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MARGINALIA

Growing up Neandertal

Pat Shipman

Tale of the Teeth

In the spring of 2004, several studies offered fascinating answers to this question. Fernando Ramírez Rozzi of the Centre national de la recherche scientifique in Paris and José Maria Bermúdez de Castro of the Museo Nacional de Ciencias Naturales in Madrid compared the rates of dental growth in several species within the genus Homo, including Neandertals. They examined the perikymata—small enamel ridges on the tooth surface—of incisor and canine teeth from 55 Neandertals, 25 Homo antecessor and Homo heidelbergensis individuals (two species that some anthropologists group together) and 39 ancient but anatomically modern humans.

Perikymata are created as a tooth grows. In humans and their close kin (such as Homo erectus), one ridge is created approximately every nine days during tooth development. The ridges of more distant relatives, including chimpanzees and gorillas, are formed at shorter intervals. By counting the number of perikymata, investigators can calculate how long the tooth took to form.

Surprisingly—and this is also the first word in the title of their paper— Ramírez Rozzi and Bermúdez de Castro found that Neandertals formed their teeth in fewer days than did H. antecessor and H. heidelbergensis. If Neandertals had been the most ancient of the lot, one might expect them to be the most ape-like. But although the other fossil species are older still, they already show the human pattern. The finding is also a surprise because some scientists still feel that Neandertals are, basically, just funny-looking humans—a judgment challenged by this fundamental difference.

Dental maturity is a common proxy for overall maturity because neurological, skeletal and sexual milestones are correlated with the pace of tooth mineralization. The authors concluded that faster dental development meant that Neandertals reached adulthood 15 percent sooner than humans, on average. To state this finding in practical terms, if humans attain physical maturity at 18 years, Neandertals were similarly grown at 15 years.

Their paper also examined the spacing of perikymata across the front surfaces of incisors and canines. Dental enamel forms first at the tip of the crown—the first point to emerge from the gum—and then proceeds toward the roots. In modern humans, the perikymata are widely spaced in the half of the tooth that formed first, indicating that lots of enamel was deposited during each nine-day increment. On the second half of each human tooth, the ridges are more closely spaced, showing a slower daily rate of enamel formation.

Like human teeth, Neandertal teeth look as if they grew rapidly at first and then slowed down. However, on the part of each Neandertal tooth that grew later, the perikymata are more spread out than in their human counterparts. In other words, although the rate of enamel formation also decreased with age in Neandertals, the slowdown was less pronounced. This pattern of dental growth resembles that of apes. We know that the apes of today reach physical maturity much faster than humans. So, presumably, did Neandertals. 

Ramírez Rozzi and Bermúdez de Castro speculated that Neandertals evolved a more rapid rate of physical maturation in response to high mortality rates. Erik Trinkaus of Washington University in St. Louis documented this subject in 1995 using the remains of 206 Neandertals whose age at death could be estimated from dental or skeletal indicators.

Astonishingly, Trinkaus found that the great majority (80 percent) of these individuals died before reaching middle age (defined as the equivalent of 40 human years). The largest fraction (40 percent) died during early adulthood—equivalent to a human age between 20 and 40 years. Even the famous "Old Man of La Chapelle-aux-Saints"—whose remains indicate that he suffered from degenerative joint disease and tooth loss during life—died around age 30. If, as these data suggest, Neandertals usually died young, then natural selection would favor individuals who grew up fast and bore their babies early. Also, earlier maturity among their offspring would shorten the period of vulnerable childhood—another selective advantage.

Growing up Neandertal was apparently a sprint compared with the endurance contest of human growth, and the fossil record shows that the finish lines, and perhaps the racecourses, were different for the two species. But how exactly did these paths diverge? Just what had to be accomplished during Neandertals' childhood years?




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