Neandertals' teeth grew faster than ours do--another indication that they were a separate species that mixed little or not at all with Homo sapiens, scientists say.
Fernando V. Ramirez Rozzi of the French national research center in Paris and Jose Maria Bermudez de Castro of the Spanish museum of natural sciences in Madrid examined front teeth (canines and incisors) from three European groups: humans dated between 8000 and 20,000 years ago; Neanderthals dated from 130,000 to 28,000 years old; and half-million-year-old teeth that belonged to the putative common ancestor of the other two groups.
The scientists looked in particular at perikymata, little striations in the enamel that occur as a result of the way enamel builds up on the crown. Before the tooth has erupted, it grows a coat of enamel starting at the tip. Enamel gets laid down over the crown in spurts lasting about 9 days, so counting the perikymata give a rough idea of how long it took for a tooth to form.
After scrutinizing several hundred teeth--including more than 100 Neandertal teeth from 54 individuals--the team found unique patterns of enamel formation. The Neandertal teeth had fewer striations than either of the other groups, indicating the shortest period of dental growth of all three groups. Because dental growth correlates with development in general, the scientists estimate that Neandertals reached adulthood by age 15, several years earlier than modern humans. And that, says Ramirez Rozzi, "can be easily seen as demonstrating that they are two different species."
Anthropologist Milford Wolpoff of the University of Michigan, Ann Arbor, who has long maintained that humans have Neandertals in their ancestry, says this work doesn't bear on the species question because "the perikymata do not always form at the same rate, even within a single species." But others welcome the research. Christopher Zollikofer of the University of Zürich in Switzerland says this work offers the first "absolute time stamp" for comparing the pace of human and Neandertal development. Tooth expert Chris Dean of University College London warns, though, that you can't go by front teeth alone--more needs to be known about Neandertal molars.