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Ancient Slow Growth: Fossil teeth show roots of human development
Bruce Bower

An extended period of childhood evolved in 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 requiring 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 upcoming 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.