Luca Bondioli and Arnaud Mazurier

Wisdom tooth.
This high resolution CT scan of a Neandertal molar shows where researchers cut out a thin section so they could see the striations (dark lines on the right) that can be read like tree rings to show how fast the tooth--and the Neandertal's body--grew each day.

Mining Ancient Molars

Ann is a contributing correspondent for Science

For the first time, researchers have been allowed to slice through Neandertal teeth. This coup is providing the best evidence yet that these creatures grew and developed at the same slow rate as modern children. Neandertals even had enough time to complete most of the development of their large brains in childhood, like modern humans.

Some paleoanthropologists have proposed that Neandertals grew up faster than modern humans so they could become fertile sooner. This would have helped ensure they would produce enough offspring to survive in the frigid climate of Europe 200,000 to 28,000 years ago, but it would have provided less time for brain development as children. In contrast, modern humans inhabited the warmer climes of Africa and western Asia until they swept into Europe 40,000 years ago. They may have lived longer as adults, enabling them to delay becoming parents so they could prolong their childhood and develop larger brains. A 2004 study supported this hypothesis by showing that Neandertals' front teeth grew faster than those of modern humans, indicating the species reached adulthood more rapidly (ScienceNOW, 28 April 2004).

A challenge came in 2005, after another team of researchers used the same method to count layers of enamel on the outside surface of Neandertal front teeth. But they also applied the method to modern humans from around the globe and found that the Neandertals' tooth development fit within the range for the diverse modern humans. Hence, both Neandertals and modern humans appeared to have similar childhood development patterns (ScienceNOW, 19 September 2005). Still, the study was limited because front teeth cannot provide detailed information about daily growth rates.

Molars, whose development is more complex, harbor this information. Like trees and shells, teeth grow in an incremental manner, preserving a record of their growth in the form of striated lines. In molars, these lines are laid down daily, and dramatic dark lines even reveal the stress of being born. Seeing these lines, however, involves slicing into the tooth--something curators of Neandertal specimens were loath to have researchers do.

Fortunately, a team of British and French researchers was able to get permission to do this thanks to a multidisciplinary effort involving curators at French museums where the molars are kept. When the researchers sliced thin sections of the molars, they noticed important similarities between Neandertals and modern humans. The dark birth line emerged at about the same time in dental development as in modern humans, indicating that Neandertal teeth developed at the same rate as modern human teeth do around the time of birth, the team reports online today in Nature. The researchers also found that the crowns and roots of the Neandertals grew at the same rate of those of modern humans, with root growth complete by age 9 as in modern children. "This all points to a dental developmental schedule that was most like that in modern humans," says anatomist and lead author Christopher Dean of University College London, who also is a dentist.

Paleoanthropologist Gary Schwartz of Arizona State University's Institute of Human Origins in Tempe says the findings will allow researchers to chart Neandertal development, "down to the level of a day." With this information, he says, scientists should be able to get a firmer handle on how Neandertals and modern humans came to differ from their ancestors late in human evolution.

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Posted in Paleontology, Archaeology