Wilson_Cann2002 - : I» § . TWa O'urQCOmmorI _...

Info iconThis preview shows pages 1–8. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 8
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: : I» § . TWa O'urQCOmmorI _ R’e’brfeéc‘a L. Canh L gandAIlanjC; Wilson POINT—COUNTERPOINT: F0: an opposing View of how humankind arose around the globe, see “The Multiragional Evolution of Humans,” on page 46. 54 SCIENTIFIC AMERICAN , ' Updatedfrom the April 1992 issue JOE LEMDNNIER (map); LAURIE 5mm: n the quest for the facts about human evolution, we molecular geneticists have engaged in two major debates with the paleon- tologists. Arguing from their fos- sils, most paleontologists had claimed the evolutionary split between humans and the great apes occurred as long as 25 million years ago. We main- tained 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 de— bate, 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 ma- ternal 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 orig— inated. But the former evidence has a cru— cial advantage in determining the struc- ture of family trees: living genes must have AFRICAN GRlGlN for all modern humans is indicated by the genetic evidence. A genealogg based on 182 current mitochondrial DNAtgpes [outeredges] points to the existence ofa common female ancestorfrom 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. 3} African 8 Asian 0 0.2 0.4 0.5 Divergence in DNA Sequence [percent] www.sciam.com A Australian Ah New Guinean I Caucasian Ancestor l—L_l_.l_l_l__l 0.6 0.4 0.2 0 Divergence in DNA Sequence [percent] SCIENTIFIC AMERICAN 55 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 ex- amine do not lead down an evolutionary blind alley. . _ The molecular approach is free from several other limitations of paleontol- ogy. 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 viti- ated 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 com- plete and objective. A genome, or full set of genes, is complete because it holds all the inherit- ed biological information of an individ- ual. Moreover, all the variants on it that appear within a population—a group of individuals who breed only with one an- other—can be studied, so specific pecu- liarities need not distort the interpreta- tion of the data. Genomes are objective because they present evidence that has not been defined, at the outset, by any particular evolutionary model. Gene se- quences are empirically verifiable and. not shaped by theoretical prejudices. a?! 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 princi- ple, be interpreted objectively: the phys- ical 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 dis- tinguished 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 no- tion that the fossils provide the most di- rect evidence of how human evolution actually proceeded. Fossils help to fill in the knowledge of how biological pro- cesses 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. ifieieeuiar merit ALL THE ADVANTAGES of our field stood revealed in 1967, when Vincent M. Sarich, working in Wilson’s labora- The Inheritance of Mitochondrial DNA Mitochondrion Nuclear DNA 55 SCIENTIFIC AMERICAN pr Fertilized egg tory at the University of California at Berkeley, challenged a fossil primate called Ramapitkecus. Paleontologists had dated its fossils to about 25 million years ago. On the basis of the enamel thickness of the molars and other skele- tal characteristics, they believed that Ramapitkecus appeared after the diver- gence of the human 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 speciesvdiverged. (At the time, it was much easier to compare proteins for subtle differences than to compare the genetic sequences that encode the pro- teins.) To check that mutations had oc- curred 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-hu— man split, dating it to between five mil— lion and seven million years ago—far lat- er than anyone had imagined. Mitochondrial DNA MOST OF AN INDIVIDUAL’S GENES are located on DNA molecules in the cell nucleus. Mitochondria, the specialized structures that provide cells with energg, also carry some genes fortheir own manufacture on a ring of DNA. When a sperm and an egg cell unite, theg contribute equallg to the DNAin the nucleus ofthe resulting cell.All the mitochondria and the DNAtheg contain, however, derive from the egg. Studies of mitochondrial DNA can reveal an individual’s maternal ancestry. NEW LOOK AT HUMAN EVOLUTION LAUTHE GRACE UTION LAUTHE GRACE 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 Ra- mapithecus is actually Sivapitbecus, a creature ancestral to orangutans and not to any of the African apes at all. More— over, the age of some sivapithecine fos- sils 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 con- tinue to reject his methods have been re- duced 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 dom— inated by such mutations, and they ac— cumulate at surprisingly steady rates in surviving lineages. In other words, evo- lution at the gene level results mainly