Cann_Wilson_Human

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Unformatted text preview: EMERGENCE the recent 54 African Genesis Genetic studies reveal that an African woman from less than 200,000 years ago was our common ancestor By Rebecca L. Cann and Allan C. Wilson of humans POINT-COUNTERPOINT: For an opposing view of how humankind arose around the globe, see “The Multiregional Evolution of Humans,” on page 46. SCIENTIFIC AMERICAN Updated from the April 1992 issue COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. I 130 140 n the quest for the facts about human evolution, we molecular geneticists have engaged in two major debates with the paleontologists. Arguing from their fossils, most paleontologists had claimed the evolutionary split between humans and the great apes occurred as long as 25 million years ago. We maintained human and ape genes were too similar for the schism to be more than a few million years old. After 15 years of disagreement, we won that argument when the paleontologists admitted we had been right and they had been wrong. Once again we are engaged in a debate, this time over the latest phase of human evolution. The paleontologists say modern humans evolved from their archaic forebears around the world over the past million years. Conversely, our genetic comparisons convince us that all humans today can be traced along maternal lines of descent to a woman who lived about 200,000 years ago, probably in Africa. Modern humans arose in one place and spread elsewhere. Neither the genetic information of living subjects nor the fossilized remains of dead ones can explain in isolation how, when and where populations originated. But the former evidence has a crucial advantage in determining the structure of family trees: living genes must have AFRICAN ORIGIN for all modern humans is indicated by the genetic evidence. A genealogy based on 182 current mitochondrial DNA types (outer edges) points to the existence of a common female ancestor from Africa. The arrows on the map (center) indicate the route and the minimum number of unrelated females ( red circles) who colonized various areas, as inferred from the branching pattern. 110 120 100 90 80 70 60 50 40 36 150 30 31 160 20 18 170 15 JOE L E MONNIER ( map ); LAURIE GRACE Ancestor 10 African 180 Asian Australian New Guinean Caucasian 0 0.2 0.4 0.6 Divergence in DNA Sequence (percent) 0.6 0.4 0.2 0 Divergence in DNA Sequence (percent) www.sciam.com SCIENTIFIC AMERICAN 55 COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. ancestors, whereas dead fossils may not have descendants. Molecular biologists know the genes they are examining must have been passed through lineages that survived to the present; paleontologists cannot be sure that the fossils they examine do not lead down an evolutionary blind alley. The molecular approach is free from several other limitations of paleontology. It does not require well-dated fossils or tools from each part of the family tree it hopes to describe. It is not vitiated by doubts about whether tools found near fossil remains were in fact made and used by the population those remains represent. And it concerns itself with a set of characteristics that is complete and objective. A genome, or full set of genes, is complete because it holds all the inherited biological information of an individual. Moreover, all the variants on it that appear within a population— a group of individuals who breed only with one another— can be studied, so specific peculiarities need not distort the interpretation of the data. Genomes are objective because they present evidence that has not been defined, at the outset, by any particular evolutionary model. Gene sequences are empirically verifiable and not shaped by theoretical prejudices. The fossil record, on the other hand, is infamously spotty because a handful of surviving bones may not represent the majority of organisms that left no trace of themselves. Fossils cannot, in principle, be interpreted objectively: the physical characteristics by which they are classified necessarily reflect the models the paleontologists wish to test. If one classifies, say, a pelvis as human because it supported an upright posture, then one is presupposing that bipedalism distinguished early hominids from apes. Such reasoning tends to circularity. The paleontologist’s perspective therefore contains a built-in bias that limits its power of observation. As such, biologists trained in modern evolutionary theory must reject the notion that the fossils provide the most direct evidence of how human evolution actually proceeded. Fossils help to fill in the knowledge of how biological processes worked in the past, but they should not blind us to new lines of evidence or new interpretations of poorly understood and provisionally dated archaeological materials. Molecular Clock ALL THE ADVANTAGES of our field stood revealed in 1967, when Vincent M. Sarich, working in Wilson’s labora- tory at the University of California at Berkeley, challenged a fossil primate called Ramapithecus. Paleontologists had dated its fossils to about 25 million years ago. On the basis of