Compared to chimps and gorillas, people take a long time to grow to maturity. A new study suggests humanity's ancestors didn't adopt that slower schedule until relatively late in evolutionary history.
Researchers found evidence that the human ancestor Homo erectus was still following an apelike fast schedule some 1.5 million years ago.
Chimps and gorillas grow up in 11 or 12 years, much faster than the 18 to 20 years people take. Prior studies indicated the earliest human ancestors followed the apelike fast track, and it's not clear when the slower pace arrived in the evolutionary lineage.
Scientists can use fossil teeth to study the question. Tiny markings act somewhat like tree rings to indicate rates of enamel formation. And that correlates to how long the body takes to reach maturity; modern humans have a slower rate than apes do.
Researchers had figured the slower pace would be detectable in Homo erectus because the erectus skeleton resembles a modern human's, said Alan Walker, a Penn State professor of anthropology and biology.
But in the Dec. 6 issue of the journal Nature, Walker and colleagues report that doesn't appear to be so. They studied tooth specimens from australopiths, which were human ancestors that lived some 2 million to 5 million years ago, from early members of the Homo group including Homo erectus, and from a Neanderthal living only 120,000 years ago.
Analysis indicated that Homo erectus was still following the rapid, apelike period of development. Only the much more recent Neanderthal specimen indicated a pace more like modern-day people.
So the shift happened sometime between Homo erectus and Neanderthals, a conclusion that fits with earlier work but narrows the gap in which the change must have happened, said Tim Bromage, anthropology professor at Hunter College in New York.
The work also sheds new light on a boy's skeleton found in Kenya. It was thought the boy had been 11 or 12 years at death. The new work suggests the age was closer to 8.