The Evolution of Proteins

One of the most remarkable accomplishments of molecular biology has been to trace the course of evolution in single molecules. According to Francisco Ayala, professor of ecology and evolutionary biology at the University of California at Irvine, this technique "has truly revolutionized the reconstruction of evolutionary history."

One of Darwin's boldest and most controversial conjectures was that all organisms are descended from common ancestors. In other words, if any two organisms living today could be traced back through evolutionary time, at some point their lines of descent would converge. The closer two organisms are in evolutionary terms, the more recent their ancestor. The most recent common ancestor of humans and chimpanzees, for example, seems to have been an ape-like creature, now extinct, that lived in Africa about 5 million years ago. The most recent ancestor of humans and mushrooms was probably a single-celled organism living in or near the water over a billion years ago.

Because of their common origins, all organisms share certain metabolic processes. We have proteins in our bodies that serve the same functions as proteins in mushrooms. However, these proteins are not identical. Over time, the DNA that codes for proteins undergoes random mutations, which supply the variation needed for evolution. These mutations can change the sequence of amino acids in a protein-replacing a glycine molecule, for instance, with one of the other 19 amino acids that commonly make up proteins. Therefore, as two evolutionary branches emerge from a common ancestor, the proteins in those branches also evolve. But because these changes are random, the proteins change in different ways. As time goes on, the proteins become more and more dissimilar.

Proteins cannot change unrestrictedly. If a protein is to serve a given function, certain amino acids must stay the same, and certain relationships among the amino acids must be maintained. If a mutation does alter a critical amino acid in a protein that is essential to an organism, the organism will not survive and the mutation will not be passed on. On the other hand, if a mutation alters the function of a protein in such a way that the fitness of the organism is enhanced, that mutation can be selected for and the mutation will spread.

Some biologists have proposed that relatively few mutations have such a positive effect. Among those mutations that do not cause a decrease in fitness, they argue, the great majority are simply neutral in their effects. Such a neutral mutation may change the sequence of amino acids in a protein, but it will not affect the protein's overall function. Other biologists disagree, contending that natural selection plays a much larger role in guiding protein evolution than the believers in the so-called neutrality theory claim.

A Molecular Clock

If the neutrality theory were correct, Ayala observes, it would have profound implications for the study of evolutionary relationships. It would mean that random changes are incorporated in DNA at a more or less constant rate. The interval between changes in a given protein would vary, in keeping with their random nature. But over a long enough time these intervals would average out.

In this way, changes in specific proteins would serve as a sort of evolutionary molecular clock. By measuring the differences in DNA or protein sequence between two different organisms, it would be possible to determine how much time has elapsed since those organisms diverged from a common ancestor.

A classic example of such a molecular clock, Ayala notes, is the protein cytochrome c. Consisting of 104 amino acids in vertebrates and a few more in some invertebrates, plants, and fungi, cytochrome c is a protein that evolved over a billion years ago to help organisms break down organic molecules and supply themselves with energy. In the 1960s, Walter Fitch and Emanuel Margoliash studied cytochrome c molecules from 20 different organisms, including the fungi Neurospora and Candida, the yeast Saccharomyces, insects, a fish, reptiles, birds, and mammals, including man. They determined the differences in amino acids among the different proteins and calculated the minimum number of substitutions that would be needed in the DNA of the organisms to account for those differences. They then used this information to arrange the organisms in evolutionary time, as shown in Figure 4-1.

FIGURE 4-1

Differences in the amino acid sequence of the enzyme cytochrome c can be used to order the evolutionary relationships among organisms. The numbers between each branch point are the minimum number of changes in the nucleotide sequence of DNA needed to (more...)

The results astonished biologists. From a single molecule, Fitch and Margoliash had reproduced centuries of work by biologists in tracing evolutionary relationships among different organisms. ''I can well remember in 1967 reading this paper in Science and being literally dumbfounded," says Ayala. "Here they were looking at a very small molecule, 104 amino acids, and in the midst of a single molecule the whole of evolutionary history could be reconstructed by and large correctly. Furthermore, there are tens of thousands of genes coding for proteins, and each one of these genes or each one of these proteins could provide us with a molecular clock, and therefore with an independent reconstruction of evolutionary history. This is what caused the enormous enthusiasms of evolutionary biologists and others with this hypothesis."

