New and provocative fossils from Kenya


August 22, 2007

Fossils from Kenya, with implications on the relationship between Homo habilis and Homo erectus, and uncovered in the year 2000, wee announced in a letter appearing in the August 9. 2007 issue of Nature. The authors, F. E. Spoor, M. G. Leakey et al. say " … We describe two new cranial fossils from the Koobi Fora Formation, east of Lake Turkana in Kenya, that have bearing on the relationship between species of early Homo. A partial maxilla assigned to H. habilis reliably demonstrates that this species survived until later than previously recognized, making an anagenetic relationship with H. erectus unlikely. The discovery of a particularly small calvaria (skull cap) of H. erectus indicates that this taxon overlapped in size with H. habilis, and may have shown marked sexual dimorphism."

The significance of this announcement is that coexistence ,in close proximity for nearly 500,000 years (between the earliest known erectus and the youngest habilis) of two species heretofore believed to have had an ancestor-descendant relationship. That is, there has been a consensus view there was straight line descent from habilis to erectus and finally to Homo sapiens. The authors are saying such a relationship, in which erectus evolved from habilis, is unlikely.

"They coexisted at the same time and in the same place for half a million years," said anthropologist Fred Spoor of University College London, a coauthor of the paper. "How likely is it that one would give rise to the other?"

Coauthor Maeve G. Leakey of Stony Brook University in New York added, "The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition."

"The situation is similar to modern Homo sapiens and Neanderthals living side by side in Europe 50,000 years ago," said anthropologist William Kimbel of the Institute of Human Origins at Arizona State University, who was not involved in the research.

"Researchers once thought that Neanderthals were a predecessor of modern humans, but it eventually became clear that they were an evolutionary dead end. Now it seems the same is true of H. habilis," Kimbel said.

The finds "are consistent with a growing consensus" that the evolutionary tree of humans is highly branched rather than a single linear trunk, he said. The diversity, he said, tells us that "there is very little in the events of the early Pleistocene that can be seen as foretelling human adaptations."

Other commentators, T. D. White and David DeGusta, teaching respectively at U.C. Berkeley and Stanford, remarked on a San Francisco radio broadcast that species overlaps, such as that pointed out by Kimbel, including multiple overlaps among species of the genus Australopithecus, were well known from the fossil record. They seemed to agree with Kimbel's analogy to a very bushy tree.

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