The Case for a Living Link

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.

Harvard's Harold Coolidge was in fact suggesting as far back as 1933 that the pygmy chimp's forebears may well have been the ancestor of both African apes and man. But evidence to support his claim was negligible. Only slightly smaller than the familiar common chimps, the pygmy chimps are restricted to a small area of Zaire, in equatorial Africa. They are quite rare and shy, and only a few are in captivity. Also, very few skeletons have been available for study. Hence, no one knew enough about them to make an effective case for the pygmy chimp as a human antecedent.

Zihlman may have changed all that. After scrutinizing every skeleton she could get her hands on and observing live pygmy chimps at the Yerkes Regional Primate Research Center in Atlanta, she concludes that the little creature fills the evolutionary niche. She noted, for instance, that pygmy chimps, like other African apes, move as easily on the ground as in trees. In captivity, they seem to walk upright even more often than common chimps--a characteristic that puts them two legs up, so to speak, as a direct kin of man. In addition, the pygmies are what anatomists would call the most generalized of all apes; they have not developed the long arms for hanging and swinging that the other apes have. Also, in such characteristics as size of canine teeth, brain capacity and robustness, there are fewer differences between the sexes than there are between males and females of common chimps or gorillas. In these respects at least, the pygmy chimps are more like humans than like apes.

If pygmy chimps are indeed the common ancestors of African apes and man, why did some of them survive essentially unchanged over millions of years while others evolved into common chimps, gorillas and man? Zihlman speculates that pygmy chimps were once far more widespread in Africa. As conditions changed, she says, some of the roving chimp bands may have found themselves in totally new environments and eventually turned into entirely new species. But stragglers who stayed behind in the central African rain forests were perfectly suited to their habitat, and so they remain today much as they always have been. If Zihlman is right, these few pygmy chimps are the living links with man's evolutionary past.

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