|
By Greta Petry
Stewart, an associate professor of biological sciences at the University, and colleague Todd Disotell, a molecular anthropologist from New York University, have proposed a controversial new model for how apes and humans (together called hominids) evolved. In their model, Stewart and Disotell argue that the most recent ancestor of the living African apes and humans evolved in Eurasia, not Africa. That’s a significant departure from the long-held view that the evolutionary history of the lineage leading to humans was confined to the African continent. Their research, which was the cover story last summer in Current Biology, has stimulated debate anew among paleoanthropologists. “If we were studying frogs, no one would care,” said Stewart. “No other animal group would arouse this kind of vehement passion.” How did Stewart and Disotell arrive at their conclusion? They took the methodology
of molecular evolution and applied it to paleoanthropology. Molecular evolution involves interpreting the history
of the molecules that make up living matter. Paleoanthropology is the study of prehistoric life forms through plant
and animal fossils. The research by Stewart and Disotell was based on an approach called parsimony analysis—that
is, the model that involves the fewest evolutionary events to explain Stewart and Disotell synthesized data from studies of DNA from living apes and monkeys, from those primate fossils that had been analyzed by parsimony, and from biogeography of the living and fossil species. The new tree strongly suggested that Eurasian apes that migrated back into Africa were the ancestors of the great apes.
Science reporter Nicholas Wade of The New York Times called the theory “ingenious and far-reaching.” “Their conclusion is that an early ape-like species dispersed out of Africa about 20 million years ago, giving rise to the gibbons, orangutans and other ape lineages. One of these descendant ape species must then have moved back to Africa about 10 million years ago, speciating into the gorilla, the chimp, and the hominid line that led to humans,” he wrote. The more widely accepted theory among paleoanthropologists is that humans, chimpanzees and gorillas are all the descendants of a species—like Kenyapithecus, Proconsul or Morotopithecus—that lived in Africa. Under the scenario offered by Stewart and Disotell, only two hominoid migrations are required—one out of and another back into Africa. The alternative scenario, of maintaining the lines of apes in Africa, means there had to be at least six migrations of hominoids out of Africa, even at times when there was no land bridge between Africa and Asia. “The novel thing we did was to analyze the data in a way that explicitly showed the number of dispersal events—or permanent moves—required by the alternative hypotheses,” explained Stewart, a 1994 winner of the National Science Foundation Presidential Faculty Fellow Award. “We laid out explicitly what the African scenario was versus the Eurasian scenario and what this meant for hominid evolution.” Reaction to the new theory has been mixed. Some paleoanthropologists, like Colin P. Groves, are delighted. In unpublished comments to the editors of Current Biology, Groves wrote, “At times I feel I am in good company. Recently, the good company I was in was that of Tom Huxley who, on reading about natural selection in The Origin of Species is said to have slapped his thigh and exclaimed, ‘How stupid of me not to have thought of that myself!’ I felt exactly that way when reading Stewart and Disotell’s zoogeographic synthesis . . .” Commenting in a Science magazine article by Ann Gibbons, Harvard paleoanthropologist David Pilbeam took issue with the findings. He noted the fossil record is spotty in Africa from the time when the living African apes evolved, and he was doubtful the answers could ever be found from the fossil bones of extinct apes. Stewart and Disotell wrote a lengthy reply in the Letters to the Editor and encouraged other scientists to pursue more data analysis in this new area.
 Back to Cover
page
John Delano / Dean Falk / Thomas Constantine / Michael Forbes / John McHugh / Robert Bellafiore / Washington Semester Program / James Jaccard / Darcie and Joe Trapasso / News & Notes / Faculty Books / Kresge Grant |
