protected species or distribute their parts (much of the hunting of birds in the past was for their feathers). Examples of protected species include northern cardinals, the red-tailed hawk, and the American black vulture.

Global warming is expected to be a major driver of biodiversity loss. Many governments are concerned about the effects of anthropogenic global warming, primarily on their economies and food resources. Since greenhouse gas emissions do not respect national boundaries, the effort to curb them is an international one. The international response to global warming has been mixed. The Kyoto Protocol, an international agreement that came out of the United Nations Framework Convention on Climate Change that committed countries to reducing greenhouse gas emissions by 2012, was ratified by some countries, but spurned by others. Two countries that were especially important in terms of their potential impact that did not ratify the Kyoto protocol were the United States and China. Some goals for reduction in greenhouse gasses were met and exceeded by individual countries, but, worldwide, the effort to limit greenhouse gas production is not succeeding. The intended replacement for the Kyoto Protocol has not materialized because governments cannot agree on timelines and benchmarks. Meanwhile, the resulting costs to human societies and biodiversity predicted by a majority of climate scientists will be high.

As already mentioned, the non-profit, non-governmental sector plays a large role in conservation effort both in North America and around the world. The approaches range from species-specific organizations to the broadly focused IUCN and Trade Records Analysis of Flora and Fauna in Commerce (TRAFFIC). The Nature Conservancy takes a novel approach. It purchases land and protects it in an attempt to set up preserves for ecosystems. Ultimately, human behavior will change when human values change. At present, the growing urbanization of the human population is a force that mitigates against valuing biodiversity, because many people no longer come in contact with natural environments and the species that inhabit them.

Conservation in Preserves

Establishment of wildlife and ecosystem preserves is one of the key tools in conservation efforts (Figure 21.16). A preserve is an area of land set aside with varying degrees of protection for the organisms that exist within the boundaries of the preserve. Preserves can be effective for protecting both species and ecosystems, but they have some serious drawbacks.

A simple measure of success in setting aside preserves for biodiversity protection is to set a target percentage of land or marine habitat to protect. However, a more detailed preserve design and choice of location is usually necessary because of the way protected lands are allocated and how biodiversity is distributed: protected lands tend to contain less economically valuable resources rather than being set aside specifically for the species or ecosystems at risk. In 2003, the IUCN World Parks Congress estimated that 11.5 percent of Earth’s land surface was covered by preserves of various kinds. This area is greater than previous goals however, it only represents 9 out of 14 recognized major biomes and research has shown that 12 percent of all species live outside preserves these percentages are much higher when threatened species are considered and when only high quality preserves are considered. For example, high quality preserves include only about 50 percent of threatened amphibian species. The conclusion must be that either the percentage of area protected must be increased, the percentage of high quality preserves must be increased, or preserves must be targeted with greater attention to biodiversity protection. Researchers argue that more attention to the latter solution is required.

A biodiversity hotspot is a conservation concept developed by Norman Myers in 1988. Hotspots are geographical areas that contain high numbers of endemic species. The purpose of the concept was to identify important locations on the planet for conservation efforts, a kind of conservation triage. By protecting hotspots, governments are able to protect a larger number of species. The original criteria for a hotspot included the presence of 1500 or more species of endemic plants and 70 percent of the area disturbed by human activity. There are now 34 biodiversity hotspots (Figure 21.17) that contain large numbers of endemic species, which include half of Earth’s endemic plants.

There has been extensive research into optimal preserve designs for maintaining biodiversity. The fundamental principles behind much of the research have come from the seminal theoretical work of Robert H. MacArthur and Edward O. Wilson published in 1967 on island biogeography. 2 This work sought to understand the factors affecting biodiversity on islands. Conservation preserves can be seen as “islands” of habitat within “an ocean” of non-habitat. In general, large preserves are better because they support more species, including species with large home ranges they have more core area of optimal habitat for individual species they have more niches to support more species and they attract more species because they can be found and reached more easily.

Preserves perform better when there are partially protected buffer zones around them of suboptimal habitat. The buffer allows organisms to exit the boundaries of the preserve without immediate negative consequences from hunting or lack of resources. One large preserve is better than the same area of several smaller preserves because there is more core habitat unaffected by less hospitable ecosystems outside the preserve boundary. For this same reason, preserves in the shape of a square or circle will be better than a preserve with many thin “arms.” If preserves must be smaller, then providing wildlife corridors between them so that species and their genes can move between the preserves for example, preserves along rivers and streams will make the smaller preserves behave more like a large one. All of these factors are taken into consideration when planning the nature of a preserve before the land is set aside.

