"The really wonderful property of teeth is the fact that how they grow mirrors how organisms grow. You're able to reconstruct whole aspects of life history," Schwartz said.
Paleontologists define adulthood by when the last permanent tooththe wisdom toothemerges.
"When you look at the modern human growth profile, humans are uniquely long-lived for apes," Schwartz said. "It takes us a long time to reach adulthood, and then we live for a long time."
Scientists consider modern humans' prolonged pattern of growth and maturation a major step forward in human evolution, because it allows extra time for learning.
In Neandertals the entire process was fast-forwarded. Their wisdom teeth emerged at around 15 years of age, according to Ramirez Rozzi, a researcher with Dynamique de l'Evolution Humaine (Dynamics of Human Evolution) in Paris, and Bermudez de Castro, of the Museo Nacional de Ciencias Naturales (National Museum of Natural Sciences) in Madrid.
Jay Kelley, a paleoanthropologist at the College of Dentistry at the University of Illinois in Chicago, wrote an accompanying commentary to the Nature article. He thinks the conclusion that Neandertals reached adulthood by 15 might be a bit of a stretch.
"I think the study shows a clear pattern difference that probably does have some implications for how Neandertals grew up," Kelley said. "I'm much more cautious about the specific conclusions about just how fast Neandertals grew up. Relationships between traits like brain size and teeth growth rate are fairly broad and have a lot of variation to them."
The authors say the results were somewhat surprising, because the growth and development of teeth and brains are very tightly correlated, and on average, Neandertals had larger brains than modern humans.
Earlier studies suggested that larger brain size drove the prolonged growth pattern. The authors suggest that the accelerated maturation rate resulted from high adult mortality rates. They also suggest Neandertals must have had a high-calorie diet and a fast metabolism to fuel such rapid growth.
"That conclusion is probably not far off in terms of life-history theory," Schwartz said. "Earlier maturation means earlier reproduction, and if you're a population at risk, then it behooves you to reproduce earlier."
The issue of whether Neandertals were direct ancestors of modern humans has basically reached a consensus, Trinkaus said.
Neandertals evolved in Europe and were a highly successful species, surviving in harsh environments for a quarter of a million years. Modern humans evolved in Africa around 130,000 years ago. They spread out of Africa to populate the rest of the world around 50,000 years ago.
"Beyond that, I don't think we're going to get much further," Trinkaus said. "The degree to which the two different groups interacted in the Pleistocene [epoch], whether and how much they interbred, and what the social and behavioral interactions were when they met remain unknown."
SOURCES AND RELATED WEB SITES