from the relentless accumulation of rap- tations that seem to be neither harmful nor beneficial. The second concept, mo— lecular clocks, stemmed from the obser— vation 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. iriiteshenririal gins WE COULD BEGIN 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 dif— ferences 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 www.sciam.com 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 protein- coated 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, muta— ‘ tion 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 mitochon- drial DNA that interest us accumulate mutations rapidly and steadily, accord- ing to empirical observations. Because many mutations do not alter the mito- chondrion’s function, they are effective- ly 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 ran- dbmly 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 con- trolling“ skeletal characters do not fall within this group. Second, unlike nuclear DNA, mito— chondrial DNA is inherited from the mother alone, unchanged except for chance mutations. The father’s contribu- tion 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 ori— gins, however, we focus on the mito— chondrial, maternal lineages. Maternal lineages are closest among siblings because their mitochondrial DNA has had only one generation in 'which to accumulate mutations. The de- gree of relatedness declines step by step as one moves along the pedigree, from first cousins descended from the mater- nal grandmother, to second cousins de- scended from a common maternal great— grandmother and so on. The farther back the genealogy goes, the larger the ' circle of maternal relatives becomes, un- til at last it embraces everyone alive. Logically, then, all human mito— chondrial DNA must have had an ulti- mate 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. Be- cause maternal lineages die out whenev- R€BECCA L. CANN and ALLAN C. WILSON applied the tools of genetics to paleontology dur- ing many oftheir collaborations. Cann is professor ofgenetics and molecular biology atthe John A. Burns School of Medicine ofthe University of l-lawaii at Manoa. She received both her bachelor’s degree in genetics and her PhD. in anthropology from the University ofCal- ifornia, Berkeley. As a postdoctoral fellow, she worked at Berkeley with Wilson and at the University of California, San Francisco. term is using mitochondrial DNA to assay the ge- netic diversity of birds in the Hawaiian islands. Until his death in 1991, Wilson was profes- sor of biochemistry at Berkeley. A native of New Zealand, he received his doctorate from Berkeley..Wilson also worked atthe Weizmann Institute ofScience in Rehovot, Israel, atthe University ofNairobi and at Harvard University. SCIENTIFIC AMERICAN 5? 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 popula- tion, the time for this fixation of the ma- ternal lineage to occur is the length of a generation multiplied by twice the popu- lation size. we in afriea ONE MIGHT REFER to the lucky wo- man 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. More— over, if we were to reconstruct the ordi— nary 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 Uni— versity, 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 ordi- nary 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 prob- ably correct, one must apply the parsi- mony principle, which requires that sub— jects be connected in the simplest possi— ble 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 ana- lyzed for evidence of the geographic his- tory inherent in elements. In 198 8 Thomas D. Kocher of Berke- ley (now at the University of New Hamp— shire) 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 consider— ations into account, he then concluded that Africa was the ultimate human 58 SCIENTIFIC AMERICAN homeland: the global distribution of mi- tochondrial DNA types he saw could then be explained most easily as the re- sult 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 c0mparison of the human mitochondrial DNAs, he also included analogous sequences from four chim— panzees. If the human lineages had dif— fered in the rate at which they accumu- lated mutations, then some of the 14 hu- man sequences would be significantly closer or farther away from the chim— panzee sequences than others. In fact, all 14 human sequences are nearly equidis- tant 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 hu— manity sprang from a relatively small stock of common ancestors. 