Thursday, October 01, 2009

Closing in on the common ancestor of humans and chimps

Here's the latest on the 4.4 million year old hominid, Ardipithecus ramidus, which is believed to be the closest we've come to finding a common ancestor of humans and chimpanzees:

Ar. ramidus, although likely millions of years more recent than the so-called missing link between chimpanzees and humans, represents "coming as close as we've ever come to that last common ancestor," Tim White of the University of California, Berkeley, one of the studies' lead authors, said in a recorded interview for Science.

Ardi is, in fact, "so rife with anatomical surprises, that no one could have imagined it without direct fossil evidence," wrote C. Owen Lovejoy, a professor of anthropology at Kent State University in Ohio, and his colleagues in a summary of one of the papers.

Among the surprises: Ardi's jaw and limbs show she was a forest-dwelling omnivore, not a fruit-eater like today's chimps or an open savanna–dweller like other early hominids. Ardi had a brain about the size of a modern chimp's relative to body size (about a third the size of a modern human's). And Ar. ramidus's foot is strikingly unlike that of a modern chimpanzee, the authors of another paper (led by Lovejoy) explain.

For a primitive cousin who likely stood at only about 120 centimeters and weighed about 50 kilograms, Ardi is likely to make a big impact in the field of paleoanthropology. For instance, Ardi's physical form also has implications for many other ancient animals, including the controversial six-million- to seven-million-year-old Sahelanthropus tchadensis, discovered in Chad in 2001. The similarities in skull size and shape among these two species now has prompted the researchers of one of the new papers (led by Gen Suwa, a professor at the University of Tokyo) to conclude that S. tchadensis was, indeed, an early hominid, rather than a female ape as others have suggested.

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