Homo erectus, the big-brained hominid thought to have made humankind's first exodus out of Africa, is accepted as one of the many intermediate stages between a chimplike ancestor and modern H. sapiens. But a study in chimp tooth growth suggests that growth patterns of H. erectus may have been closer to chimp than human.
Dental development among primates, including humans, correlates tightly with other important traits including life span, age at reproduction, and brain growth. In this respect H. erectus has been assumed to be intermediate between chimps and humans, with the emergence of the first molar (M1) at about 4.5 years of age, according to work on fossils by Christopher Dean of University College London and colleagues.
But most such cross-species comparisons have been made using captive chimps, says anthropologist Adrienne Zihlman of the University of California, Santa Cruz. Now she and colleagues have done a comparison with wild chimps. They found that in 18 wild African chimps, M1 emerges at about 4 years of age--almost a year later than in captive chimps and in the same league as H. erectus. That leaves slow-growing H. sapiens--whose first molars appear around age 6--an exception.
"Because brain size in Homo erectus is intermediate between that of chimps and modern humans, we have assumed that the timing of dental development was intermediate also," says Zihlman, whose study appears in the 20 July Proceedings of the National Academy of Sciences. So "the data came as a surprise to us." Anthropologist Barry Bogin of the University of Michigan, Dearborn, calls the study "very persuasive." Before this, he says, the only aspect of development known about wild chimps was their reproductive activity.
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