the enamel thickness of the molars and other skeletal characteristics, they believed that Ramapithecus appeared after the divergence of the human and ape lineages and that it was directly ancestral to humans. Sarich measured the evolutionary distance between humans and chimpanzees by studying their blood proteins, knowing the differences reflected mutations that have accumulated since the species diverged. (At the time, it was much easier to compare proteins for subtle differences than to compare the genetic sequences that encode the proteins.) To check that mutations had occurred equally fast in both lineages, he compared humans and chimpanzees against a reference species and found that all the genetic distances tallied. Sarich now had a molecular clock; the next step was to calibrate it. He did so by calculating the mutation rate in other species whose divergences could be reliably dated from fossils. Finally, he applied the clock to the chimpanzee-human split, dating it to between five million and seven million years ago—far later than anyone had imagined. The Inheritance of Mitochondrial DNA Egg Fertilized egg 37 genes Mitochondrial DNA Mitochondrion Nuclear DNA MOST OF AN INDIVIDUAL’S GENES are located on DNA molecules 56 SCIENTIFIC AMERICAN NEW LOOK AT HUMAN EVOLUTION COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. LAURIE GRACE Sperm in the cell nucleus. Mitochondria, the specialized structures that provide cells with energy, also carry some genes for their own manufacture on a ring of DNA. When a sperm and an egg cell unite, they contribute equally to the DNA in the nucleus of the resulting cell. All the mitochondria and the DNA they contain, however, derive from the egg. Studies of mitochondrial DNA can reveal an individual’s maternal ancestry. Mitochondrial DNA is inherited from the mother alone, so all of it today had one female ancestor. At first, most paleontologists clung to the much earlier date. But new fossil finds undermined the human status of Ramapithecus: it is now clear that Ramapithecus is actually Sivapithecus, a creature ancestral to orangutans and not to any of the African apes at all. Moreover, the age of some sivapithecine fossils was downgraded to only about six million years. By the early 1980s almost all paleontologists came to accept Sarich’s more recent date for the separation of the human and ape lines. Those who continue to reject his methods have been reduced to arguing that Sarich arrived at the right answer purely by chance. Two novel concepts emerged from the early comparisons of proteins from different species. One was the concept of inconsequential, or neutral, mutations. Molecular evolution appears to be dominated by such mutations, and they accumulate at surprisingly steady rates in surviving lineages. In other words, evolution at the gene level results mainly from the relentless accumulation of mutations that seem to be neither harmful nor beneficial. The second concept, molecular clocks, stemmed from the observation that rates of genetic change from point mutations (changes in individual DNA base pairs) were so steady over long periods that one could use them to time divergences from a common stock. convert food into a form of energy the rest of the cell can use. Unlike the DNA of the nucleus, which forms bundles of long fibers, each consisting of a proteincoated double helix, the mitochondrial DNA comes in small, two-stranded rings. Whereas nuclear DNA encodes an estimated 100,000 genes— most of the information needed to make a human being— mitochondrial DNA encodes only 37. In this handful of genes, every one is essential: a single adverse mutation in any of them is known to cause some severe neurological diseases. For the purpose of scientists studying when lineages diverged, mitochondrial DNA has two advantages over nuclear DNA. First, the sequences in mitochondrial DNA that interest us accumulate mutations rapidly and steadily, according to empirical observations. Because many mutations do not alter the mitochondrion’s function, they are effectively neutral, and natural selection does not eliminate them. This mitochondrial DNA therefore behaves like a fast-ticking clock, which is essential for identifying recent genetic changes. Any two humans chosen randomly from anywhere on the planet are so alike in most of their DNA sequences that we can measure evolution in our species only by concentrating on the genes that mutate fastest. Genes controlling skeletal characters do not fall within this group. Second, unlike nuclear DNA, mitochondrial DNA is inherited from the mother alone, unchanged except for chance mutations. The father’s contribution ends up on the cutting-room floor, as it were. The nuclear genes, to which the father does contribute, descend in what we may call ordinary lineages, which are of course important to the transmission of physical characteristics. For our studies of modern human origins, however, we focus on the mitochondrial, maternal lineages. Maternal lineages are closest among siblings because their mitochondrial DNA has had only one generation in which to accumulate mutations. The degree of relatedness