EVOLUTION MAKING SENSE OF LIFE PDF

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PDF | A comprehensive, engaging textbook about evolution for biology majors now in its second edition. PDF | On Jan 11, , Carl Zimmer and others published Ebook Evolution: Making Sense of Life By. These separate lines of evidence all support the same scenario for the evolution of marsupials (Springer et al. ). Marsupial-like mammals were living in.


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by カール・ジンマー. Carl Zimmer; Douglas J Emlen; Isao Sarashina; Makiko Ishikawa; Yoshiki Kunitomo. Print book. Japanese. 講談社, Tōkyō: Kōdansha. (Download) Evolution: Making Sense of Life By Carl Zimmer PDF #Audiobook computerescue.info?book= #readOnline. Evolution: Making Sense of Life site Barnes & Noble IndieBound Co-author Doug Emlen is interviewed by Nature about creating an ebook version.

Sean B. Carroll, University of Wisconsin, Madison A richly illustrated and very clearly written text, Evolution: Making Sense of Life brings forth the excitement, power, and importance of modern evolutionary biology in an accessible, yet sophisticated overview of the field.

Peter R. Grant, Princeton University Two master craftsmen in the art of scientific communication have combined to produce an excellent basic text on Evolution: it informs, explains, teaches, and inspires. The illustrations are outstanding. John N. Thompson, University of California, Santa Cruz Carl Zimmer and Douglas Emlen have captured in this stunning new book the excitement and richness of twenty-first-century evolutionary biology.

They describe clearly and elegantly not only what, but also how, we are learning about evolutionary processes and the patterns they produce. Carl Zimmer is an award-winning writer on evolution, with several books for the masses under his belt, whereas Douglas Emlen is a professor of evolutionary biology at the University of Montana in the United States.

According to Emlen's professional homepage, the collaboration had one main goal: a revolutionary new textbook designed from the start to be an enjoyable and engaging read. Evolution reflects our shared vision for what modern textbooks can be: exciting, relevant, concept-oriented, and gorgeously illustrated; a reading adventure designed to grab the imagination of students, showing them exactly why it is that evolution makes such brilliant sense of life.

This goal is an important concept for me, because my earliest book reviews in the early s concerned themselves very much with the search for a textbook that presented systematics as an interesting science rather than solely as one component of academic intellectual activity.

I failed to find any such thing at the time. So, Zimmer and Emlen have their hearts in the right place, as far as I am concerned. My concern is that they may have missed their goal simply by trying too hard.

The writing in the book itself is smooth and engaging, which is perhaps the book's greatest strength. There is, sadly, no way to make a textbook read like a novel, but making it readable is itself a meaningful goal in science. The book is organized in an innovative way, starting with the science and philosophy of evolution, then proceeding through fossils, phylogenetics, genotypes, phenotypes, selection, adaptation, microevolution, macroevolution, and behavior, and ending with human evolution and medicine.

I have seen few books that try to create an interesting storyline throughout, and yet this one does. The objective has not been to present evolutionary studies but rather to make studying evolution interesting.

This is a laudable goal, and it has been amply achieved. The content is as current as you could expect, but obviously only until last year, and only from the authors' perspective. This does create a few problems, arising from the fact that the book leads you to expect an unreasonable degree of currency. From Sepkoski Carboniferous Perm. To do so, we must be lineage able to count separate species in a reliable and accurate way.

This is with different no easy task. As we saw in Chapter 13, biologists who study living morphology species use several criteria to delineate their boundaries, such as breeding ability, morphological or genetic differences, geographi- Gap in fossil record cal separation, and ecological differentiation.

Paleontologists, on the due to erosion other hand, can look only at the morphology of fossils to determine whether they belong to a previously described species or represent Older fossil lineage a new one.

Using morphology to identify paleontological species creates some special challenges when we try to study the process of spe- ciation in the fossil record. Box Figure The vertical axis represents the position of the fossils in the rock matrix. The fossils at the bot- tom are older, and the ones at the top are younger.

