Human Evolution Put Brakes on Tooth Growth. (Science News of the Week)

Article excerpt

Scientists have looked our ancestors in the mouth and extracted a new insight about human evolution. Slowed-down tooth growth, a marker of extended childhood development in humans, emerged by only around 100,000 years ago, the investigators have found.

Even fossil species treated as direct or close ancestors of Homo sapiens, such as Homo erectus and Homo ergaster, exhibited relatively rapid tooth growth, more like that of apes than of people, report anatomist Christopher Dean of University College London and his colleagues. Overall, people take 18 to 20 years to reach full growth, while chimpanzees and gorillas take 11 to 12 years.

"I was quite surprised to find that tooth enamel grew at such different rates in early Homo species, [especially] Homo erectus, compared with Homo sapiens," Dean says. In his view, this dental disparity indicates that prolonged childhood development emerged when our ancestors evolved brains and bodies similar to those of people today. Dean's group presents its results in the Dec. 6 NATURE.

Steven R. Leigh, an anthropologist at the University of Illinois at Urbana-Champaign, calls the new study "a fine piece of work with important implications for understanding the evolution of human-life histories."

In particular, Dean's data highlight the "unusual nature" of individual development in H. sapiens and Neandertals, Leigh says. He suspects that delayed dental growth accompanied a substantially longer lifespan in these species.

The scientists focused on microscopic features of teeth to establish a time scale for dental growth throughout human evolution. Children's permanent teeth grow in layers that preserve a record of their developmental pace. One sheet of enamel gets laid down daily, the process forming a criss-cross pattern inside the tooth. Ridges on the enamel surface accumulate every 8 or 9 days.

Daily enamel layers grow more slowly and therefore are thinner in people than in apes, reflecting our longer period of physical development.

Of 13 fossil teeth examined by Dean and his coworkers, those attributed to australopithecines--members of the human evolutionary family that lived between 5 million and 1 million years ago--and to three Homo species--Homo habilis, Homo rudolfensis, and H. …