declines step by step as one moves along the pedigree, from first cousins descended from the maternal grandmother, to second cousins descended from a common maternal greatgrandmother and so on. The farther back the genealogy goes, the larger the circle of maternal relatives becomes, until at last it embraces everyone alive. Logically, then, all human mitochondrial DNA must have had an ultimate common female ancestor. But it is easy to show she did not necessarily live in a small population or constitute the only woman of her generation. Imagine a static population that always contains 15 mothers. Every new generation must contain 15 daughters, but some mothers will not produce a daughter, whereas others will produce two or more. Because maternal lineages die out whenev- Mitochondrial Clue W E C O U L D B E G I N to apply these methods to the reconstruction of later stages in human evolution only after 1980, when DNA restriction analysis made it possible to explore genetic differences with high resolution. Workers at Berkeley, including Wes Brown, Mark Stoneking and us, applied the technique to trace the maternal lineages of people sampled from around the world. The DNA we studied resides in the mitochondria, cellular organelles that REBECCA L. CANN and ALLAN C. WILSON applied the tools of genetics to paleontology during many of their collaborations. Cann is professor of genetics and molecular biology at the John A. Burns School of Medicine of the University of Hawaii at Manoa. She received both her bachelor’s degree in genetics and her Ph.D. in anthropology from the University of California, Berkeley. As a postdoctoral fellow, she worked at Berkeley with Wilson and at the University of California, San Francisco. Cann is using mitochondrial DNA to assay the genetic diversity of birds in the Hawaiian Islands. Until his death in 1991, Wilson was professor of biochemistry at Berkeley. A native of New Zealand, he received his doctorate from Berkeley. Wilson also worked at the Weizmann Institute of Science in Rehovot, Israel, at the University of Nairobi and at Harvard University. SCIENTIFIC AMERICAN THE AUTHORS www.sciam.com 57 COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. er there is no daughter to carry it on, it is only a matter of time before all but one lineage disappears. In a stable population, the time for this fixation of the maternal lineage to occur is the length of a generation multiplied by twice the population size. Eve in Africa to the lucky woman whose lineage survives as Eve. Bear in mind, however, that other women were living in Eve’s generation and that Eve did not occupy a specially favored place in the breeding pattern. She is purely the beneficiary of chance. Moreover, if we were to reconstruct the ordinary lineages for the population, they would trace back to many of the men and women who lived at the same time as Eve. Population geneticists Daniel L. Hartl, now at Harvard University, and Andrew G. Clark, now at Cornell University, estimate that as many as 10,000 people could have lived then. The name “Eve” can therefore be misleading— she is not the ultimate source of all the ordinary lineages, as the biblical Eve was. From mitochondrial DNA data, it is possible to define the maternal lineages of living individuals all the way back to a common ancestor. In theory, a great number of different genealogical trees could give rise to any set of genetic data. To recognize the one that is most probably correct, one must apply the parsimony principle, which requires that subjects be connected in the simplest possible way. The most efficient hypothetical tree must be tested by comparison with other data to see whether it is consistent with them. If the tree holds up, it is analyzed for evidence of the geographic history inherent in elements. In 1988 Thomas D. Kocher of Berkeley (now at the University of New Hampshire) applied just such a parsimonious interpretation to the interrelatedness of the mitochondrial DNA of 14 humans from around the world. He determined that 13 branching points were the fewest that could account for the differences he found. Taking the geographic considerations into account, he then concluded that Africa was the ultimate human ONE MIGHT REFER homeland: the global distribution of mitochondrial DNA types he saw could then be explained most easily as the result of no more than three migration events to other continents. A crucial assumption in this analysis is that all the mitochondrial lineages evolve at the same rate. So when Kocher conducted his comparison of the human mitochondrial DNAs, he also included analogous sequences from four chimpanzees. If the human lineages had differed in the rate at which they accumulated mutations, then some of the 14 human sequences would be significantly closer or farther away from the chimpanzee sequences than others. In fact, all 14 human sequences are nearly equidistant from the chimpanzee sequences, which implies that the rates of change among humans are fairly uniform. The chimpanzee data also illustrated how remarkably homogeneous humans are at the genetic level: chimpanzees commonly show as much as 10 times the genetic variation of humans. That fact alone suggests that all of modern humanity sprang from a relatively small stock of common ancestors. 