In Box Figure As we move up in the section, we encounter some variability in this species, but nothing remarkable.

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Then we see a gap where there are no preserved fossils. Above the gap, there are more fossils. They are similar to the lower ones, but measurably different in the shape of their shell. Do these younger fossils belong to the same species as the C older ones? Now two species existed where there had once been Gradual only one. Otherwise, we can choose between two possibilities.

On morphological the one hand, the old species might have split in two, but this split change without speciation occurred when no fossils were being deposited. The original species went extinct during this time, while the new species survived. The other possibility is that during the undocumented time, the shape of the shell in the original species evolved into the new form.

In the latter case, no speciation would Box Figure Above a discontinuity, they find would be dealing with a case transformation of a lineage from a new species with morphological differences, as shown in panel A.

In macro- There are two possible explanations for this pattern: This between species, as illustrated in Box Figure If the fossil record were complete, it would show a gentle ria Box Figure Equilibria referred to the stasis in lineages transformation from one form to the next. They argued that most of the lineages documented most species undergo relatively little change for most of their in the fossil record experienced stasis; in other words, they exhib- geologic history.

These periods of stasis are punctuated by brief ited little or no directional change for millions of years. The stasis periods of rapid morphological change, often associated with was punctuated by relatively rapid change—enough to produce the speciation. Eldredge and Gould argued that these bursts were consis- fossil and living bryozoans separate into species in a similar man- tent with a principal model of speciation recognized by population ner?

The researchers compared morphological differences in fossil biologists—the peripheral isolate model.

Instead of the gradual and living bryozoans against molecular and genetic assessments divergence of a big population into two new lineages, the periph- of difference in living forms. They found that morphology was a eral isolate model proposed that geographic isolation can cause very good guide to the taxonomy of living bryozoans, as it was for fairly rapid evolution if a relatively small portion of the species fossil ones.

Jackson and Cheetham later extended their study to a vari- and Gould argued, then we should expect abrupt morphological ety of other marine invertebrates and microorganisms and found breaks in the fossil record. Paleobiologist cies in the act of splitting. Gene Hunt of the Smithsonian Institution found a similar pattern, In Chapter 8 we saw how evolutionary biologists have docu- in which stasis dominated the fossil record Hunt Conse- mented that natural selection can change allele frequencies in a quently, punctuated equilibria endures as an influential model of matter of years or less.

Such examples of rapid natural evolution are macroevolution. Hatched areas marked A—D are preserved tern in the fossil record with gradual anagenesis, as in this diagram sediment; white spaces are inferred gaps in the stratigraphic record. Adapted from Eldredge and Gould Species that best fit the punctuated equilibria model. This diagram shows how experienced long periods of stasis, punctuated by rapid morphological a lineage of bryozoans Metrarabdotos evolved rapidly into new spe- change during speciation.

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This is in contrast to the traditional model cies, but changed little once those species were established. Adapted of slow, steady directional change in fossil lineages, which turns out from Benton Most of these groups never recovered, and some major groups became extinct.

The Modern fauna has its roots in the Cambrian era, although its members were of relatively low diversity at the time. After the Cambrian, it climbed gradually in numbers. The Modern fauna, which has been dominant since the end-Permian extinc- tion, consists mainly of gastropods snails and bivalves clams. Scientists have found evidence for two types of factors involved in macroevolutionary change.

Intrinsic factors, such as the physi- ology of clades, can play a role. Extrinsic factors in the environment can as well. Sepkoski, for example, proposed that the transition between faunas may have been driven by their differences in origination a and extinction V rates.

The inver- tebrates that dominated the Cambrian fauna especially trilobites had very high turnover rates, those of the Paleozoic fauna had moderate ones, and those of the Modern fauna particularly clams and snails had low ones.