However, the diagram produced by Fitch and Margoliash is not perfect. It shows turtles, a reptile, being closer to birds than to the rattlesnake, another reptile. Within the birds, chickens appear to be more closely related to penguins, whereas they are in fact more closely related to ducks and pigeons. Finally, the primates, including humans, seem to have branched away from the mammalian limb before the kangaroo did, whereas in fact just the opposite occurred. Could it be, Ayala asks, that these mistakes point toward more serious problems with molecular clocks?

An Erratic Clock

One way in which Ayala and his colleagues have been studying this problem is by analyzing a protein known as superoxide dismutase. A protein involved in protecting cells from the reactive effects of oxygen, superoxide dismutase consists of two identical subunits of 153 amino acids in humans, horses, yeast, and mold and 151 amino acids in rats, cows, swordfish, and fruit flies. A total of 55 of the 153 possible amino acid sites are identical in all eight organisms, indicating that parts of the protein need to be conserved to maintain its function. The other 98 sites vary from species to species.

The amino acid differences between humans and the other three mammals—rats, cows, and horses—are fairly uniform, ranging from 25 to 30 amino acids (Figure 4-2). This makes sense, since the fossil record indicates that primates diverged from rodents about 63 million years ago, that the lineages leading to cows and horses diverged around the same time, and that the common ancestor to the four mammals lived about 75 million years ago.

FIGURE 4-2

The evolutionary relationships derived from the fossil record for eight organisms can be compared with the differences in amino acid sequences for the superoxide dismutase enzymes they contain. For instance, only 25 of the 153 amino acids in the superoxide (more...)

The difference between humans and fish is about 48 amino acids, or nearly twice as much as between humans and the other mammals. If superoxide dismutase were an accurate molecular clock, this would mean that fish and humans diverged about twice as long ago as humans and the other mammals, or approximately 150 million years ago. However, the fossil record indicates otherwise: the most common ancestor to humans and fish seems to have lived about 450 million years ago.

The situation is even worse for the other organisms. Human superoxide dismutase differs by 69 amino acids from the superoxide dismutase of yeast and molds, or about three times as much as within the mammals. Yet the lines leading to humans and to yeast became distinct an estimated 1.2 billion years ago, 20 times more distant than the common ancestor among the mammals.

The raw numbers of amino acid differences are not a completely accurate measure of evolutionary distance, Ayala points out. For instance, if an amino acid mutated at some point during evolution and then mutated back to the original amino acid, the double substitution would be hidden. Various statistical corrections can be applied to the numbers to correct for such events. But these corrections are not nearly enough to account for the discrepancies in the substitution rate of superoxide dismutase.

Another possibility is that the amino acid substitutions in superoxide dismutase are constrained in some way that is not yet thoroughly understood. For instance, there are 98 variable sites in the superoxide dismutases of the eight organisms studied. Perhaps this value approximates a maximum number of differences and the rate of amino acid substitutions slows down as it approaches that value. However, the greatest number of substitutions between any two organisms is only 69, nowhere near the maximum. Also, a maximum number of substitutions does not seem to be a factor with comparable proteins, like cytochrome c. Overall, Ayala says, one is forced to conclude that superoxide dismutase "is not a very good molecular clock."

The Value of Molecular Clocks

"What is more common," Ayala asks, "the relative regularity of cytochrome c or the capriciousness of superoxide dismutase? I think the answer is that we don't know. The amount of information that we have is very limited. If the situation that we have with superoxide dismutase prevails, we are not going to be able to use the evolutionary clock very readily, unless we become more sophisticated about it and learn some things."

Nevertheless, there are certain circumstances in which molecular clocks can still provide valuable results, Ayala contends. For instance, some proteins, such as cytochrome c, appear to be better clocks than others. By comparing many such clocks, biologists can learn which proteins provide the most valid results. Such information will become increasingly available as DNA sequencing projects for humans and other species get under way.

Individual molecular clocks can also be valuable within certain limits. By looking at relatively similar groups of organisms, in which evolutionary processes are presumably similar, and by considering relatively long periods of time, in which variations in the clock can average out, molecular clocks can yield accurate results. Even superoxide dismutase is a good molecular clock if one looks just at the evolution of mammals. But to apply such clocks more widely, Ayala concludes, biologists need to learn more about the factors that shape protein evolution.

Two Periods of Life

Based on the geologic record, the history of the earth can be divided into two very unequal periods (Figure 4-3). About 600 million years ago, fossils suddenly appear in great profusion in sedimentary rock-first marine plants and animals, and later their terrestrial descendants. The period since then is known as the Phanerozoic, from the Greek word for visible or manifest.