In addition to the physical specifications of a preserve, there are a variety of regulations related to the use of a preserve. These can include anything from timber extraction, mineral extraction, regulated hunting, human habitation, and nondestructive human recreation. Many of the decisions to include these other uses are made based on political pressures rather than conservation considerations. On the other hand, in some cases, wildlife protection policies have been so strict that subsistence-living indigenous populations have been forced from ancestral lands that fell within a preserve. In other cases, even if a preserve is designed to protect wildlife, if the protections are not or cannot be enforced, the preserve status will have little meaning in the face of illegal poaching and timber extraction. This is a widespread problem with preserves in the tropics.

Some of the limitations on preserves as conservation tools are evident from the discussion of preserve design. Political and economic pressures typically make preserves smaller, never larger, so setting aside areas that are large enough is difficult. Enforcement of protections is also a significant issue in countries without the resources or political will to prevent poaching and illegal resource extraction.

Climate change will create inevitable problems with the location of preserves as the species within them migrate to higher latitudes as the habitat of the preserve becomes less favorable. Planning for the effects of global warming on future preserves, or adding new preserves to accommodate the changes expected from global warming is in progress, but will only be as effective as the accuracy of the predictions of the effects of global warming on future habitats.

Finally, an argument can be made that conservation preserves reinforce the cultural perception that humans are separate from nature, can exist outside of it, and can only operate in ways that do damage to biodiversity. Creating preserves reduces the pressure on human activities outside the preserves to be sustainable and non-damaging to biodiversity. Ultimately, the political, economic, and human demographic pressures will degrade and reduce the size of conservation preserves if the activities outside them are not altered to be less damaging to biodiversity.

Concepts in Action

Check out this interactive global data system of protected areas. Review data about specific protected areas by location or study statistics on protected areas by country or region.

Habitat Restoration

Habitat restoration holds considerable promise as a mechanism for maintaining or restoring biodiversity. Of course once a species has become extinct, its restoration is impossible. However, restoration can improve the biodiversity of degraded ecosystems. Reintroducing wolves, a top predator, to Yellowstone National Park in 1995 led to dramatic changes in the ecosystem that increased biodiversity. The wolves (Figure 21.18) function to suppress elk and coyote populations and provide more abundant resources to the guild of carrion eaters. Reducing elk populations has allowed revegetation of riparian (the areas along the banks of a stream or river) areas, which has increased the diversity of species in that habitat. Suppression of coyotes has increased the species previously suppressed by this predator. The number of species of carrion eaters has increased because of the predatory activities of the wolves. In this habitat, the wolf is a keystone species, meaning a species that is instrumental in maintaining diversity within an ecosystem. Removing a keystone species from an ecological community causes a collapse in diversity. The results from the Yellowstone experiment suggest that restoring a keystone species effectively can have the effect of restoring biodiversity in the community. Ecologists have argued for the identification of keystone species where possible and for focusing protection efforts on these species. It makes sense to return the keystone species to the ecosystems where they have been removed.

Other large-scale restoration experiments underway involve dam removal. In the United States, since the mid-1980s, many aging dams are being considered for removal rather than replacement because of shifting beliefs about the ecological value of free-flowing rivers. The measured benefits of dam removal include restoration of naturally fluctuating water levels (often the purpose of dams is to reduce variation in river flows), which leads to increased fish diversity and improved water quality. In the Pacific Northwest, dam removal projects are expected to increase populations of salmon, which is considered a keystone species because it transports nutrients to inland ecosystems during its annual spawning migrations. In other regions, such as the Atlantic coast, dam removal has allowed the return of other spawning anadromous fish species (species that are born in fresh water, live most of their lives in salt water, and return to fresh water to spawn). Some of the largest dam removal projects have yet to occur or have happened too recently for the consequences to be measured. The large-scale ecological experiments that these removal projects constitute will provide valuable data for other dam projects slated either for removal or construction.

