Parents who think their kids are growing up too fast should be glad they're not Neandertals. A new study of the fossilized teeth of eight Neandertal children finds that their permanent teeth grew significantly faster and erupted earlier than those of our own species, Homo sapiens. Taken with recent studies showing subtle differences in the brain maturation and developmental genes in Neandertals and H. sapiens, the new data suggest Neandertal kids may have reached adulthood a few years faster than modern human children do.
Researchers have long known that humans grow up slowly. We take almost twice as long as chimpanzees to reach adulthood. Our distant ancestors were more like chimps; Lucy and other australopithecines, for example, matured quickly and died young. When—and why—did we evolve the ability to prolong childhood?
One important clue came in 2007, when paleoanthropologists Tanya Smith of Harvard University and Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, analyzed growth lines in a slice of molar from an 8-year-old Neandertal from Belgium. They concluded that its teeth developed more rapidly than those of our species, including teeth from a 160,000-year-old H. sapiens child from Jebel Irhoud, Morocco. But the findings were based on only one individual Neandertal, so it's possible that child was an early bloomer. And one other study of a Neandertal child's tooth showed that it grew at the same rate as a modern human's. Researchers needed additional samples to settle the debate, but curators of Neandertal fossils were hesitant to let them slice more precious fossil teeth.
Fortunately, Smith and Hublin found a powerful new tool that would let them "see" inside fossil teeth without damaging them—the 52 kilo-electron-volt synchrotron x-ray beam at the European Synchrotron Radiation Facility (ESRF). Working with paleontologist Paul Tafforeau of ESRF, the multinational team used the beam to x-ray teeth from dozens of Neandertals, including 11 juveniles, and several fossil H. sapiens. By counting daily growth lines laid down in the enamel and comparing the number of days that passed between birth and key developmental events, such as the eruption of the first, second, and third molars, the researchers could calculate the age at death as well as the rate of dental growth.
They found that it took the Neandertals 2.5 years to form their first molar crowns, compared with 3 years on average in modern humans. Second molars appeared by age 8 in Neandertals, and 10 to 12 years on average in modern humans. This suggests that Neandertals reached adulthood a few years earlier than modern humans, they report online today in the Proceedings of the National Academy of Sciences.
Paleoanthropologist Gary Schwartz of the Institute of Human Origins at Arizona State University (ASU), Tempe, agrees that "mounting evidence suggests that perhaps Neandertals were biologically quite distinct from Homo sapiens." But, he adds, although it now seems that Neandertal teeth grew faster, the "jury is still out" on whether they actually reached adulthood faster than H. sapiens.
Part of the problem is that there are no definitive studies showing whether children whose teeth erupt early also reach puberty and adulthood faster than those whose teeth erupt later, says neurobiologist Christoph Zollikofer of the University of Zurich in Switzerland. And more data are needed to show the precise magnitude of the difference in growth rate between Neandertals and us, says ASU paleoanthropologist Jay Kelley.
Sorting out the link between dental development and overall growth rates is important, Smith agrees, because a longer childhood may have given humans a reproductive advantage over Neandertals. Indeed, studies have shown that women who wait until at least age 19 produce more babies that survive to adulthood, perhaps because they have more time to learn how to be better parents. "This may have been one of the few defining things that set us apart from other species and gave us a competitive advantage," she says.