Wilson da Silva, Sydney
CHIMPANZEES may have once walked upright, but lost the ability and returned to the trees, according to an Australian researcher.
His controversial claim is that humans diverged from chimpanzees between 3.6 and 4 million years ago, much later than the 5 million years or earlier that anthropologists generally believe.
Simon Easteal and postgraduate student Genevieve Herbert of the Australian National University in Canberra arrived at their conclusions after conducting a broad-based recalibration of the “molecular clock” in a number of mammalian species.
Many evolutionary biologists assume that natural mutations of a given gene in a group of related species happen at a fairly constant rate as they diverge and evolve. So by taking two or more living species for which there is good fossil evidence of the date of their divergence, it is possible to work out the most likely genetic sequence of the common ancestor, and the number of mutations that probably occurred between the common ancestor and the living descendants.
This gives an average rate of mutations for each gene, which can then be applied to those species that do not have a good fossil record in order to find divergence dates in the past. Previous estimates of mutation rates in human evolution give a figure of about 1.5, meaning 1.5 × 10-9 mutations per year for each DNA base, a rate that implies that humans and chimpanzees diverged just over 5 million years ago.
However, Easteal argues that the rate of genome mutations should be roughly the same in all mammals. Yet when the rate of 1.5 is applied to the divergence of marsupials from other mammals, for instance, it gives a date of 330 million years ago.
This is “obviously absurd”, says Easteal, because the fossil evidence suggests a date of no more than 125 million years. “All you can really do is find the best fit across all of the different taxa comparisons,” he says. “You can pick any rate constant you want, but if you do that you have to accept the implications for all of these species.”
To find the best single value for the mutation rate in mammals as a whole, Easteal compared data gathered over the past 12 years for various genes in several species. These included Old and New World monkeys, humans and chimpanzees, orang-utans and gibbons, rats and mice, and marsupials. They found that the only way that they could fit the fossil record with a single molecular clock was if the mutation rate was between 2 and 2.25.
To his surprise, Easteal’s average mutation rate suggested that humans and chimpanzees diverged less than 4 million years ago. “My feeling is that these molecular dates of 3.6 to 4 million are not definitive, but I think this is much more likely to be the date,” he says. He reports the results in this month’s issue of the Journal of Molecular Evolution (vol 44, supplement 1, p S121).
If Easteal is right, then Australopithecus afarensis, assumed to be an early hominid, is possibly the ancestor of both chimpanzees and humans. A. afarensis was a biped, so early chimpanzees must have walked upright. “We didn’t come down out of the trees—they went up into the trees,” says Easteal. The earliest fossils that appear to be human are about 4.4 million years old. But if Easteal is right, these “human” fossils are not human after all.
His results also suggest that Australopithecus africanus, a descendant of A. afarensis, did not die out, as most anthropologists believe, but was the ancestor of chimps. Another descendant of A. afarensis, Australopithecus robustus, which anthropologists believe also died out, could be the ancestor of gorillas. This would also explain why there are no fossil records of ancient chimpanzees and gorillas, says Easteal.
The work has attracted criticism from anthropologists, who point out that the large apes that lived 5 million years ago looked apelike and walked on four legs. Yet A. africanus and A. robustus were much more human-like. If they are ancestors of chimpanzees and gorillas, their features must have undergone a reversal of evolution.
“It may be true that, despite walking upright, A. africanus and A. robustus might have nothing to do with human evolution,” says Alan Walker, an anthropologist at Pennsylvania State University. “But that doesn’t make them ancestral to modern African apes—the evolutionary reversals are just far too great.”
Morris Goodman, a molecular anthropologist at Wayne State University, says that Easteal’s assumption of a universal molecular clock is flawed. “It just doesn’t exist,” he says. However, Thomas Loy of the University of Queensland in Brisbane is supportive. “When it comes to morphology, the molecular approach will be more accurate than the fossil record,” he says. He hopes anthropologists will re-examine the fossil record in the light of Easteal’s work.