On the other hand, Shanan Peters, a paleontologist at the University of Wiscon- sin, has found possible physical factors involved in these changes: He observed that most fossils of the Paleozoic fauna Figure Most of the Modern fauna fos- tion from the sun and the chemical sils are found in rocks known as silicoclastics, which formed from the sediments composition of the atmosphere.

The ecosystems built on these rocks may favor preser- Large changes in the climate—both vation of certain clades over others. The numbers by each clastic rocks have become more common, possibly because of sediment delivered to arrow shows the watts per square the oceans by rivers. Peters proposed that as the seafloor changed, the Modern fauna meter being absorbed or released by could expand across a greater area while the Paleozoic fauna retreated to a shrinking the atmosphere and Earth.

Solar radiation Thermal radiation Another physical factor drives long-term changes in biodiversity: Earth is warmed by incoming radiation from the sun Fig- Earth: Once this energy reaches Earth, it Space surface: The chemical composition of the atmosphere can change Atmosphere captured by the amount of heat it traps. Warm water has a higher concentra- tion of the isotope oxygen than cool water does, for example, and Earth so rocks that form in warm water will lock in those isotopes as well.

Over a span of million years, the global mean temperature red circles fluctuated dramatically. Global levels of biodiversity also fluctuated blue circles; both shown as deviations from their mya average. Warmer periods had higher global levels of standing diversity than cooler periods. Adapted from Mayhew et al.

One major source of this variation is the amount of carbon dioxide in the atmosphere. When certain types of volcanoes erupt more, they deliver more of these heat-trapping greenhouse gases to the atmosphere.

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Peter Mayhew of the University of York examined how these changes in climate might affect the diversity of life. As we saw earlier, differences in climate have been proposed to explain the different levels of diversity found today in the tropics and in temperate zones. By carefully comparing the fossil record with the climate record, Mayhew and his colleagues did indeed find a correla- tion.

After correcting for sampling biases in the fossil record, they found that periods with warmer ocean temperatures also had increased standing diversity of marine invertebrates Figure Statistical analyses can control for known biases and help scientists make and test predictions about the processes that shaped the observed patterns.

Sep- koski and Raup, for example, studied marine fossils collected across the whole world. But even on a local scale, macroevolution can produce striking patterns that intrigue scientists.

Take, for example, the islands of Hawaii. As we discussed in Chapter 13, Hawaii is home to 37 species of swordtail crickets found nowhere else. Hawaii is also home to other remarkable clades, such as more than 50 species of honeycreeper birds.

Silversword plants also diversified into an equally impressive range of forms Figure Clades like More than half of these species have since gone extinct. Adapted from Losos and 8 7 6 5 4 3 2 1 0 Ricklefs ; Lerner et al. Dubautia arborea D. Hawaiian silversword alliance D. After the ancestors of modern D. From A. Argyroxiphium grayanum these, which have rapidly diversified by adapting to a wide range of resource zones, are known as adaptive radiations Losos The Great ary lineages that have undergone Lakes of East Africa are geologically very young, in many cases having formed in just exceptionally rapid diversification the past few hundred thousand years Sturmbauer et al.

Once the lakes formed, into a variety of lifestyles or ecologi- cichlid fishes moved into them from nearby rivers. The fishes then diversified explo- cal niches. Along the way, the cichlids adapted to making a living in an astounding range of ways—from crushing mollusks to scraping algae to eating the scales off other cichlids Kocher ; Salzburger et al.

During adaptive radiations, new lineages expand to occupy new ecological roles. As a result, adaptive radiations produce some of the most striking examples of evo- lutionary convergence. Cichlid fish diversified independently within adjacent African Great Lakes, and these simultaneous radiations resulted in striking parallels in feeding ecology and morphology.

The African Great Lakes cichlids also experienced convergent evolution as lineages adapted to the same lifestyles and habitats in different lakes Figure Adaptive radiations may occur when clades evolve to occupy ecological niches in the absence of competition. These opportunities can arise with the emergence of a new island or lake. But they can arise in other ways as well. When extinctions remove certain species from an ecological resource zone, other lineages can evolve that take their place.