The Role of Zoos and Captive Breeding

Zoos have sought to play a role in conservation efforts both through captive breeding programs and education (Figure 21.19). The transformation of the missions of zoos from collection and exhibition facilities to organizations that are dedicated to conservation is ongoing. In general, it has been recognized that, except in some specific targeted cases, captive breeding programs for endangered species are inefficient and often prone to failure when the species are reintroduced to the wild. Zoo facilities are far too limited to contemplate captive breeding programs for the numbers of species that are now at risk. Education, on the other hand, is a potential positive impact of zoos on conservation efforts, particularly given the global trend to urbanization and the consequent reduction in contacts between people and wildlife. A number of studies have been performed to look at the effectiveness of zoos on people’s attitudes and actions regarding conservation at present, the results tend to be mixed.


Evolution of extreme parasites explained by scientists

Extreme adaptations of species often cause such significant changes that their evolutionary history is difficult to reconstruct. Zoologists at the University of Basel in Switzerland have now discovered a new parasite species that represents the missing link between fungi and an extreme group of parasites. Researches are now able to understand for the first time the evolution of these parasites, causing disease in humans and animals. The study has been published in the latest issue of the scientific journal Proceedings of the National Academy of Sciences (PNAS).

Parasites use their hosts to simplify their own lives. In order to do so, they evolved features that are so extreme that it is often impossible to compare them to other species. The evolution of these extreme adaptations is often impossible to reconstruct. The research group lead by Prof. Dieter Ebert from the Department of Environmental Science at the University of Basel has now discovered the missing link that explains how this large group of extreme parasites, the microsporidia, has evolved. The team was supported in their efforts by scientists from Sweden and the U.S.

Microsporidia are a large group of extreme parasites that invade humans and animals and cost great damage for health care systems and in agriculture over 1,200 species are known. They live inside their host's cells and have highly specialized features: They are only able to reproduce inside the host's cells, they have the smallest known genome of all organisms with a cell nucleus (eukaryotes) and they posses no mitochondria of their own (the cell's power plant). In addition, they developed a specialized infection apparatus, the polar tube, which they use to insert themselves into the cells of their host. Due to their phenomenal high molecular evolution rate, genome analysis has so far been rather unsuccessful: Their great genomic divergence from all other known organisms further complicates the study of their evolutionary lineage.

Between fungi and parasite

The team of zoologists lead by Prof. Dieter Ebert has been studying the evolution of microsporidia for years. When they discovered a new parasite in water fleas a couple of years ago, they classified this undescribed species as a microsporidium, mostly because it possessed the unique harpoon-like infection apparatus (the polar-tube), one of the hallmarks of microsporidia. The analysis of the entire genome had several surprises in store for them: The genome resembles more that of a fungi than a microsporidium and, in addition, also has a mitochondrial genome. The new species, now named Mitosporidium daphniae, thus represents the missing link between fungi and microsporidia.

With the help of scientists in Sweden and the U.S., the Basel researchers rewrote the evolutionary history of microsporidia. First, they showed that the new species derives from the ancestors of all known microsporidians and further, that the microsporidians derive from the most ancient fungi thus its exact place in the tree of life has finally been found. Further research confirms that the new species does in fact have a microsporidic, intracellular and parasitic lifestyle, but that its genome is rather atypical for a microsporidium. It resembles much more the genome of their fungal ancestors.

Genome modifications

The scientists thus conclude that the microsporidia adopted intracellular parasitism first and only later changed their genome significantly. These genetic adaptations include the loss of mitochondria, as well as extreme metabolic and genomic simplification. "Our results are not only a milestone for the research on microsporidia, but they are also of great interest to the study of parasite-specific adaptations in evolution in general," explains Ebert the findings.


Overproduction

Overproduction is a serious issue that everyone needs to pay attention to. Between pollution and deforestation, we are doing a lot of harm to our planet without even realizing it. Overproduction is causing more waste and that waste is filling our air and water. This is causing pollution that can easily be stopped. Deforestation is causing us to cut down trees to make more homes for people, leaving our animals without homes. Animals have been suffering pollution and deforestation and we are only starting to notice because we are in danger to overproduction next. How can we as a community stop this? Are we really in serious danger?

Overproduction is production of more of a product, commodity, or substance than is wanted or needed. An example of this would be: when working at a restaurant you often make more food than you need, which then causes you to throw away the extra food. That extra food could go to someone else who really needs it, instead of in the trash. Or another idea would be to only make what is needed.