1 III IIIIIIIII 2 w" 3 II IIIIIIIII IIIIIIIII 4 III IIIIIIII IIIIIIIIII 7 I ' e I g 9 III llIIIIIIIINllNl ‘3 10 In 14 III 15 l‘ ' Working at Berkeley with Stoneking, we expanded on Kocher’s work by ex- amining a larger genealogical tree made up of 182 distinct types of mitochondri- al DNA from 241 individuals. The mul- tiple 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 con~ structed had two main branches, both of which led back to Africa, it, too, sup— ported the hypothesis that Africa was the place of origin for modern humans. One noteworthy point that jumps out of our study is that although geo- graphic barriers do influence a popula- tion’s mitochondrial DNA, people from a given continent do not generally all be- long to the same maternal lineage. The New Guineans are typical in this respect. Their genetic diversity had been suspect- ed from linguistic analyses of the re- markable 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 AMERI— CAN, July 1991]. On our genealogical tree, N I» I \WIIII “ ” T I “llllllllv I .. \\IIII\\\ ‘lllllllllli llllllllllllw UNIVERSAL MATERNAL ANCESTDR can be found for all the members ofang 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. Eventuallg, bg chance, one maternal lineage [dark blue] replaces all the others. NEW LOOK AT HUMAN EVOLUTION LA“R\E GRACE LAURIE GRACE ing, V8.8 1ps LAumE GRACE LAURIE spurs nlkely—would have been needed for multiregionl. New Guineans showed up on several dif- ferent 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 moth- ers whose maternal lineages were most closely related to those in Asia. That finding is what one would ex- pect if the African origin hypothesis were true: as people walked cast 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 re- lated by membership in a common geo— graphic race need not be very closely re- lated in their mitochondrial DNA. Mi- tochondrially 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 peo- ple colonized the island, chiefly as the re— sult 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 af- ter that continent was colonized by Africans. During the early 19805, when we were constructing our genealogical tree, we had to rely on black Americans as substitutes for Africans, whose mito— chondrial DNA was difficult to obtain in the required quantities. Fortunately, the development of a technique called the polymerase chain reaction has eliminat- African human Q Non-African person W Chimpanzee E] ngmg chimpanzee INTERRELATEDNESS of 14 humans and four chimpanzees was inferred . _ lZl \ . from similarities discovered in their mitochondrial DNA sequences. The L2 chimpanzee data help researchers to measure when various evolutionarg divergences in the human lineages occurred. www.sciam.com ed that constraint. The reaction makes it possible to duplicate DNA sequences easily, ad infinitum; a small starting sam- ple of DNA can expand into an endless supply. The polymerase chain reaction en- abled Linda Vigilant of Pennsylvania State University to redo our study using mitochondrial DNA data from 120 Af- ricans, 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 Af— ricans and non-Africans. The non—Afri— cans lie on shallow secondary branches stemming from the 15th branch. Con~ sidering the number of African and non- African mitochondrial DNAs surveyed, the probability that the 14 deepest branches would be exclusively African is one in 10,000 for a tree with this branch- ing 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 few- er parts of Africa; they, too, found that the mitochondrial lineages led back to Africa. We estimate the odds of their ar- riving at that conclusion accidentally were only four in 100. Although these statistical evaluations are not strong or rigorous tests, they do make it seem like- ly that the theory of an African origin for human mitochondrial DNA is now fair— ly secure. 200,800 Years at Less BECAUSE OUR COMPARISONS 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 mil- SCIENTIFIC AMERlCAN 59 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 diver— gence 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 there- fore 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 mito- chondrial DNA called the control re- gion, 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 multi- ple mutations that occur within the con- trol 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 ex- ample, that intervals of genetic stagna- tion might be interrupted by short bursts of’change when, say, a new mutagen en- ters the environment, or a virus infects the germ-line cells, or intense natural se- lection affects all segments of the DNA. ‘ To rule out the possibility that the clock r in PRESENT AFRICAN EUROPEAN 100,000 7‘ 5 Q I 3 ~ : , < . 700,000 0lduvai European « Neandertal might run by fits and starts, we ran a test to measure how much mitochondrial DNA has evolved in populations found- ed 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 evolu- tion 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. Anthropol- ogiSts 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 Emer- gence of Modern Humans,” by Christo— pher 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 I,“ EAST ASIAN , AUSTRALIAN a. Ngandong Zhoukoudian (“Beijingul Sambungmachan Lantian Java Hr! ;_____N____———J 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 occupied by archaic people who had mi- grated from Africa to Asia nearly a mil- lion years ago. Such famous fossils asava Man and Beijing Man are of this type. This finding and the hypothesis that