Such appears to be the case for mammals. When large dinosaurs became extinct at the end of the Cretaceous, large mammals rapidly evolved and diversified Smith et al. In other cases, clades may radiate because new adaptations, known as key inno- vations, evolve that allow them to occupy habitats or adaptive zones that were simply off limits to earlier clades Table That seems to be what happened in the most diverse clade of animals on Earth, the insects.

Insects first evolved about million years ago, and today a million species of insects have been named.

Probably millions more have yet to be described. Their closest relatives are a group called the entogna- thans, which includes springtails. The entognathans comprise only 10, species. And although entognathans generally look very similar to one another, insects have diversified impressively, from carnivorous dragonflies to ants that tend mushroom gardens to wasps that inject their eggs into living hosts.

Unlike the entognathans, the insects evolved wings that allowed them to occupy ecological roles unavailable to flightless invertebrates. Wings permitted insects to occupy new adaptive zones, and they also permitted them to colonize new habitats Grimaldi and Engel ; Mayhew ; Nicholson et al.

Other potential fac- tors in the success of insects may include the evolution of herbivory. Cambrian radiation of Environmental change; Increased O2 availability; increased animals key innovations genetic developmental capacity to diversify toolkit, body segments, in form; colonization of new lifestyles skeletal structures e.

Key innovations can transform how organisms interact with their environments in ways that take them into new and undercontested habitats or permit them to exploit novel ways of life. These opportunities can trigger explosive subsequent diversification and adaptive radiation. Macroevolution at the Dawn of the Animal Kingdom As spectacular as the adaptive radiation of insects may have been, it was, in some respects, a modest event.

Every species of insect retains the same body plan. It gave rise to many of the major groups of animals found on Earth today, each with its own distinctive body plan.

Known as the Cambrian Explo- sion, it stands as one of the most important macroevolutionary events in the history of life Erwin and Valentine The first signs of the Cambrian Explosion emerged in the nineteenth century, as paleontologists began to organize the fossil record. They found that remains of ani- mals reached back to the Cambrian period. At their earliest appearance, animals were The interval from to million years ago is known as the Cambrian Explosion.

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Adapted from Erwin and Valentine But when they looked at rocks that formed before the Cambrian, the paleontologists found nothing. Darwin predicted that older fossils of simpler organisms would someday emerge, and he was right Schopf As we saw in Chapter 3, paleontologists have found fossils of microbes dating back some 3. Researchers have even found some animal fossils from before the Cambrian period.

Nevertheless, even after another years of fossil hunting since Darwin, the Cambrian remains striking Figure Between about and million years ago, many major taxonomic groups of animals appear for the first time in the fossil record.

Scientists refer to this interval of evolution as the Cambrian Explosion. The Cambrian Explosion is such a vast, complex event that scientists who seek to explain it need to gather evidence from a wide range of scientific disciplines as varied as ecol- ogy and geochemistry.

Molecular phylogenetics, for example, has helped researchers place the Cam- brian Explosion in the broader context of animal evolution.

As shown in Figure The first major split in animal evolution was the divergence of sponges and all other animals. The ancestors of cnidarians and bilaterians diverged about million years ago. The major lineages of living bilaterians diverged from each other between about and million years ago. Paleontologists have also used phylogenies to better understand the step-by-step anatomical transformations that produced the body plans of living animals.

Some of the first Cambrian arthropod fossils, such as trilobites, share these key syn- apomorphies. The colored circles indicate the common ancestor of living members of animal phyla. Just as we saw in Chapter 4 how dinosaurs gave rise to birds and how lobe-finned fish gave rise to tetrapods, here we see that early bilaterians gave rise to true arthropods Figure Biologists can then use these phylogenies as a framework for investi- gating how mutations in developmental genes produced innovations in animal body plans Chapter These studies show that the Cambrian Explosion occurred after some million years of animal evolution through a stepwise emergence of new body plans.