FIGURE 4-3

Organisms capable of leaving easily visible fossils evolved only in the last one eighth of the earth's history. Fossils of soft-bodied multicellular organisms have also been found, but they extend back to only about 700 million years. Before that the (more...)

The first geologic period in the Phanerozoic is known as the Cambrian. Everything before the Phanerozoic is therefore known simply as the Precambrian. The earth seems to have coalesced from a cloud of matter circling the sun 4.5 billion years ago. So the Precambrian period on earth spans about seven-eighths of its history.

For well over a century, biologists searched without success for conclusive evidence of fossils in Precambrian rocks. The sudden appearance of living things 600 million years ago posed a serious problem for the creators of evolutionary theory, who believed that all organisms arose through a process of gradual change from other organisms. In On the Origin of Species, Darwin wrote, "To the question why we do not find rich fossiliferous deposits belonging to . . . periods prior to the Cambrian system, I can give no satisfactory answer. ... The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained."

Only in the past few decades has the puzzle finally been solved, according to J. William Schopf, professor of paleobiology at the University of California at Los Angeles. It is not the case that fossils do not exist in Precambrian rocks. The problem was that people were looking for the wrong kind of fossils.

The key to the puzzle of Precambrian life begins with a certain kind of rock deposit known as a stromatolite (Figure 4-4). Shaped like a stack of mattresses (or stromas in Greek), stromatolites were first described in the early 1800s. Almost immediately, some biologists began to speculate whether they might have been formed by living organisms. But they contain no visible fossils, and most biologists concluded that stromatolites were caused by nonbiological processes. "This debate went on and on, and many geologists simply refused to investigate these structures, largely because they had been taught by their professors that they weren't worth the time," says Schopf. "But they turn out to be worth the time."

FIGURE 4-4

Stromatolites in the fossil record typically resemble stacked pancakes rising in pillars or mounds. These specimens occur in limestone deposits about 1.3 billion years old from Glacier National Park, Montana. Photograph courtesy of J. William Schopf. (more...)

Several things came together to change biologists' minds about the origins of stromatolites. For one thing, biologists began to find and examine colonies of bacteria that produce structures remarkably like the stromatolites seen in the geologic record. In a few dry, salty, and sunny places in the world, such as the coasts of Baja California and northwestern Australia, bacteria grow in columns rising from shallow water (Figure 4-5). Typically these columns are not hard; they can be cut with a machete, Schopf says. But in some places, conditions are such that calcium carbonate, the mineral constituent of limestone, adheres to their surfaces, forming a hard pillow-shaped structure. If covered with sediments and compacted, these deposits would be virtually indistinguishable from geologic stromatolites.

FIGURE 4-5

One of the few places in the world where stromatolites still live is in Shark Bay, Western Australia. For much of its history, the earth's surface probably looked something like this. Photograph courtesy of J. William Schopf.

Living stromatolites are complex ecosystems composed of several different kinds of bacteria. The top layer consists of a type of bacteria known as cyanobacteria, the same kind of bacteria responsible for pond scum on bodies of stagnant water. Like all bacteria, the cyanobacteria are prokaryotes—that is, they do not have a distinct nucleus containing their DNA. For this reason, a more common name for cyanobacterial—blue-green algae—is not entirely accurate, since all true algae have nuclei and are therefore eukaryotes. The cyanobacteria are also photosynthesizers, like green plants. They use light from the sun to convert carbon dioxide and water into the organic compounds that they use to grow.

These compounds in turn support different kinds of bacteria that live beneath the cyanobacteria in modern stromatolites. These bacteria get their energy as animals do, by feeding off the organic molecules produced by photosynthesizers.

Schopf points out that modern stromatolites might seem to lead a rather precarious existence. "They grow near the sediment-water interface," he says, "which is not a good place for organisms that rely on the sun to exist. They can get buried when the spring rains come and detritus covers them up. They can no longer see the sun and should die."

Modern stromatolites get around this problem by growing. The cyanobacteria making up the top layer of the column are phototactic—they move in response to light. "So indeed they do get buried, but as they do they glide up through the accumulated detritus and make a new layer," Schopf explains. "And there they sit, happy as clams, or happy as blue-green algae, until they get buried again and again, layer after layer. And that's how a stromatolite forms."

