Science: The Case for a Living Link

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Pygmy chimp may be the common ancestor of man and ape

Since the publication more than a century ago of Darwin's The Descent of Man, scientists have become increasingly convinced that man shares a common ancestor with chimpanzees and gorillas. But who, or what, was that kindred beast? And when did the momentous split occur? At what point did primate evolution begin taking one route that led to the great apes of Africa, another to man? Paleontologists generally believe, on the basis of bits and pieces of fossils millions of years old, that the common ancestor may have been the small, long extinct apelike animal named Dryopithecus (from the Greek for "oak" and "ape"). They also speculate that the evolutionary parting of the ways that resulted in Homo sapiens occurred some 15 million years ago.

But a few scientists have resisted this vision of man's family tree, and have proposed that the African apes and man branched off on their separate paths much more recently. Their evidence is not ancient bones, but what the University of California's Vincent Sarich calls a "genetic clock." That timepiece is based on comparative studies, done since the early 1960s, of the blood proteins, immunology and DNA (the genetic molecule) of various mammals, including the primates. Out of this work scientists have been able to measure the degree of genetic kinship among different species. They have found, for example, that while the genes of horse and Homo sapiens differ by as much as 20%, those of chimps and man vary by only 1%.

By using such genetic differences almost as if they were tree rings, Sarich and John Cronin have gone on at Berkeley to produce a chronology for the appearance of various creatures. Their research has provided hitherto unavailable biochemical support for the traditional idea that the Asian apes, the gibbons and orangutans, branched off from the common primate evolutionary tree much earlier than chimps, gorillas and man. But it also offers what Sarich and Cronin consider strong evidence that the split between man and African apes occurred only 4 million to 6 million years ago.

That timetable is highly controversial. For one thing, it would knock out of the running as the earliest hominid, or manlike creature, a favorite contender of many paleontologists, the small and apelike Ramapithecus (for the Hindu epic hero Rama), whose bones were first found in India and who died out some 10 million years ago. Perhaps more important, so recent a split would seem to allow far too little time for the development of a creature as sophisticated as modern man.

Now a young anthropologist from the University of California at Santa Cruz has added new fire to the debate. Adrienne Zihlman not only supports the molecular chronology for the crucial split, but also nominates the probable common ancestor: an animal that looked, and perhaps behaved, very much like the contemporary pygmy chimp Pan paniscus.