Scientists who study the Cambrian Explosion are investigating why animal evo- lution proceeded at a relatively slow pace for hundreds of millions of years before accelerating around million years ago.

To develop hypotheses, they gather many lines of evidence. Besides examining fossils, they also look at living animals to recon- struct the evolution of the animal toolkit see Chapter They reconstruct changing Figure Arthropods—a group that includes insects, spiders, and crustaceans—share a number of traits, such as jointed exoskeletons. Adapted from Budd Living velvet worms Aysheaia Common ancestor Hallucigenia of velvet worms and arthropods Appendages differentiate Kerygmachela Complex segments Opabinia Lateral lobes Lever muscles, Anomalocaris compound eyes Legs harden Living arthropods Hardening complete chapter fourteen macroevolution: They also examine fossils for signs of ecological change.

We saw on page 78 that during the Cambrian Explosion, many such new ecological features emerged. The evolution of the bilaterian genetic toolkit was crucial, because it enabled lineages of animals to evolve dramatically new body plans with relatively modest mutations to developmental genes.

But molecular phy- logenetic studies suggest that this toolkit was in place over million years before the Cambrian Explosion. During that interval, bilaterians were probably small, worm- like creatures living alongside sponges, jellyfish-like animals, and Ediacaran species anchored to the seafloor Figure 3.

Recent studies indicate that early in dramatically. Scientists have found the Cambrian, around million years ago, the oceans underwent a major rise in million-year-old fossils of an sea level due to tectonic activity. Large swaths of coastal regions were submerged, and animal called Cloudina that bear holes marine animals swiftly colonized these new habitats.

At that time, Smith and Harper note, an abundance of small shells appear in the fossil record. Shells might have evolved originally as a defense against calcium poisoning, allowing animals to safely remove the mineral from their tissues.

But eventually, the mineralization of animals took on new functions such as hardened weapons like mandibles and claws for predators and thick defenses for prey. At the same time, oxygen levels in the oceans were rising for reasons that are not yet entirely clear Sahoo et al.

The extra oxygen was a great boon to bilaterian animals. For one thing, animals need to burn fuel to make collagen, a protein that binds cells together in their bodies. And as animals began to move around in the ocean, powering their muscles demanded even more energy. The genetic toolkit, Smith and Harper propose, enabled bilaterians to rapidly evolve into new forms to take advantage of all the new ecological niches that were opening up.

And their biological evolution altered the chemical evolution of the oceans. Some bilaterians evolved into burrowers, and for the first time in the history of the oceans, the seafloor became shot through with tunnels. The sediments on the seafloor became oxygenated, enabling many more animals to move into this vast habitat. Together, these processes drove the expansion of habitats for animals and spurred the increased complexity of the food web.

Once ani- mals began to evolve rapidly, they may have become caught in a feedback loop. Bigger predators evolved to eat smaller ones, for example. Both predators and prey may have evolved new sensory organs, like eyes, to detect their prey and their enemies.

The Cambrian Explosion likely resulted because a developmental innovation at the microevolutionary level allowed lineages to radiate and occupy a tremendous diversity of new ecological opportunities. Adapted from Smith and Harper From Background Noise to Mass Die-Offs Insights gained from studying microevolution can help scientists better understand macroevolution.

In Chapter 13, we explored the origin of new species by looking at research on living populations that are reproductively isolated.

These insights help us interpret the origination of new species in the fossil record and explore the factors that may drive adaptive radiations. Likewise, we can gain some clues about the mac- roevolutionary patterns of extinction over hundreds of millions of years by examin- ing how species move toward extinction in our own time.

A species is a lineage made up of linked populations. It can endure for millions of years, even though the total number of individuals in the species may fluctuate wildly over time—booming when a new source of food becomes available or shrink- ing under attack from a parasite.

Even if one population completely disappears, there are other populations to sustain the species and expand its range. If the total number of individuals in a species shrinks too far, however, it faces the risk of disappearing altogether.