The discovery of modern stromatolites was a suggestive piece of evidence, but it was not enough to prove that the stromatolites in the geologic record were also made by bacteria. To do that, biologists had to find evidence of the organisms that built the geologic stromatolites. They did this by examining stromatolites preserved in silicon dioxide—the mineral quartz. If such stromatolites are sliced with a diamond-impregnated saw and ground clown by hand, it is possible to make thin layers that can be seen through with a microscope. By exhaustively searching through such translucent layers, researchers managed to find the remains of fossilized cells (Figure 4-6). Sometimes the cells are joined into rows, or flattened into disks, just like the bacteria in modern stromatolites. The fossils of the Precambrian era had finally been found.

FIGURE 4-6

Fossilized cells from 3.5-billion-year-old deposits in northwestern Australia still exhibit cell walls and a filamentous form. These fossils are among the oldest now known. Photomicrograph courtesy of J. William Schopf.

It was immediately obvious why earlier biologists had found no evidence of Precambrian life. "The Precambrian was the age of microscopic life," Schopf points out. "People were asking the wrong question. They were looking for macroscopic organisms, for the equivalent of trilobites and clams. But such organisms had not yet evolved. The planet was dominated for three billion years by microbes, which set the stage for all subsequent evolution."

Bacteria and the Atmosphere

Stromatolites first appear in the geologic record about 3.5 billion years ago, and already they contain fossilized cells (this is the earliest direct evidence for life on earth, as described in the box below). But what kinds of cells were they? Were they photosynthesizing bacteria, like the ones that form stromatolites today? Or did some other kind of organism form these early stromatolites?

Box

Extinction and the Fossil Record. Molecular clocks can only be used with living creatures and those few organisms for which perishable DNA or protein samples have been preserved. For creatures that are now extinct, biologists must rely on the fossil record (more...)

To search for evidence of photosynthesis, paleobiologists have studied the rocks in which these early cells are embedded. Carbon dioxide, which photosynthesizers absorb to create organic compounds, has always been abundant in the earth's atmosphere, because it spews into the atmosphere and oceans from volcanoes and deep-sea vents. But not all carbon dioxide is alike. The carbon in carbon dioxide consists of two different isotopes—carbon-12 (with six protons and six neutrons) and carbon-13 (with six protons and seven neutrons). When organisms photosynthesize, they tend to slightly prefer carbon-12 to carbon-13.

...........................................................................................

Therefore, when these organisms die and are fossilized, the carbon in those fossils tends to be enriched in carbon-12 compared with the carbon in nonbiological deposits.

Researchers have measured the ratio of carbon-12 to carbon-13 in early stromatolites, and the results indicate a definite enrichment in carbon-12. ''The carbon isotopes are consistent with the presence of photosynthesis going back three and a half billion years ago," says Schopf.

The question then becomes what kind of photosynthesis the early bacteria engaged in. Plants and cyanobacteria combine carbon dioxide with water to get organic molecules, giving off oxygen in the process. But other kinds of photosynthesis are possible. In particular, certain bacteria that evolved before cyanobacteria use hydrogen sulfide rather than water in photosynthesis, releasing sulfur as a waste product. The bond between sulfur and hydrogen is easier to break than the one between oxygen and hydrogen, and the biochemical machinery needed is less involved. Perhaps these bacteria were responsible for the early stromatolites.

Schopf and his colleagues have attacked this question by examining the size and shape of the fossils found in the earliest known stromatolites. They are larger than modern sulfur-producing bacteria, he points out, being more along the lines of modern cyanobacteria. They are also organized into rows or globular colonies encased by thick, layered sheaths, a common feature among cyanobacteria and rare among other prokaryotes. Altogether, he says, the evidence supports the idea that oxygen-producing bacteria had evolved by 3.5 billion years ago, but it is not conclusive.

To examine the issue of when oxygen-producing photosynthesizers first appear, paleobiologists again turn to the geologic record. When photosynthesizers give off oxygen, it enters the atmosphere and oceans. There it influences the formation of certain minerals, leaving a trace of its presence. By examining these minerals, it is possible to get a fairly good idea of when oxygen first appeared in abundance in the earth's atmosphere.