Deforestation

When considering overproduction and pollution, we have to think about what causes us to overproduce/pollute so much and where we get our resources from. Deforestation is perhaps the main cause of overproduction and is driven by the demands of human consumption. Deforestation is the removal of a forest or stand of trees from land, which is then converted to a non-forest use. Humans have been using trees for building material, paper products, tools, and many other uses throughout history, but all these things come at a cost, and that is the effect deforestation and overproduction have on the environment.

When it comes to deforestation, there are many reasons it happens and various techniques used in the process. The main reasons include land for agriculture, settlements, roads, and infrastructure. Overproduction due to the constantly growing population is one of the main contributors to deforestation.The growing population leaves us with no other option but to build more houses which leads to more infrastructure, etc. According to National Geographic, if the current rate of deforestation continues the world’s rainforests may completely vanish in as little as 100 years. Overproduction and the earth’s population has caused us to lose millions of acres of forests per year. Deforestation throughout the past and currently will also have a noticeable effect on the climate in the near future.

Deforestation first became a serious concern in the 1950s and 1960s, especially in Brazil. However, deforestation has been occurring for hundreds of thousands of years since humans first began using fire. As civilization and urbanization increased, the demand for more land and resources resulted in more and more clearing of the forests. Slash and burn is one of the techniques used in the deforestation process. This is when existing vegetation is cut down and burned off before new crops are planted. Prior to the 1960s, restrictions kept deforestation in the Amazon to a minimum, until farmers began colonizing this tropical area using slash-and-burn techniques. This greatly diminishes the soil’s nutrients and lessens the possibility for future growth in that area. Techniques such as slash-and-burn have a huge impact on the environment and the overall health of our ecosystems, but deforestation and these techniques lead to other problems like the earth’s water cycles.

Another major concern we are seeing with deforestation, is the effect it has on weather, especially water cycles. The elimination of trees decreases the amount of transpiration, which means that the amount of rainfall in that area will decrease. Water cycles are connected to clouds, precipitation patterns, runoff and stream flow patterns which effects all of nature and us as humans. We have seen the effects these changes are having throughout the world and according to the U.S. Drought Monitor, 38.4% of the U.S. is in a drought. Greenhouse gases produced by deforestation influence and create drought and changes in water cycles. This has an ongoing effect on the earth’s ecosystem which affects crops, wildlife, people, etc. The effects deforestation has on the climate is evident and if we continue on this path, eventually their won’t be anything left.

The last aspect of deforestation, is logging, which is one of the primary drivers of deforestation. Logging is the cutting down of trees to harvest timber, products, fuel, etc. There are many reasons for logging including materials, infrastructure like roads, and many other uses but it leads to many negative consequences for wildlife, and the overall integrity of the forest. Plants store carbon dioxide and the deforestation because of logging is one of the biggest contributors to greenhouse gases because trees store carbon dioxide which is all released into the environment when they are cut down. This is one of the main contributors to global warming which is one of the major problems we are seeing around the world today. Deforestation is a product of overproduction and we are seeing the tole this takes on the environment. Something has to change in the near future to preserve the forests we have left as well as beginning to rebuild everything we have destroyed.

Effects on Animals

Overproduction has proven to have a detrimental effect on wildlife. According to statistics, humans have wiped out nearly 60% of mammals, birds, fish, and reptiles since the 1970s. The World Wildlife Foundation equates this massive decline in animal populations to the continually growing number of food and resource consumption. This increase in consumption is a direct result of the ever growing human population. According to scientists, the world has already begun the process of its sixth mass extinction, the first to ever be caused by an animal species. That species is the human race. Extinction is always occurring, with a natural rate of about one to five species disappearing each year. At present time, however, extinction is occurring at nearly 1,000 to 10,000 times the natural rate per year. This number averages out to dozens of extinctions daily. At present moment, humans have eliminated almost half of the plant life in the world and nearly 83% of all mammals. The destruction caused by overproduction has already gone too far, with scientists predicting that, at present moment, it could take the natural world 5 to 7 million years to recover from this annihilation of plants and wildlife.