1 2 3 4 5 6 7 Generation 8 9 10 11 12 13 14 15 16 Working at Berkeley with Stoneking, we expanded on Kocher’s work by examining a larger genealogical tree made up of 182 distinct types of mitochondrial DNA from 241 individuals. The multiple occurrences of mitochondrial DNA types were always found among people from the same continent and usually in persons who lived within 100 miles of one another. Because the tree we constructed had two main branches, both of which led back to Africa, it, too, supported the hypothesis that Africa was the place of origin for modern humans. One noteworthy point that jumps out of our study is that although geographic barriers do influence a population’s mitochondrial DNA, people from a given continent do not generally all belong to the same maternal lineage. The New Guineans are typical in this respect. Their genetic diversity had been suspected from linguistic analyses of the remarkable variety of language families— usually classified as Papuan— spoken on this one island [see “The Austronesian Dispersal and the Origin of Languages,” by Peter Bellwood; Scientific American, July 1991]. On our genealogical tree, UNIVERSAL MATERNAL ANCESTOR can be found for all the members of any population. The example shown here traces the lineages of 15 females in a stable population. In each generation, some maternal lineages proliferate and others become extinct. Eventually, by chance, one maternal lineage (dark blue) replaces all the others. NEW LOOK AT HUMAN EVOLUTION 58 SCIENTIFIC AMERICAN C OPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. LAURIE GRACE Huge levels of gene flow between early continents—very unlikely—would have been needed for multiregionalism. New Guineans showed up on several different branches, which proved that the common female ancestor of all New Guineans was not someone in New Guinea. The population of New Guinea must have been founded by many mothers whose maternal lineages were most closely related to those in Asia. That finding is what one would expect if the African origin hypothesis were true: as people walked east out of Africa, they would have passed through Asia. Travel was probably slow, and during the time it took to reach New Guinea, mutations accumulated both in the lineages that stayed in Asia and in those that moved on. Thus, people who are apparently related by membership in a common geographic race need not be very closely related in their mitochondrial DNA. Mitochondrially speaking, races are not like biological species. We propose that the anatomical characteristics uniting New Guineans were not inherited from the first settlers. They evolved after people colonized the island, chiefly as the result of mutations in nuclear genes spread by sex and recombination throughout New Guinea. Similarly, the light skin color of many whites is probably a late development that occurred in Europe after that continent was colonized by Africans. During the early 1980s, when we were constructing our genealogical tree, we had to rely on black Americans as substitutes for Africans, whose mitochondrial DNA was difficult to obtain in the required quantities. Fortunately, the development of a technique called the polymerase chain reaction has eliminated that constraint. The reaction makes it possible to duplicate DNA sequences easily, ad infinitum; a small starting sample of DNA can expand into an endless supply. The polymerase chain reaction enabled Linda Vigilant of Pennsylvania State University to redo our study using mitochondrial DNA data from 120 Africans, representing six diverse parts of the sub-Saharan region. Vigilant traced a genealogical tree whose 14 deepest branches lead exclusively to Africans and whose 15th branch leads to both Africans and non-Africans. The non-Africans lie on shallow secondary branches stemming from the 15th branch. Considering the number of African and nonAfrican mitochondrial DNAs surveyed, the probability that the 14 deepest branches would be exclusively African is one in 10,000 for a tree with this branching order. Satoshi Horai and Kenji Hayasaka of the National Institute of Genetics in Mishima, Japan, analogously surveyed population samples that included many more Asians and individuals from fewer parts of Africa; they, too, found that the mitochondrial lineages led back to Africa. We estimate the odds of their arriving at that conclusion accidentally were only four in 100. Although these statistical evaluations are not strong or rigorous tests, they do make it seem likely that the theory of an African origin for human mitochondrial DNA is now fairly secure. African human Non-African person Chimpanzee Pygmy chimpanzee 200,000 Years or Less BECAUSE OUR COMPARISONS INTERRELATEDNESS of 14 humans and four chimpanzees was inferred from similarities discovered in their mitochondrial DNA sequences. The chimpanzee data help researchers to measure when various evolutionary divergences in the human lineages occurred. www.sciam.com with the chimpanzee data showed