Guatelli-Steinberg, though, says the earlier study did not take into account how variable modern populations are in their dental growth—a criticism that was also raised at the time of the 2004 study's publication.
"We examined a much broader range of modern humans, from three different regions of the world," the anthropologist added. "When we did this we found that Neandertal [front teeth] formation spans are comparable to those of modern humans."
Both studies are important in shedding new light on Neandertal development, according to Christopher Dean, professor of anatomy and developmental biology at University College London.
He adds that teeth are the key to finding out when prolonged childhood first emerged in humans.
Teeth "are all we've got, really, because in the fossil record there are no birth certificates," he said. "There's no way of getting a handle on maturation other than looking at teeth tissues, which grow incrementally."
But Dean says much more work is needed, particularly on understanding the development of molar teeth.
"It is molar-emergence times that correlate so closely with life history events such as weaning," he said.
Dying Younger
Neandertals and modern humans may have shared similarly prolonged childhoods. But the fossil record from the late Stone Age (around 40,000 years ago)—when both species where living in Europe and western Asia—suggests that Neandertal adults were dying much younger.
Short life spans, Dean says, would have put populations under serious pressure.
"Neandertals presumably went extinct because they couldn't reproduce fast enough to survive," he said.
Neandertals had much in common with early modern humans—using stone tools to hunt, harnessing fire, burying their dead. But researchers believe other cultural differences may hold the clue to why one group thrived while the other died away.
Modern humans exploited a wider range of materials, including bone and ivory, to build up a bigger arsenal of hunting weaponry. They also appear to have learned how to fish—a skill that Neandertals apparently didn't have.
Even more important may have been the ability of early modern humans to express themselves through art and language.
Early modern humans produced sophisticated forms of both abstract and figurative art, including ivory statuettes and elaborate cave paintings, according to Paul Mellars, professor of prehistory and human evolution at Cambridge University in England (see "Neandertals Beaten by Rivals' Word Skills, Study Says").
In a study published the journal Nature in November 2004, he wrote, "Expression at this level of complexity would be almost inconceivable in the absence of complex language systems and in the absence of brains structured very similarly, if not identically, to our own."
Mellars said complex language would have given modern humans a crucial, competitive edge over Neandertals. In contrast with Homo sapiens, there is no direct archaeological evidence for complex language among Neandertals.
So Neandertals and modern humans might have shared similar sized brains and lengthy childhoods. But it in adulthood the two groups likely had very different outlooks on life.
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