The recent african physical characteristics by which they are classified necessarily reflect the models the paleontologists wish to test If one classifies, say, a pelvis as human Genesis of humans because it supported an upright pos ure, then one is presupposing that bi pedalism distinguished early from apes. Such reasoning tends to cir Genetic studies reveal that an African woman of tuve the ethr e cloeoatols g st s perspea 200,000 years ago was our common ancestor that limits its power of observation As such biologists trained in mod- ern evolutionary theory must reject the by Allan C. wilson and Rebecca L Cann notion that the fossils provide the most direct evidence of how human evolu tion actually proceeded. Fossils help to n the quest for the facts about hu- lived about 200,000 years ago, proba- fill in the knowledge of how biological man evolution, we molecular geneti- bly in Africa. Modern humans arose in processes worked in the past, but they cists have engaged in two major one place and spread elsewhere should not blind us to new lines of evi- debates with the paleontologists. Argu- Neither the gene tic information of liv- dence or new interpretations of poorly ing from their fossils, most paleontolo- ing subjects nor the fossilized remains understood and provisionally dated gists had claimed the evolutionary split of dead ones can explain in isolation chaeological materials between humans and the great apes how, when and where populations orig- occurred as long as 25 million years inated But the former evidence has a 11 the advantages of our field ago We maintained human and ape crucial advantage in determining the stood revealed in 1967. when enes were too similar for the schism structure of family trees: living genes Vincent m. sarich. working in to be more than a few million years must have ancestors, whereas dead fos- Wilsons laboratory at the University of old. After 15 years of disagreement, we sils may not have descendants. Molec- California at berkeley challenged a fos won that argument, when the paleon- ular biologists know the genes they are sil primate called Ramapithecus. Pale ologists admitted we had been right examining must have been passed ontologists had dated its fossils to and they had been wrong through lineages that survived to the about 25 million years ago. On the ba- Once again we are engaged in a de- present; paleontologists cannot be sure sis of the enamel thickness of the mo- bate, this time over the latest phase of that the fossils they examine do not lars and other skeletal characteristics, human evolution. The paleontologists lead down an evolutionary blind alley. they believed that Ramapithecus say modem humans evolved from their The molecular approach is free from peared after the divergence of the hu- r c cs ay can be traced along ma- ly tree it hopes to describe. It is not vi- distance between humans and chim- haic forebears around the world over several other limitations of paleontolo-. man and ape lineages and that it was parsons convince us that all sils or tools from each part of the fami- Sarich measured the evolutionary umans ternal lines of descent to a woman who tiated by doubts about whether tools panzees by studying their blood pro- found near fossil remains were in fact teins knowing the differences reflected made and used by the population those mutations that have accumulated since he late ALLAN C. WILSON and Rl-1 remains represent. Finally, it concems the species diverged. (At the time, it BECCA L. CANN applied the tools of itself with a set of characteristics that was much easier to compare proteins netics to paleontology during many of is complete and objective for subtle differences than to compare a genome, or full set of genes, is com- the genetic sequences that encode the 91, Wilson was professor of biochem- plete because it holds all the inherited proteins ) To check that mutations had istry at the University of California, I biological information of an individual. occurred equally fast in both lineages Berkeley. a native of New Zealand, he Moreover, all the variants on it that ap- he compared humans and chimpanzees |/限h四mm四 eceived his undergrads Berkel other-can be studied as well, so specif- Sarich now had a molecular clock; the son also worked at the Weizmann ic peculiarities need not distort the next step was to calibrate it. He did so stitute of Science, the University of Nai-I interpretation of the data. Genomes are by calculating the mutation rate in oth robi and Harvard University. Cann is as- objective sources of data because they er species whose divergences could be sociate professor of genetics and present evidence that has not been de- reliably dated from fossils. Finally, he lar biology at the John A. Bums School I fined, at the outset, by any particular applied the clock to the at Manoa. She received both her bache- evolutionary model, Gene sequences are human split, dating it to between five lor's degree in The fossil record, on the other hand, At first, most paleontologists clung with Wilson and at the University of Cal- is infamously spotty because a handful to the much earlier date. But new fossi San