One factor that is the direct result of overproduction, as well as a direct cause of this animal species destruction, is air pollution. The existence of air pollution has been present since Roman Times and has only continued to worsen up until present day. The effects of air pollution have proven detrimental on all animal habitats. One of these negative results is acid rain, which is rainfall made acidic due to environmental strain and harm. It ultimately causes a change in both the chemistry and quality of soil and water. Acid rain produces aluminum and, in turn, increases the pH balance of lakes, streams, and ponds, among other bodies of water. Higher pH levels can result in the inability for fish eggs to hatch, while lower pH levels can cause the death of some adult fish. Acid rain also negatively affects plant life, as it can over produce aluminum and strip away important nutrients from the soil. Without these essential nutrients, both plants and trees are unable to grow.

Heavy metals are another result of air pollution, ultimately entering into animal food chains and having a damaging effect on food quality and supply. Mercury is a heavy metal that, at high levels of exposure, can produce many repercussions. Fish populations are especially affected by high mercury concentrations, as researchers have observed a reduction in loon chick egg production in ponds where high levels of mercury are present. Smog is another consequence of air pollution. While not perfectly understood by scientists, it is thought that it has the same effect on wildlife as it does humans. Smog is thought to cause harm to the lungs and cardiovascular systems of animal species.

It is not just animal species that are affected by air pollution, however. The human population, specifically children, experience certain detriments caused by unhealthy air quality. These detriments can even begin in the womb, with children being more likely to experience preterm birth when born into communities with higher particle pollution levels. Children are also more susceptible to a lack of lung growth in their early years when exposed to poor air quality. Eighty percent of the alveoli, a tiny air sac found in the lungs that is essential to the intake of oxygen, does not become fully developed until adulthood. Exposure to air pollution can potentially stunt the growth of the alveoli, resulting in future respiratory problems. This lack of lung growth can also make individuals more susceptible to infection and disease.

Food Industry

According to Google, the food industry is defined as “a complex, global collective of diverse businesses that supplies most of the food consumed by the world’s population.” When it comes to overproduction, the food industry plays a big role. Much research has found that humans are wasteful when it comes to food. Seeing as though more food is being produced than what the population needs, majority of the time, the extra food goes to waste. Reports from the nonprofit organization, ReFED, have demonstrated that 25 billion dollars worth of food is produced as waste each year from U.S. restaurants alone. Becoming aware of such a fact can come as a shock to many people as none of us are truly conscience about the waste we produce daily. It has been many years coming for this global issue to expand into what it is today, but surely we can all play a role in bettering the situation.

Although our world is currently being greatly affected by overproduction of food, it has not always been so bad. Back in time when the food industry was not yet as industrialized as it is today, sustainable living prevented the effects we are seeing today. Seeing as though humans were forced to hunt and grow their food, they were not as wasteful as we are today. Generally, the only food they were consuming was the amount they needed in order to survive. As a matter of fact, even today, those who survive off of what they grow or hunt are considered to be outside of the scope of what is considered to be the food industry today. With the growth of population and the development of lifestyle that has occured over time, these results have all changed.

Studies have found that the U.S. spends over 220 billion dollars a year growing, transporting, and processing almost 70 millions tons of food that goes to waste. Scientists have estimated that growing food that ends up waste uses up to 21% of our fresh water, 19% of our fertilizer, 18% of our cropland, and 21% of our landfill volume. These facts have proven that when we waste food, it is not the only resource that we are being wasteful of. Not only is this hurting our planet, but it is also paving the way for the future of our people, our Earth and our animals. Research has also found that the growing food system is one of the biggest contributors to climate change due to the fact that it is responsible for approximately one-third of all human caused greenhouse gas emissions. How can this be some may ask? Well according to research, when food is disposed of in a landfill, as it rots it becomes a source of methane, a greenhouse gas that carries 21 times the potential for global warming than carbon dioxide. Overall, studies have determined that in the U.S. we waste 40% of our food.

Possible solutions to food waste:

  • Plan appropriately
  • Becoming aware of food portions
  • Save leftovers
  • Store food correctly to prevent expiration
  • Keep track of waste
  • Reuse, Reduce, Recycle

In the 1960s environmental movements began because of the pollution that was coming into our planet. It all started with an oil spill that caused many Americans to panic. They soon realized that there was an increase in demand and limited resources to fill that demand. In the 1970’s, cars filled up roads all across America. This caused carbon monoxide to fill the air. Automobiles and factory industries began to pollute our air. This “environmental crisis” was known as the Smog in Los Angeles and New York City. “In the Great Smog of 1952, pollutants from factories and home fireplaces mixed with air condensations killed at least 4,000 people in London over the course of several days.” Events such as Earth Day and Acts such as the Clean Air Act (1970) and the Clean Water Act (1972) were put into place to help keep the Earth cleaner. Water pollution happens when things such as chemicals or microorganisms get into our water and create a toxic environment for humans. Water pollution is caused by “toxic substances from farms, towns, and factories readily dissolve into and mix with it causing water pollution.”

