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What a Tooth Reveals

19 September 2005 (All day)
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Long in the tooth. The horizontal lines in this scanning electron microscope image are the growth layers (perikymata) of a lower incisor from a 130,000 to 140,000 year old Neandertal specimen from Krapina, Croatia.

The typical Neandertal may have enjoyed a long childhood similar to that of modern humans. This finding, based on a comparison of tooth growth, calls into question a similar study in 2004 that claimed Neandertals grew up quickly, as fast-maturing primates do. The conflicting results may prompt a wider investigation of Neandertal fossil teeth to learn how closely related we are to these big-boned hominids.

Neandertals, whose fossils are between about 28,000 and 150,000 years old, had larger brain cavities on average than modern humans. In hominids and other primates, a bigger brain is usually associated with slow postnatal body development. It was therefore a surprise last year when a report in Nature indicated that the front teeth of Neandertals grew 15% faster than modern humans (ScienceNOW, 28 April 2004). Because tooth development is a widely used gauge of overall body growth--one of the only ones that anthropologists have--the researchers inferred that Neandertals reached adulthood by age 15, about 3 years earlier than modern humans normally do. This accelerated growth added to other evidence that these ancient hominids were a separate species from us. That Neandertals matured faster is disputed, however, by Debbie Guatelli-Steinberg from Ohio State University in Columbus and her colleagues, who, in a study published online this week by the Proceedings of the National Academy of Sciences, claim that Neandertal front teeth grew at a similar rate to modern humans.

The time required to grow a tooth is determined by counting layers of enamel, called perikymata, on its surface. Like tree rings, these layers build up incrementally, with each one corresponding to about a week. Guatelli-Steinberg and collaborators found that Neandertal front teeth grew faster than those of modern Alaskans and English people, but slower than modern South Africans--placing them within the modern human range. Without a clear distinction in dental development, "we would say there is no reason to expect that Neandertals had abbreviated childhoods," Guatelli-Steinberg says.

But the comparison of Neandertals to such a wide range of modern humans is not meaningful, claims anthropologist Fernando Ramirez Rozzi of the French national research center in Paris, lead author of the 2004 Nature study. "Modern human variation is larger than that of any other mammal," he says. The reason for this spread is not entirely understood, so Ramirez Rozzi advocates using teeth from modern humans that lived several thousand years ago. Tooth expert Chris Dean of University College London says the best way to resolve this issue is to analyze molars, which apparently are more indicative of life history. Unfortunately, back teeth have proven harder to study because they often have to be cut open to count their growth layers, so it may take some time to settle what growing up Neandertal was really like.

Related site:
Debra Guatelli-Steinberg's site

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