Once a species falls below this threshold, any number of different factors may drive it extinct. If a lizard species is made up of just 50 individuals living on a single tiny island, a big hurricane can kill them all in one fell swoop.

Small populations also face threats from their own genes. Small populations also have less genetic variation, which can leave them less prepared to adapt quickly to a changing environment. When Dutch explorers arrived on the island of Mauritius in the s, for example, they discovered a big, flightless bird called the dodo Figure The explorers killed dodos for food and also inad- vertently introduced rats to Mauritius.

The dodo became extinct in the late s, probably due to hunting and predation by introduced species. The Carolina parakeet became extinct in the early s, due in part to logging, which removed the logs where it built its nests. As adult and young dodos alike were killed, the population shrank until only a single dodo was left. When it died, the species was gone forever Rijsdijk et al. Simply killing off individuals is not the only way to drive a species toward extinction.

Habitat loss—the destruction of a particular kind of environment where a species can thrive—can also put a species at risk. The Carolina parakeet once lived in huge numbers in the southeastern United States. Loggers probably hastened its demise in the early s by cutting down the old-growth forests where the parakeets made their nests in hollow logs.

A smaller habitat supported a smaller population, until the entire species collapsed. These extinctions, and many other recent ones, have humans as their ultimate cause. But we humans have been capable of driving extinctions for only a geologi- cally short period of time. Species have become naturally extinct through similar processes. Some species have become extinct through competition with other species.

Some have been wiped out because they could not withstand changes to their local physical environment.

The fossil record shows us that extinction is a continuous pro- cess. The typical rate of extinctions is called background extinction: A clade can survive background extinctions if lineages branch to form new species at a greater rate than the background extinction rate. Higher extinction rates V also increase the risk of extinction.

Extinction rates that rise for many clades all at once produce what are known as mass extinctions. Mass extinctions can affect the biodiversity of the entire world, or they may affect only a region. For example, biologists can discuss a mass extinction of crinoids in the Indian Ocean during an interval of the Tertiary period.

The existence of a mass extinction in the fossil record depends not on absolute magnitude, but on the relative change from the normal conditions. Scientists can determine the regularity of this process and use that background extinction rate to examine how departure from that rate affects the diversity of life on Earth. Raup and Sepkoski measured the extinction rate for families of marine invertebrates and vertebrates.

They identified five mass extinction events that A 20 a c e were significantly higher than the 15 background extinction rate.

Here, the mass extinction events are marked in Total extinction rate purple. Background extinction rate measurements are noted by red dots. Standing diversity through time 10 for families of marine invertebrates and vertebrates. The letters mark the Big Five mass extinction events. Adapted from Raup and Sepkoski Cambrian Ordovician Sil. Carboniferous Dev. Jurassic Perm. Jurassic Cretaceous Tertiary Cretaceous Tertiary 0 Million years before present B e Number of families b a d c 0 Cambrian Ord.

Carboniferous Carbon. Jurassic Cambrian Ordovician Sil. Jurassic Cretaceous Tertiary Cretaceous Tertiary 0 Million years before present In Dave Raup and Jack Sepkoski charted the total extinction rate for families of marine invertebrates per million years through time see Figure They found five peaks of extinction far above all the rest, including major drops in standing diversity of marine families.

The Big Five mass extinctions were truly catastrophic. The biggest of them all, which occurred at the boundary of the Permian and Triassic periods, million years ago, is estimated to have claimed 96 percent of all species on Earth.

The end-Ordovician, end-Permian, and end-Cretaceous events each resulted from a dramatic rise in extinction rates see Figure The other two resulted from a drop in origination rates as well as heightened extinction. The Big Five also differed in the ecological profile of their victims Bam- bach et al. The end-Ordovician mass extinctions took their heaviest toll on trilobites, for example, while the greatest losses in the end-Permian mass extinctions were experienced by brachiopods, crinoids, and anthozoan corals.