The best evidence for atmospheric oxygen comes from a geologic feature known as banded-iron formations. Between about 3 billion and 2 billion years ago, a series of red bands formed by the mineral hematite, or iron oxide, appear in the geologic record. "No matter where you go on the earth's surface, 2 to 3 billion years ago, you're going to find this type of rock," says Schopf. These bands appear to have been formed by deposition in the earth's oceans. Before the earth's atmosphere contained oxygen, the oceans were saturated with ferrous iron, which can exist dissolved in water. But as photosynthetic bacteria began giving off oxygen in large quantities, this oxygen combined with ferrous iron to produce ferric oxides, which are insoluble in water and dropped to the ocean floor. There they were compacted by other sediments to form the banded-iron formations.

Today, most of the world's commercially important deposits of iron come from these formations. "Why were there steel mills in Pittsburgh, or an automobile industry in Michigan?" Schopf asks. "Because that's where the iron was processed." This iron was deposited as banded iron formations 2.2 billion years ago, along the shores of an ancient sea. As Schopf says, "The world rusted.''

Stromatolites appear in abundance in the geologic record about 2.8 billion years ago. It is reasonable to assume, Schopf asserts, that they were built by the same oxygen-producing photosynthesizers responsible for the banded-iron formations.

For several hundred million years, iron in the oceans and other geologic sinks absorbed the oxygen given off by these early photosynthesizers. But eventually these sinks for oxygen got used up. At that point, oxygen began to accumulate in the atmosphere. The result was a biological revolution that would forever change the course of evolution.

An Oxygenic Atmosphere

The introduction of oxygen into the atmosphere was one of the most momentous events in earth's history. Today the earth's atmosphere is about a fifth oxygen, with virtually all of that oxygen produced biologically by green plants and cyanobacteria. But this abundance of oxygen disguises the fact that it can be a deadly toxin to life. Oxygen reacts with organic molecules, destroying the functions of proteins, nucleic acids, and other essential molecules. Essentially, oxygen burns these substances, removing their biological activity.

The development of an oxygenic atmosphere undoubtedly drove many organisms to extinction. Others found ways to avoid oxygen by retreating to anaerobic, or oxygenless, environments. Still others managed to develop biochemical defenses against the reactivity of oxygen. For instance, superoxide dismutase, the subject of Ayala's molecular clock studies, evolved to protect intracellular systems from toxic derivatives of oxygen.

But evolution is endlessly opportunistic, and while some organisms suffered because of oxygen, others thrived. In the upper atmosphere, oxygen atoms formed a layer of ozone, which absorbs ultraviolet light 7 and keeps it from reaching the earth's surface. Previously, organisms had been forced to protect themselves from the biologically damaging effects of this radiation by shielding themselves from direct sunlight or evolving elaborate biochemical defenses. With the ultraviolet light all but gone, they could spread into ecological niches that had been closed to them. (The shutting off of ultraviolet light also eliminated the plentiful source of energy that many biologists believe contributed to the formation of life, as described in the box below.)

Box

The Origins of Life. The fossil record contains many gaps, but none is longer or more exasperating than the very first one. The oldest known rocks date from about 3.8 billion years ago, when the earth was just 700 million years old. But the first sedimentary (more...)

Even more important, organisms began to develop ways to use oxygen to their advantage. They evolved biochemical mechanisms that used oxygen to break down foodstuffs, resulting in a much more efficient use of organic molecules to get energy. They developed biochemical pathways in which oxygen was an essential participant, resulting in such molecules as steroids, carotenoids, and unsaturated fatty acids. These organisms could not only tolerate the presence of atmosphere; they used its growing abundance to establish their biological dominance.

The development of an oxygenic atmosphere by about 1.7 billion years ago set in motion an entirely new era in evolution. By as early as 1.5 billion years ago, cells with nuclei and other internal structures began to appear. To this day, almost all of these eukaryotic cells are aerobic, requiring the presence of oxygen, and the exceptions are clearly descended from earlier aerobic eukaryotes. About 700 million years ago, these cells began to form integrated multicellular colonies, and individual cells acquired specialized functions. At first these multicellular organisms had soft bodies that were rarely preserved as fossils. But about 600 million years ago organisms began to evolve hard skeletons and other body parts, which when buried by sediments left easily visible traces. Today, all multicellular organisms, including all plants and animals, are composed of eukaryotic cells.