that the human mitochondrial DNA clock has ticked steadily for millions of years, we knew it should be possible to calculate when the common mother of humanity lived. We assumed that the human and chimpanzee lineages diverged five milSCIENTIFIC AMERICAN LAURIE GRACE 59 C OPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. lion years ago, as Sarich’s work had shown. We then calculated how much humans had diverged from one another relative to how much they had diverged from chimpanzees— that is, we found the ratio of mitochondrial DNA divergence among humans to that between humans and chimpanzees. Using two different sets of data, we determined that the ratio was less than 1:25. Human maternal lineages therefore grew apart in a period less than 1⁄ 25 as long as five million years, or less than 200,000 years. With a third set of data on changes in a section of the mitochondrial DNA called the control region, we arrived at a more ancient date for the common mother. That date is less certain, however, because questions remain about how to correct for multiple mutations that occur within the control region. One might object that a molecular clock known to be accurate over five million years could still be unreliable for shorter periods. It is conceivable, for example, that intervals of genetic stagnation might be interrupted by short bursts of change when, say, a new mutagen enters the environment, or a virus infects the germ-line cells, or intense natural selection affects all segments of the DNA. To rule out the possibility that the clock PRESENT AFRICAN EUROPEAN might run by fits and starts, we ran a test to measure how much mitochondrial DNA has evolved in populations founded at a known time. The aboriginal populations of New Guinea and Australia are estimated to have been founded less than 50,000 to 60,000 years ago. The amount of evolution that has since occurred in each of those places seems about one third of that shown by the whole human species. Accordingly, we can infer that Eve lived three times 50,000 to 60,000 years ago, or roughly 150,000 to 180,000 years ago. All our estimates thus agree that the split happened not far from 200,000 years ago. Those estimates fit with at least one line of fossil evidence. The remains of anatomically modern people appear first in Africa, then in the Middle East, and later in Europe and east Asia. Anthropologists have speculated that in east Africa the transition from anatomically archaic to modern people took place as recently as 130,000 years ago [see “The Emergence of Modern Humans,” by Christopher B. Stringer; Scientific American, December 1990]. On the other hand, a second line of evidence appears to conflict with this view. The fossil record shows clearly that the southern parts of Eurasia were EAST ASIAN AUSTRALIAN 100,000 Age (years) Klasies Neandertal Ngandong Dali occupied by archaic people who had migrated from Africa to Asia nearly a million years ago. Such famous fossils as Java Man and Beijing Man are of this type. This finding and the hypothesis that the archaic Eurasian population underwent anatomical changes that made them resemble more modern people led to the multiregional evolution model: similar evolutionary changes in separate geographic regions converted the inhabitants from archaic small-brained types to modern big-brained types. Huge levels of gene flow between continents, however, would be necessary to maintain human populations as one biological species. The multiregional evolution model also predicts that at least some genes in the modern east Asian population would be linked more closely to those of their archaic Asian predecessors than to those of modern Africans. We would expect to find deep lineages in Eurasia, especially in the Far East. Yet surveys in our laboratories and in others, involving more than 1,000 people from Eurasia and its mitochondrial DNA satellites (Australia, Oceania and the Americas), have given no hint of that result. It therefore seems very unlikely that any truly ancient lineages survive undetected in Eurasia. We simply do not see the result predicted by the regional model. Moreover, geneticists such as Masatoshi Nei of Pennsylvania State University, Kenneth K. Kidd of Yale University, James Wainscoat of the University of Oxford and Luigi L. Cavalli-Sforza of Stanford University have found support for an African origin model in their studies of nuclear genes. 300,000 Saldanha Petralona Zhoukoudian (“Beijing”) Sambungmachan Multiregional Mystery P R O P O N E N T S O F the multiregional evolution model typically emphasize that they have documented a continuity of anatomical morphologies between the archaic and modern residents of different regions; they insist that these morphologies would be unlikely to evolve independently in any invading people. For that argument to hold true, however, it must also be shown that the cranial features in question are truly indepen- 700,000 Olduvai European Lantian Java Homo erectus ARCHAIC HUMAN GROUPS were gradually replaced throughout the Old World by modern humans who arose in Africa. Archaic females do not seem to have contributed mitochondrial genes to the modern people of Europe, east Asia and Australia. 