Teenage aspirations aside, modern humans are in no rush to grow up. We reach adulthood later than any other primate, allowing children 18 years to grow their brains and develop complex behaviors, such as creating art, speaking languages, and playing video games. It turns out that our ancestors were slow growers, too: A new study finds that an 8-year-old Homo sapiens living 160,000 years ago was just as immature as today's 8-year-old. The findings indicate that a prolonged childhood may always have been a distinguishing trait of our species.
Our early human ancestors grew up fast. Australopithecines, including the famous fossil "Lucy" who walked upright 3.1 million years ago, reached adulthood in about 12 years, similar to chimpanzees. Even the early members of our genus Homo, whose brains began to expand and who were long thought to have developed like us, were fully grown at 14 to 16 years. Studies of the teeth of juvenile hominids showed that extended youth emerged relatively late in human evolution.
One international team of scientists had a unique chance to see just how late by examining the fossilized teeth and lower jaw of an 8-year-old child, which was discovered in 1968 in Morocco. The researchers used several methods to reconstruct the life history of the child. Most notably, they employed a powerful new technique called synchrotron microtomography, in which x-ray images reveal the incremental growth lines laid down daily in the child's teeth and enamel formation--information that only could be obtained before by drilling samples out of the tooth or making less reliable estimates from ridges on the outside of the teeth. By counting these lines, which are laid down like annual rings in a tree, and detecting molars that had not erupted, the team determined that the fossil tooth had grown as slowly as that of a modern child. "I'm seeing a kid that looks just like a European child of age 8," says paleoanthropologist Tanya Smith of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, leader of the study, which appears online this week in Proceedings of the National Academy of Sciences.
The new findings provide "some badly needed data that our distinctive pattern of growth and development is likely to have been part of the package that marked out anatomically modern humans," says paleoanthropologist Bernard Wood of George Washington University in Washington D.C. This suggests that a longer childhood has always been a hallmark of H. sapiens--and might even have given our species sufficient advantage to replace Neandertals who lived in Europe circa 50,000 years ago, he says.