Imagine how different our world would be without any pollution. The sky at night and the ocean during the day would not even look real. They would be so clear and beautiful. Water pollution is when chemicals or microorganisms infect our water. This is caused by “universal solvent” which is when water cannot break down any substances other than a liquid that enters water. “According to the most recent surveys on national water quality from the U.S. Environmental Protection Agency, nearly half of our rivers and streams and more than one-third of our lakes are polluted and unfit for swimming, fishing, and drinking.” If we keep polluting our waters, it will only get worse.

Below are pictures of how the sky used to look before air pollution and what it looks like now. This is the air that we are breathing in and someday our children will be breathing it in if we do not put a stop to this. Air pollution is just as dangerous as water pollution. From cars to factory buildings, they are making the air we breathe worse.

How can overproduction be stopped?

  • Eliminate Waste
  • Carpool
  • Recycle
  • Plant more trees
  • Be less wasteful
  • Conserve energy

With proper awareness of this issue we as a society can help minimize the effects we are causing to our planet. All of these things on this list seem pretty easy and simple, right? So why are we not doing them? If we recycle, there is a smaller chance that our waste pollutes our oceans. This includes saving our animals as well. Carpooling will provide less toxic pollution from coming into our air. If we simply plant more trees, we would not have to worry about deforestation.


Contents

Species go extinct constantly as environments change, as organisms compete for environmental niches, and as genetic mutation leads to the rise of new species from older ones. Occasionally biodiversity on Earth takes a hit in the form of a mass extinction in which the extinction rate is much higher than usual. [9] A large extinction-event often represents an accumulation of smaller extinction- events that take place in a relatively brief period of time. [10]

The first known mass extinction in earth's history was the Great Oxygenation Event 2.4 billion years ago. That event led to the loss of most of the planet's obligate anaerobes. Researchers have identified five major extinction events in earth's history since: [11]

    : 440 million years ago, 86% of all species lost, including graptolites : 375 million years ago, 75% of species lost, including most trilobites , "The Great Dying": 251 million years ago, 96% of species lost, including tabulate corals, and most extant trees and synapsids : 200 million years ago, 80% of species lost, including all of the conodonts : 66 million years ago, 76% of species lost, including all of the ammonites, mosasaurs, ichthyosaurs, plesiosaurs, pterosaurs, and nonavian dinosaurs

(Dates and percentages represent estimates.)

Smaller extinction-events have occurred in the periods between these larger catastrophes, with some standing at the delineation points of the periods and epochs recognized by scientists in geologic time. The Holocene extinction event is currently under way. [12]

Factors in mass extinctions include continental drift, changes in atmospheric and marine chemistry, volcanism and other aspects of mountain formation, changes in glaciation, changes in sea level, and impact events. [10]

In this timeline, Ma (for megaannum) means "million years ago," ka (for kiloannum) means "thousand years ago," and ya means "years ago."

Hadean Eon Edit

Date Event
4600 Ma The planet Earth forms from the accretion disc revolving around the young Sun, with organic compounds (complex organic molecules) necessary for life having perhaps formed in the protoplanetary disk of cosmic dust grains surrounding it before the formation of the Earth itself. [13]
4500 Ma According to the giant impact hypothesis, the Moon originated when the planet Earth and the hypothesized planet Theia collided, sending a very large number of moonlets into orbit around the young Earth which eventually coalesced to form the Moon. [14] The gravitational pull of the new Moon stabilised the Earth's fluctuating axis of rotation and set up the conditions in which abiogenesis could occur. [15]
4400 Ma First appearance of liquid water on Earth.
4374 Ma The age of the oldest discovered zircon crystals.
4280 Ma Earliest possible appearance of life on Earth. [16] [17] [18] [19]

Archean Eon Edit

Bacteria develop primitive forms of photosynthesis which at first did not produce oxygen. [32] These organisms generated Adenosine triphosphate (ATP) by exploiting a proton gradient, a mechanism still used in virtually all organisms, unchanged, to this day. [33] [34] [35]

