They found that the rate of tooth growth present in the Neanderthal fossils they examined was comparable to that of three different populations of modern humans.
And since the rate of tooth growth has become a more-accepted tool for estimating the length of childhood among hominids, the finding is the latest evidence suggesting that Neanderthals may not have been as different from modern humans as some researchers have thought.
The study by Debbie Guatelli-Steinberg, assistant professor of anthropology at Ohio State , appeared in the current issue of the Proceedings of the National Academy of Sciences. Donald J. Reid, lecturer in oral biology at the University of Newcastle , Thomas A. Bishop, associate professor of statistics, and Clark Larsen, professor and chair of anthropology, both at Ohio State , were co-authors in the study.
"Based on our study of the enamel of these Neanderthal teeth and other modern ones, we can't support the claim that Neanderthals grew up more quickly than do modern humans," she said.
Key to this conclusion are microscopic lines on the outside of teeth that mark the incremental growth of enamel on a young tooth. Like tree rings that can gauge the age of a redwood, these striations – called perikymata – record new growth on the surface of the tooth.
Researchers know from earlier work that these markings are present in all forming teeth, signifying six to 12 days of growth. By multiplying that interval by the number of perikymata on a tooth's surface, researchers can gauge how long it took for the tooth to mature. And that gives them an indication of the length of an individual's childhood.
Neanderthals, Homo neanderthalensis, were the dominant hominid inhabiting most of what is now Europe and western Asia . Remains have been found as far south as Iraq and as far north as Great Britain . Fossil skulls reveal the distinctively prominent brows and missing chins that set them apart from later humans.
They thrived from about 150,000 to 30,000 years ago until their lineage failed for as-yet unknown reasons. Most researchers have argued that their life in extremely harsh, Ice Age-like environments, coupled with their limited technological skills, ultimately led to their demise.
In a study published last year in the journal Nature, other researchers contended that Neanderthal teeth took 15 percent less time to reach maturity than those in later Homo sapiens, suggesting to them that a Neanderthal childhood would be shorter than our own.
But Guatelli-Steinberg's team wanted a broader comparison and therefore compared the teeth from Neanderthals to those of three modern populations – people currently living in Newcastle-upon-Tyne , U.K. ; indigenous people from southern Africa, and Inuit from Alaska dating from 500 B.C. until the present.
"We chose these three groups since they would provide a good cross-section of various populations from different regions of the world," she said. "We feel that they give us some insights into the variation that exists within modern humans."
For the study, the researchers used precise dental impressions Guatelli-Steinberg and Larsen made of 55 teeth believed to come from 30 Neanderthal individuals. These were compared to 65 teeth from 17 Inuit, 134 teeth from 114 southern Africans and 115 teeth from as many Newcastle residents. In all cases, the researchers tallied the number of perikymata on the enamel surface of the teeth.
Guatelli-Steinberg said that the results showed that the enamel formation times for the Neanderthals fell easily within the range of time shown by teeth from the three modern populations – a conclusion that did not support a shorter childhood for the Neanderthals.
Enticing though it may be, these new findings haven't convinced the researchers that a Neanderthal childhood was equal to a modern human's.
"The missing key bit of data to show that would be evidence for when the first molar tooth erupted in the Neanderthals, and we simple have no evidence of when that occurred," she said.
The length of time is important, the researchers say, because unlike all other primates, humans have an extended period of childhood growth, during which brain matures both in size and through experiences. Some earlier hominids matured far more quickly than modern humans.
"The question is when exactly did that pattern of development evolve in the growth of humans," she said.
Support for this research came from a grant from the Leakey Foundation and from the College of Social and Behavioral Sciences at Ohio State .
Contact: Debbie Guatelli-Steinberg, (614) 292-9768; guatelli-steinbe.1@osu.edu
Written by Earle Holland, (614) 292-8384; holland.8@osu.edu.
Journal
Proceedings of the National Academy of Sciences