A Modern View

The anaerobic portion of earth's history-from its creation 4.5 billion years ago until about 2 billion years ago-may seem a rather sedate time in the history of evolution. But that is because evolution tends to be equated with changes in the shape of organisms. During the first half of earth's history, the dramatic events in evolution occurred inside cells. The period was characterized by what Schopf calls a "Volkswagen syndrome" -the tendency for outward appearances to remain the same while internal mechanisms are undergoing substantial change. The ancient prokaryotes developed all of the basic biochemical machinery on which later life depends. And in doing so they transformed the earth's environment from one hostile to life to one in which advanced organisms could prosper.

Unraveling the threads of Precambrian evolution has been a difficult process and is far from over, says Schopf. It is an interdisciplinary undertaking, drawing on fields from throughout biology and beyond. "To address the problems of three and a half billion year old pond scum, one has to worry about such things as geology and minerology, microbiology and paleontology, organic chemistry, biochemistry, atmospheric evolution, a little bit of comparative climatology, and the history of science." Progress in the field has sometimes been slowed, Schopf says, by the tendency of scientists to focus on particular fields and ignore their interdisciplinary connections. But "nature is not compartmentalized," Schopf observes. "There's a great imperfection in our science, and I think it's a function of the way we educate our students. There's something to be said for an interdisciplinary education, and perhaps my remarks in some small way illustrate that point."

Human societies are now reaching a point where the interconnectedness of the biological and the nonbiological can no longer be ignored. By burning fossil fuels and destroying vegetation, we are increasing the amount of carbon dioxide in the earth's atmosphere. Carbon dioxide acts as a greenhouse gas, trapping infrared radiation and raising the temperature of the globe. If computer models of atmospheric processes are accurate, global temperatures will rise several degrees over the next century, shifting agricultural and ecological zones and possibly raising sea levels.

Humans are also modifying one of the influences that made the modern ecosystem possible. Industrial chemicals known as chlorofluorocarbons have been breaking down ozone molecules in the upper atmosphere, allowing more ultraviolet radiation to reach the ground. Besides increasing skin cancer rates, this increased ultraviolet radiation could eventually harm terrestrial and marine plants and animals, with untold ecological consequences.

By studying the geologic, atmospheric, oceanographic, and biologic processes that have shaped our modern world, biologists hope to learn more about how these forces will continue to interact. "The past determines the present," says Schopf, "and in the same sense the present determines the future."

ESSAY-Preserving Biological Diversity

Global change-the worldwide modification of the environment as a result of human activities-has become front-page news. Three broad trends seem responsible for this new-found concern with the environment. Industrial pollution, long a problem at the local level, has become national and international in scope, particularly through its contributions to acid rain. Increasing levels of greenhouse gases in the atmosphere are coinciding with a gradual warming of global temperatures, raising fears about the effects of future warmings on agriculture, rainfall, and sea levels. And an observed thinning of the ozone layer, including its virtual disappearance over Antarctica in the spring, has sparked concern that continued releases of chlorofluorocarbons could dramatically increase the ultraviolet radiation reaching the earth's surface.

However, discussions of global change often overlook another critical trend, according to Edward 0. Wilson, professor of science at Harvard University. "There is a fourth horseman in the environmental apocalypse, which needs to be much more closely monitored and acted upon. Unlike the others, it is truly irreversible and hence unpredictable in its consequences. I'm speaking of the extinction of species caused by habitat destruction, especially the destruction of tropical forests."

Human beings are now causing a mass extinction that rivals any of the extinction events that have occurred in the earth's 4.5-billion-year history. Over the course of a hundred years-little more than a human lifetime-as many as half of the species living on the earth could become extinct. The biological diversity of the world is being irrevocably reduced, not through any conscious decision to reduce diversity but because no decision has been made to preserve it. "This is the folly our descendents are least likely to forgive us," Wilson believes.

Ethical beliefs inevitably shape a person's attitudes toward the extinction of species. Some people may hold with the Book of Genesis that God gave humans dominion over all living things to use as we see fit. Others may believe that we are charged with the stewardship of other species and are responsible for their welfare. Some people may see humans as a self-contained species, with no intrinsic responsibility toward other species except as they influence human welfare. Others might feel that because humans are a product of evolution, it diminishes humanity to let the other products of evolution be destroyed.

"The field biologist is impatient with these niceties of moral reasoning," says Wilson. "He is like a molecular biologist watching the laboratory burn down." Species are disappearing much too fast to withhold action until an ethical consensus emerges, Wilson says. Given the many known benefits of biological diversity and the unknown consequences of reducing that diversity, simple prudence would dictate that we act to preserve the world's biological heritage.