Human evolution put brakes on tooth growth

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.

erectus–displayed signs of speedier, apelike enamel growth.

In a separate study, Spanish researchers identified a modern sequence of tooth development in an 800,000-year-old Homo species (SN: 6/2/01, p. 346: Evolution’s Youth Movement).

However, the new results suggest that this dental sequence occurred more rapidly than in modern people, Dean holds.

Scientists had expected that H. erectus, the first fossil ancestor with body proportions and tooth sizes like those of modern people, grew relatively slowly. Yet using measurements of the fossil teeth, Dean’s group determined the time needed for tooth formation. The results indicate early and rapid growth for the teeth of H. erectus and earlier Homo species. For example, the first permanent molar, which erupts at around

6 years in modern humans and at about 3 years in apes, erupted between 4 and 4 years in H. erectus, Dean says.

The new study included teeth from a juvenile H. erectus skeleton found in Kenya and dating to 1.5 million years ago. Many researchers regard this boy, who was 5 feet 3 inches tall, as having been close to 12 years old. Evidence of more-rapid dental growth in H. erectus now places this boy at closer to 8 years old when he died, according to Dean. The earliest evidence for slower tooth growth appeared in a 100,000-year-old Neandertal specimen from an Israeli site, he adds.

A slowing of dental growth may have contributed to a tendency for the skulls of H. sapiens adults to look like larger versions of juvenile H. erectus skulls, comments anthropologist Susan C. Anton of Rutgers University in New Brunswick, N.J. Even with refined theories about tooth growth throughout human evolution, she says the developmental picture for H. erectus remains fuzzy.

Tooth growth may have proceeded at varying paces in far-flung H. erectus groups, Anton suggests. For instance, African and Asian H. erectus specimens display shape differences in their skulls and teeth that may reflect disparities in the timing of enamel growth, Anton says.

Nonetheless, it’s now clear that the thick tooth enamel in H. sapiens develops more slowly than comparably thick tooth enamel in our ancient ancestors grew, remarks Italian anthropologist Jacopo Moggi-Cecchi of the University of Florence.