Info

Zhoukoudian ("Beijing")

Ngandong

i

Sambungmachan

Lantian

Java y

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.

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 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 undetected in Eurasia. We simply do not see the result predicted by the regional model. Moreover, geneticists such as Masa-toshi 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.

Multiregional Mystery

PROPONENTS OF 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

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 [blueline) leads back to only one person in every generation.

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 [blueline) leads back to only one person in every generation.

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. 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 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

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 pro-genitor—"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-

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