Australasia

Lagar Velho Predmostí Mladec

Vindija Kebara La Ferrassie La Chapelle

Qafzeh Krapina

Ehringsdorf Biache Zuttiyeh

Sima de los Huesos Petralona

Arago Steinheim

Gran Dolina

Afalou Lukenya

Dar es Soltan

Klasies Omo Kibish

Ngaloba Florisbad

Kabwe Ndutu

Bodo Ternifine Olduvai 12

Dmanisi

Buia, Bouri Olduvai 9

Konso Gardula Lake Turkana (east) 992

Lake Turkana (east) 730, 3883, 3733 (west) 15000

Shandingdong Ziyang Liujiang

Maba

Dingcun Xujiayao

Dali Jinniushan

Zhoukoudian H Hexian Nanjing

Zhoukoudian D, E, L Chenjiawo Yunxian

Gongwangling Yuanmou

Kow Swamp

Keilor Willandra Lakes 50

Lake Mungo 1, 3

Sambungmachan 1, 3 Ngandong

Sangiran 2, 10, 12, 17 Trinil

Sangiran 4, 27, 31

Mojokerto

WELL-DATED FOSSILS point to the continuous, linked evolution of modern humans at sites around the world. Modern human groups in different regions developed distinct anatomical identities. Nevertheless, gene flow between the groups through interbreeding spread important changes throughout and was sufficient to maintain humans as a single species.

could not—those lost were gone forever.

Human populations with dissimilar demographic histories can therefore be expected to preserve different numbers of mutations since their last common mi-tochondrial DNA ancestor. They cannot be used together in a model that assumes the lengths of mitochondrial lineages reflect the age of their divergence. One cannot assume that all the variation in a population's mitochondrial DNA stems solely from mutations: the history of the population is also important.

No Molecular Clock

A MAJOR PROBLEM with the Eve theory, therefore, is that it depends on an accurate molecular clock. Its accuracy must be based on mutation rates at many different loci, or gene positions. Yet genes in the mitochondrial DNA cannot recombine as genes in the nucleus do. All the mitochondrial DNA genes are the equivalent of a single locus. The molecular clock based on mitochondrial DNA is consequently unreliable.

Mitochondrial DNA may not be neutral enough to serve as the basis for a molecular clock, because some data suggest that it plays a role in several diseases. Because of random loss and natural selection, some vertebrate groups have rates of mitochondrial DNA evolution that are dramatically slower than Wilson and his colleagues have claimed for humans. A number of molecular geneticists disagree with Wilson's interpretation of the mitochondrial genetic data.

The molecular clock has, we believe, major problems: its rate of ticking has probably been overestimated in some cases and underestimated in others. Rebecca L. Cann of the University of Hawaii at Manoa and Mark Stoneking of Pennsylvania State University, two of Wilson's students, have acknowledged that their clock was able to date Eve to only between 50,000 and 500,000 years ago. Because of the uncertainty, we believe that for the past half a million years or more of human evolution, for all intents and purposes, there is no molecular clock.

Putting aside the idea of a clock, one can interpret the genetic data in a much more reasonable way: Eve carried the most recent common ancestor of all existing human mitochondria, but she is not the most recent common ancestor of all living people. Mitochondrial history is not population history, just as the history of names mentioned earlier is not the same as the history of populations. Such an interpretation can fully reconcile the fossil record with the genetic data. We propose that future research might more productively focus on attempts to disprove this hypothesis than on attempts to recalibrate a clock that does not work.

The dramatic genetic similarities across the entire human race show the consequences of linkages between people that extend to when our ancestors first populated the Old World. They are the results of an ancient history of population connections and mate exchanges that has characterized the human race since its inception. Human evolution happened everywhere because every area was always part of the whole.

Neither anatomical nor genetic analyses provide a basis for the Eve theory. Instead the fossil record and the interpretation of mitochondrial DNA variation can be synthesized to form a view of human origins that does fit all the currently known data. This synthetic view combines the best sources of evidence about human evolution by making sense of the archaeological and fossil record and the information locked up in the genetic variation of living people all over the world. The richness of human diversity, which contrasts with the closeness of human genetic relationships, is a direct consequence of evolution. We are literally most alike where it matters—under the skin.

Epilogue

IN THE DECADE since this article originally appeared in Scientific American, significant discoveries and analyses have changed the nature of the debate about the pattern of human evolution. The finding of a 25,000-year-old Portuguese child from Lagar Velho who has a combination of Neandertal and "modern European" characteristics suggests that Neandertals mixed with other populations and therefore were the same species. A million-year-old Ethiopian skull found in Bouri that is similar to Asian Homo erectus remains, and is anatomically intermediate between earlier and later Africans, suggests that the evolving Homo lineage in the early and middle Pleistocene was a single species, not a mix of different species evolving in different places. Early specimens of "moderns" are also instructive. In the Australian case, significant ancestry in the Ngandong fossils from Indonesia could not be excluded. In the European case, a 50 percent contribution by Neandertals for the earliest moderns could not be excluded. These anatomical studies support the idea of multiregional evolution.

Meanwhile genetic research has become more definitive. The rate of change of mitochondrial DNA (mtDNA) was first estimated over millions of years from comparisons with chimpanzees, but with modern intergenerational studies the rates have been found to be many times as fast. The effects of accidental loss of mtDNA variation were greatly underestimated. Then came the realization that because mtDNA is a single molecule, it cannot recombine or have crossover, so selection on any part of it is selection on the whole. Natural selection has repeatedly reduced its variation; the same has been found in the nonrecombining parts of the nuclear chromosomes. If selection and not population history accounts for mtDNA variation, it does not address the Eve theory.

MtDNA has also been recovered from Neandertals and from ancient Australians, and some of it is unlike the modern form. This evidence addresses the issues of how, and how quickly, mtDNA changes, but it does not help resolve the pattern of evolution. Also less than helpful is the possibility that all the Neandertal mtDNA recovered so far may have been altered by contamination or DNA breakdown. This is suspected because the most recent Neandertal mtDNA is most like that of living humans, whereas the oldest is least alike— the opposite of what we would expect from unaltered Neandertal mtDNA evolving in a separate genetic line.

More recently, researchers have obtained sequences of nuclear DNA, and they provide a different picture. Most fundamentally, nuclear genes prove to be older than the mitochondrial gene, in some cases by millions of years. If the origin of today's mtDNA was also the origin of a new species, all the older nuclear variations should have been eliminated, and most genes should be the approximate age of the species or younger. This is the most significant disproof of the Eve theory. Nuclear genes are much older than Eve and preserve evidence of past migrations, mostly out of Africa but also from some other regions, followed by population mixtures that preserve past variation. This genetic evidence significantly supports multiregional evolution.

A greater focus on epistemology also has made it clear that the original debate over modern human origin was indeed a debate about the pattern of human evolution. The multiregional model is an in-traspecific, network model, fundamentally different from the tree-based Eve theory. This was important because an assumption that tree (branching) attributes describe population histories underlies the acceptance of gene trees as population trees. The increasing molecular and anatomical evidence against recent speciation underscores the appropriateness of such a network model. Molecular and anatomical variation reflect something different than the time since the separation of populations. They include the complexities of gene flow between groups, different histories of selection, and different population structures across space and over time. S3

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