Proterozoic Eon Edit

Date Event
2500 Ma Great Oxidation Event led by cyanobacteria's oxygenic photosynthesis. [37] Commencement of plate tectonics with old marine crust dense enough to subduct. [20]
By 1850 Ma Eukaryotic cells appear. Eukaryotes contain membrane-bound organelles with diverse functions, probably derived from prokaryotes engulfing each other via phagocytosis. (See Symbiogenesis and Endosymbiont). Bacterial viruses (bacteriophage) emerge before, or soon after, the divergence of the prokaryotic and eukaryotic lineages. [39] The appearance of red beds show that an oxidising atmosphere had been produced. Incentives now favoured the spread of eukaryotic life. [40] [41] [42]
1400 Ma Great increase in stromatolite diversity.
1300 Ma Earliest land fungi [43]
By 1200 Ma Meiosis and sexual reproduction are present in single-celled eukaryotes, and possibly in the common ancestor of all eukaryotes. [44] Sex may even have arisen earlier in the RNA world. [45] Sexual reproduction first appears in the fossil records it may have increased the rate of evolution. [46]
1000 Ma The first non-marine eukaryotes move onto land. They were photosynthetic and multicellular, indicating that plants evolved much earlier than originally thought. [47]
750 Ma First protozoa (ex: Melanocyrillium) beginning of animal evolution [48] [49]
850–630 Ma A global glaciation may have occurred. [50] [51] Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution. [52] [53] [54] It is believed that this was due to evolution of the first land plants, which increased the amount of oxygen and lowered the amount of carbon dioxide in the atmosphere. [55]
600 Ma The accumulation of atmospheric oxygen allows the formation of an ozone layer. [56] Prior to this, land-based life would probably have required other chemicals to attenuate ultraviolet radiation enough to permit colonisation of the land. [38]
580–542 Ma The Ediacara biota represent the first large, complex aquatic multicellular organisms — although their affinities remain a subject of debate. [57]
580–500 Ma Most modern phyla of animals begin to appear in the fossil record during the Cambrian explosion. [58] [59]
550 Ma First fossil evidence for Ctenophora (comb jellies), Porifera (sponges), Anthozoa (corals and sea anemones). Appearance of Ikaria wariootia (an early Bilaterian).

Phanerozoic Eon Edit

The Phanerozoic Eon, literally the "period of well-displayed life," marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic, Mesozoic and Cenozoic, which are divided by major mass extinctions.

Palaeozoic Era Edit

Date Event
535 Ma Major diversification of living things in the oceans: chordates, arthropods (e.g. trilobites, crustaceans), echinoderms, molluscs, brachiopods, foraminifers and radiolarians, etc.
530 Ma The first known footprints on land date to 530 Ma. [63]
525 Ma Earliest graptolites
511 Ma Earliest crustaceans
510 Ma First cephalopods (nautiloids) and chitons
505 Ma Fossilization of the Burgess Shale
500 Ma Jellyfish have existed since at least this time.
485 Ma First vertebrates with true bones (jawless fishes)
450 Ma First complete conodonts and echinoids appear
440 Ma First agnathan fishes: Heterostraci, Galeaspida, and Pituriaspida
420 Ma Earliest ray-finned fishes, trigonotarbid arachnids, and land scorpions [64]
410 Ma First signs of teeth in fish. Earliest Nautilida, lycophytes, and trimerophytes.
395 Ma First lichens, stoneworts. Earliest harvestmen, mites, hexapods (springtails) and ammonoids. The first known tetrapod tracks on land.
365 Ma Acanthostega is one of the earliest vertebrates capable of walking.
363 Ma By the start of the Carboniferous Period, the Earth begins to resemble its present state. Insects roamed the land and would soon take to the skies sharks swam the oceans as top predators, [65] and vegetation covered the land, with seed-bearing plants and forests soon to flourish.

Four-limbed tetrapods gradually gain adaptations which will help them occupy a terrestrial life-habit.

Mesozoic Era Edit

From 251.4 Ma to 66 Ma and containing the Triassic, Jurassic and Cretaceous periods.

Major extinctions in terrestrial vertebrates and large amphibians. Earliest examples of armoured dinosaurs