Francisco. As one of her cur- of surviving bones may not represent finds undermined the human status of cts. cann is using mitochon- he majority of organisms that left no Ramapithecus: it is now clear Rama to assay the senetic diversity trace of themselves. Fossils cannot, in pithecus is actually Sivapithecus, a crea- principle, be interpreted objectively: the ture ancestral to orangutans and not to 68 SCIENTIFIC AMERICAN April 1992The Recent African Genesis of Humans Genetic studies reveal that an African woman of 200,000 years ago was our common ancestor by Allan C. Wilson and Rebecca L. Cann I n the quest for the facts about hu￾man evolution, we molecular geneti￾cists have engaged in two major debates with the paleontologists. Argu￾ing from their fossils, most paleontolo￾gists 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 paleon￾tologists admitted we had been right and they had been wrong. Once again we are engaged in a de￾bate, this time over the latest phase of human evolution. The paleontologists say modem 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 ma￾ternal lines of descent to a woman who The late ALLAN C. WILSON and RI￾BECCA L. CANN applied the tools of ge￾netics to paleontology during many of their collaborations. Until his death In 1991, Wilson was professor of biochem￾istry at the University of California, Berkeley. A native of New Zealand, he received his undergraduate degree from the University of Otago in 1955, his master’s from Washington State Univer￾sity and his doctorate from Berkeley. Wilson also worked at the Weizmann In￾stitute of Science, the University of Nai￾robi and Harvard University. Cann is as￾sociate professor of genetics and molec￾ular biology at the John A. Bums School of Medicine of the University of Hawaii at Manoa. She received both her bache￾lor’s degree in genetics and her Ph.D. in anthropology from Berkeley. As a post￾doctoral fellow, she worked at Berkeley with Wilson and at the University of Cal￾ifornia, San Francisco. As one of her cur￾rent projects, Cann is using mitochon￾drial DNA to assay the genetic diversity of birds In the Hawaiian Islands. 68 lived about 200,000 years ago, proba￾bly in Africa. Modern humans arose in one place and spread elsewhere. Neither the gene tic information of liv￾ing subjects nor the fossilized remains of dead ones can explain in isolation how, when and where populations orig￾inated. But the former evidence has a crucial advantage in determining the structure of family trees: living genes must have ancestors, whereas dead fos￾sils may not have descendants. Molec￾ular 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 paleontolo-. gy. It does not require well-dated fos￾sils or tools from each part of the fami￾ly tree it hopes to describe. It is not vi￾tiated by doubts about whether tools found near fossil remains were in fact made and used by the population those remains represent. Finally, it concerns itself with a set of characteristics that is complete and objective. A genome, or full set of genes, is com￾plete because it holds all the inherited biological information of an individual. Moreover, all the variants on it that ap￾pear within a population-a group of in￾dividuals who breed only with one an￾other-can be studied as well, so specif￾ic peculiarities need not distort the ‘interpretation of the data. Genomes are objective sources of data because they present evidence that has not been de￾fined, 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 pos￾ture, then one is presupposing that bi￾pedalism distinguished early hominids from apes. Such reasoning tends to cir￾cularity. The paleontologist’s perspec￾tive therefore contains a built-in bias that limits its power of observation. As such, biologists trained in mod￾ern evolutionary theory must reject the notion that the fossils provide the most direct evidence of how human evolu￾tion 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 evi￾dence or new interpretations of poorly understood and provisionally dated ar￾chaeological materials. 11 the advantages of our field Astood revealed in 1967, when Vincent M. Sarich, working in Wilson’s laboratory at the University of California at Berkeley, challenged a fos￾sil primate called Ramapithecus. Pale￾ontologists had dated its fossils to about 25 million years ago. On the ba￾sis of the enamel thickness of the mo￾lars and other skeletal characteristics, they believed that Ramapithecus ap￾peared after the divergence of the hu￾man and ape lineages and that it was directly ancestral to humans. Sarich measured the evolutionary distance between humans and chim￾panzees by studying their blood pro￾teins, 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 oth￾er species whose divergences could be reliably dated from fossils. Finally, he applied the clock to the chimpanzee￾human split, dating it to between five and seven million years ago-far later than anyone had imagined. At first, most paleontologists clung to the much earlier date. But new fossil finds undermined the human status of Ramapithecus: it is now clear Rama￾pithecus is actually Sivapithecus, a crea￾ture ancestral to orangutans and not to