the archaic Eurasian population un- derwent anatomical changes that made them resemble more modern people led to the multiregional evolution model: similar evolutionary changes in separate geographic regions converted the inhab- itants from archaic small-brained types to modern big-brained types. Huge levels of gene flow between continents, however, would be neces- sary to maintain human populations as one biological species. The multiregion— - al 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 mitochon- drial 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 unde- tected in Eurasia. We simply do not see the result predicted by the regional mod- el. Moreover, geneticists such as Masa— toshi Nei of Pennsylvania State Univer— sity, Kenneth K. Kidd of Yale Universi— ty, 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 stud- ies of nuclear genes. Muitiregienai Mgatsrg PROPONENTS OF the multiregional evolution model typically emphasize that they have documented a continuity of anatomical morphologies between the archaic and modern residents of dif— ferent regions; they insist that these mor- phologies would be unlikely to evolve independently in any invading people. For that argument to hold true, howev- er, it must also be shown that the cranial features in question are truly indepen- NEW LOOK AT HUMAN EVOLUT|0N LAUR\E GRACE 1'??? w wa-h-r-svwi—a LAUR|E GRACE LAURIE GRACE dent of one another——that is, that natur- al selection would not tend to favor cer- tain constellations of functionally relat- ed features anyway. Yet we know that powerful jaw muscles may impose changes on the mandible, the browridge and other points on the skull; circum- stances 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 charac- teristics that evolved uniquely in the fos- sil record. Instead fossils and modern populations are united by their shared retention of still older ancestral charac- teristics. The continuity seen by believ- ers 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 I mixture may sound unlikely. Neverthe— less, 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?" compelling mystery. One of us (Cann) suspects that infectious diseases could have contributed to the process by help- ing to eliminate one group. Cavalli- Sforza 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 se- quences ultimately influence. Even before then, further studies of both nuclear and mitochondrial DNA will render more informative genetic trees. Particularly enticing are the se- quences on the Y chromosome that de- termine maleness and that are therefore inherited from the father alone. Gerard www.sciam.com a}, Male 0 Female Mitochondrial DNA source PEDIGREE ofone individual illustrates the difference between the patterns ofnuclear and mitochondrial inheritance. All 32 ancestors from five generations ago contributed equally to his nuclear DNA. His mitochondrial lineage [blue line] leads backto only one person in every generation. Lucotte, while at the College of France, and his colleagues have indirectly com- pared such sequences in an effort to trace paternal lineages to a single pro- genitor—“Adam,” if you will. Those preliminary results also point to an African homeland, and with further re- finements this work‘ on paternal lineages may be able to provide an invaluable check on our results for maternal lin— eages. Unfortunately, base changes ac- ciimulate slowly on useful regions of the Y chromosome, making it technically difficult to conduct a detailed genealog- icaFanalysis. More progress can be expected soon, as molecular biologists learn to apply their techniques to materials uncovered by our friendly rivals, the paleontolo— gists. Preliminary molecular studies have already beenconducted 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 ap- pear close at hand. With them, we may i begin building the family tree from a root that was alive when the human family was young. Epfiegue SINCE THIS ARTICLE was first pub- lished, further genetic work on the mi- tochondrial DNA sequences of three Neandertal specimens upholds our con— clusions about the lack of a mixture be- tween ancient and modern Homo supi— ens. 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 EXPLORE , 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—?,198?. Mitochondrial DNA. M. Stoneking and A. C. Wilson in The Colonization ofthe 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 ofthe NationalAcademy ofSciences USA, Vol. 86, No. 23, pages 9350—9354; December 1989. Sequence Evolution ofMitochondrial DNAin Humans and Chimpanzees. T. D. Kocher and A. C. Wilson in Evolution ofLife. Edited by S. Usawa and T. Honjo. Springer-Verlag, Tokyo, 1991. SCIENTIFIC AMERICAN 61 ...
View Full Document

This note was uploaded on 01/06/2010 for the course NS 2750 taught by Professor Haas&gu during the Fall '08 term at Cornell University (Engineering School).

Page1 / 8

Wilson_Cann2002 - : I» § . TWa O'urQCOmmorI _...

This preview shows document pages 1 - 8. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online