To explain how these mass extinctions occurred, paleontologists have acted like forensic scientists who gather clues from a crime scene to infer the cause of death. But because the deaths they study occurred millions of years ago, their detective work is far more challenging. Over the course of many decades of research, scientists have identified a number of compelling candidates for the causes of mass extinctions.

Making matters more complex, the evidence indicates that several different causes have often interacted to cause a single bout of mass extinctions Table A range of physical causes appear to be involved in mass extinctions. Sea-level regressions, for example, are associated with many mass extinctions.

They reduced the available surface area on the continental shelves. Exactly how sea-level regres- sions may have caused extinctions is still a matter of debate. On the other hand, sea-level transgression—in which the ocean rises and spreads over land—may sometimes cause extinctions as well.

Transgression can deliver oxygen- poor water from the deep ocean into coastal regions, making it difficult for many animals to survive. Along with changes in sea level and ocean chemistry, the climate can also play a major role in mass extinctions.

If climate-altering gases are introduced quickly enough into the atmosphere—through volcanoes, for example—they can create cli- mate change so rapid that many species cannot adapt and become extinct. Biological causes can also play a part in mass extinctions. Losing individual spe- cies can eventually put a whole ecosystem at risk.

Evolution: Making Sense of Life

Removing a species can endanger its ecological partners as well. The result can be ecological collapse. Scientists have been able to document this change thanks to the discovery of geological formations in southern China from just before and after the mass extinction.

The rocks have a wealth of fossils, and they are arranged in a dense stack of thin layers, many of which can be precisely dated using uranium and lead isotopes.

The most recent analysis of these rocks reveals that the end-Permian extinctions occurred in a geological flash—less than 60, years Burgess et al. The rocks also reveal a massive shift in carbon isotopes over just 10, years that occurred shortly before the extinctions.

This carbon may have been injected by volcanoes, which are known to have been unusually active just before the mass extinctions. Scientists have also found evidence that at the end of the Permian period, the ocean warmed drastically—possibly due to the heat-trapping gases released by the volca- noes.

Carbon dissolving in the oceans acidified the water, disrupting the physiology of many marine organisms. At the same time, the carbon dioxide and methane in the atmosphere warmed the planet. The high temperatures in the ocean drove out much of the free oxygen in the surface waters. Some researchers have suggested that in these acidic, low-oxygen waters, once-rare types of bacteria thrived, releasing toxic gases such as hydrogen sulfide Erwin There is also evidence for extraterrestrial causes playing a part in mass extinc- tions.

Over the years, scientists have proposed a number of these causes, including Adapted from Barnosky et al.

Table Uplift and weathering of the Appalachians af- fecting atmospheric and ocean chemistry. Sequestration of carbon dioxide, lowering aver- age global temperatures. Evidence for widespread deep-water anoxia and the spread of anoxic waters by trans- gressions. Some evidence exists of impacts of an asteroid or comet, but their timing and importance are a subject of debate.

The Permian Event Ended million years ago; in less than Siberian volcanism. Spread of deep marine anoxic waters.

Elevated hydrogen sulfide and carbon dioxide concen- trations in both marine and terrestrial realms. Ocean acidification. Evidence for an impact still debated. The Triassic Event Ended million years ago; within 8.Although our sample is not very large, we can see that 12 extinctions in stage C is more than usual, and that on average the extinc- tion rate is slightly greater than the origination rate.

Stable URL: The extra oxygen was a great boon to bilaterian animals. Warm water has a higher concentra- tion of the isotope oxygen than cool water does, for example, and Earth so rocks that form in warm water will lock in those isotopes as well.

Williams pp. Tomiya, G. Carbon dioxide, as we saw earlier, is also a heat-trapping gas. The researchers compared morphological differences in fossil biologists—the peripheral isolate model. Breathtakingly illustrated, this book covers not only the usual topics in evolution—adaptation, drift, phylogenetic analysis—but also a host of new and exciting areas where groundbreaking research is occurring.

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