60 SCIENTIFIC AMERICAN NEW LOOK AT HUMAN EVOLUTION COPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. LAURIE GRACE dent of one another— that is, that natural selection would not tend to favor certain constellations of functionally related features anyway. Yet we know that powerful jaw muscles may impose changes on the mandible, the browridge and other points on the skull; circumstances that promoted the evolution of these features in one population might do so again in a related population. Other paleontologists also dispute the evidence for continuity. They argue that modern populations are not linked to past ones by morphological characteristics that evolved uniquely in the fossil record. Instead fossils and modern populations are united by their shared retention of still older ancestral characteristics. The continuity seen by believers in multiregional evolution may be an illusion. The idea that modern humans could cohabit a region with archaic ones and replace them completely without any mixture may sound unlikely. Nevertheless, some fossil finds do support the idea. Discoveries in the caves at Qafzeh in Israel suggest that Neandertals and modern humans lived side by side for 40,000 years, yet they left little evidence of interbreeding. How one human population might have replaced archaic humans without any detectable genetic mixing is still a compelling mystery. One of us (Cann) suspects that infectious diseases could have contributed to the process by helping to eliminate one group. CavalliSforza has speculated that the ancestors of modern humans may have developed some modern trait, such as advanced language skills, that effectively cut them off from breeding with other hominids. This and related questions may yield as molecular biologists learn how to link specific genetic sequences to the physical and behavioral traits that those sequences ultimately influence. Even before then, further studies of both nuclear and mitochondrial DNA will render more informative genetic trees. Particularly enticing are the sequences on the Y chromosome that determine maleness and that are therefore inherited from the father alone. Gerard www.sciam.com Son Father Mother Male Female Mitochondrial DNA source PEDIGREE of one individual illustrates the difference between the patterns of nuclear and mitochondrial inheritance. All 32 ancestors from five generations ago contributed equally to his nuclear DNA. His mitochondrial lineage (blue line) leads back to only one person in every generation. Lucotte, while at the College of France, and his colleagues have indirectly compared such sequences in an effort to trace paternal lineages to a single progenitor —“Adam,” if you will. Those preliminary results also point to an African homeland, and with further refinements this work on paternal lineages may be able to provide an invaluable check on our results for maternal lineages. Unfortunately, base changes accumulate slowly on useful regions of the Y chromosome, making it technically difficult to conduct a detailed genealogical analysis. More progress can be expected soon, as molecular biologists learn to apply their techniques to materials uncovered by our friendly rivals, the paleontologists. Preliminary molecular studies have already been conducted on DNA from mummified tissues found in a Florida bog and dated to 7,500 years ago. Im- proved methods of extracting DNA from still older fossilized bone now appear close at hand. With them, we may begin building the family tree from a root that was alive when the human family was young. Epilogue S I N C E T H I S A R T I C L E was first published, further genetic work on the mitochondrial DNA sequences of three Neandertal specimens upholds our conclusions about the lack of a mixture between ancient and modern Homo sapiens. Furthermore, whole mitochondrial genome sequencing— all 16,569 base pairs from more than 50 donors— gives more precise resolution to the timescale of our emergence. It now seems that the earliest migration out of Africa is closer to 120,000 years ago than 200,000 years ago— more recent, yet still within the range we had originally estimated. MORE TO E XPLORE Mitochondrial DNA and Human Evolution. Rebecca L. Cann, Mark Stoneking and Allan C. Wilson in Nature, Vol. 325, No. 6099, pages 31–36; January 1–7, 1987. Mitochondrial DNA. M. Stoneking and A. C. Wilson in The Colonization of the Pacific: A Genetic Trail. Edited by Adrian V. S. Hill and Susan W. Serjeantson. Oxford University Press, 1989. Mitochondrial DNA Sequences in Single Hairs from a Southern African Population. Linda Vigilant, Renee Pennington, Henry Harpending, Thomas D. Kocher and Allan C. Wilson in Proceedings of the National Academy of Sciences USA, Vol. 86, No. 23, pages 9350–9354; December 1989. Sequence Evolution of Mitochondrial DNA in Humans and Chimpanzees. T. D. Kocher and A. C. Wilson in Evolution of Life. Edited by S. Osawa and T. Honjo. Springer-Verlag, Tokyo, 1991. SCIENTIFIC AMERICAN LAURIE GRACE 61 C OPYRIGHT 2003 SCIENTIFIC AMERICAN, INC. ...
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