Ancient slow growth: fossil teeth show roots of human development.

An extended period of childhood evolved people at least 160,000 years ago, according to a new analysis of a fossil child's teeth. That's the earliest evidence to date of a modern-human life history requiting intensive parental care and a wide range of early-life learning opportunities, the researchers say.

A lower jaw holding several teeth of a nearly 8-year-old early Homo sapiens child displayed tooth development comparable to that of same-age European kids today, report anthropologist Tanya M. Smith of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and her colleagues. They employed a new X-ray technique to peer inside teeth and count layers of enamel that form at regular intervals as teeth develop. Researchers previously had to cut sections out of fossil teeth to probe enamel formation.

Earlier measurements of the decay of radioactive uranium in the fossil, found at a Moroccan site called Jebel Irhoud in 1968, yielded the estimate of when the child lived. In an upcomimg Proceedings of the National Academy of Sciences, Tanya Smith and her coworkers describe their analysis of the Jebel Irhoud child's teeth.

"The [new] study pushes back clear evidence for humanlike growth to 160,000 years ago," remarks anthropologist B. Holly Smith of the University of Michigan in Ann Arbor.

Using a similar X-ray method, another team reported in the Dec. 7, 2006 Nature that molar teeth of Neandertals dating to roughly 127,000 years ago developed much as those of modern humans do.

Tanya Smith's team examined an erupted molar, an incisor in the process of erupting, and a canine that had yet to erupt in the jaw of the Jebel Irhoud child. Tallies of enamel layers that typically form every 7 to 9 days as teeth develop, as well as counts of daily growth bands in the enamel, enabled the scientists to estimate that the child died at age 7 years, 10 months.

The extent of tooth eruption and rate of tooth formation in the fossil youngster resembled corresponding measures for European 7-year-olds today, the researchers say. The Moroccan specimen showed relatively few similarities to fossil teeth of earlier Homo species and Neandertals, the team adds.

The X-ray technique used by Tanya Smith's team can now be applied to teeth from an 800,000-year-old Homo species found in Spain, suggests anatomist Christopher Dean of University College London. Other researchers have argued that those ancient choppers bear signs of prolonged, humanlike, childhood development, although anthropologists have yet to count enamel layers in the teeth.

Mounting dental evidence suggests that a lengthening of childhood growth began by around 1.6 million years ago in Homo erectus, Dean says. However, it's not clear whether the developmental pattern of people today emerged in the first H. sapiens about 200,000 years ago or in earlier Homo species.

The new, noninvasive approach to fossil-tooth analysis will enable researchers to establish landmarks of dental development for every species of human ancestor, Holly Smith says.

Unfortunately for scientists, human teeth stop forming by early adulthood and don't reflect late-life events, she adds. Mastodon tusks, which grew throughout adulthood, have revealed much about those prehistoric creatures' entire life spans. "If only we had tusks," Holly Smith laments.