The dental evidence does not settle two thornier questions, however: Were the Neanderthals -- who died out 30,000 years ago -- a separate species, and why did they become extinct?

Still, the finding that the two groups reached puberty at similarly slow rates "gives valuable, partial support to people who see Neanderthals as extremely close to modern humans," said Neanderthal craniofacial expert Jeffrey Laitman, a professor of otolaryngology and director of the department of anatomy at Mount Sinai School of Medicine, in New York City.

Laitman, who was not involved in the teeth study, said he leans toward the theory that the Neanderthals were a separate species.

This latest evidence again points to "the beauty of the Neanderthals," he said. "That this is a group that was so similar to us in some ways, yet so different in others -- and how majestic that is, a different species of human ancestors existing in our not-so-distant past."

Experts believe the classic Neanderthal arose in Europe, the Middle East and parts of Western Asia about 150,000 years ago, thriving and surviving up till the last Ice Age. Their brains were as large -- or even slightly larger than -- those of modern humans. And they possessed a much stockier, more muscular build, according to Debbie Gautelli-Steinberg, lead researcher on the teeth study and an assistant professor of anthropology at Ohio State University in Columbus.

"The shape of their skull was very different -- Neanderthals did not have a vertical forehead, it really sloped back," she said. "They also had very heavy arches over their eyes and a cranium that bulged in the back, whereas ours comes up more vertically."

But then, for reasons that remain unclear, the Neanderthals became extinct.

One theory behind their demise rested on the idea that they matured at a faster rate than modern humans.

On the one hand, a quick maturity can be an advantage -- shorter childhoods mean more individuals will survive to breed. But for primates, growing up too fast can be a disadvantage, too.

"A long period of maturity does one great thing -- it provides time for the brain to grow," Gautelli-Steinberg explained. "The brain is such an energetically expensive organ and so complex. And long maturation provides time for learning, which really means a reorganization of the synapses in the brain."

It's our bigger, more complex brains "that have really helped us survive," she said. And a relatively long time to puberty would foster that.

However, in a study published in Nature in 2004, a team of German researchers compared the growth of layers of enamel on Neanderthal front teeth with a sample from modern humans.

They found that the Neanderthal enamel was laid down in a 15 percent shorter time span than that seen in the modern human teeth. Because front-tooth growth is thought to be correlated with maturation rates, it seemed as if Neanderthals did, indeed, grow up faster than humans do.

But the new Ohio State study contradicts those findings. The key difference: Instead of just looking at tooth-enamel samples from one type of modern human, "the populations we looked at were from different regions of the globe," Gautelli-Steinberg said.

In the study, published in this week's Proceedings of the National Academy of Sciences, her team compared enamel growth from a Neanderthal front teeth ranging from 150,000 to 40,000 years in age, to teeth from three types of modern humans: the Arctic Inuit people, the English and South Africans.

"We found that the Neanderthal's teeth did not grow more quickly than modern humans, if you look at modern humans from different places in the world," Gautelli-Steinberg said. In fact, Neanderthal front-tooth enamel formation falls well within the normal range of various modern human populations -- in fact, it was very similar to that of modern South Africans, she said.

"So, looking at that diversity, we found that you can't generalize and say that the front teeth of Neanderthals grow more quickly than those of modern humans," Gautelli-Steinberg said. And, if front tooth enamel deposition is correlated with maturation rates, modern humans and Neanderthals probably matured at more or less the same rate, she said.

The finding brings humans a little closer to these long-extinct relatives, but still leaves unanswered the twin questions of whether or not Neanderthals were a separate species, and why they died out.

"Right now, in terms of their becoming extinct, it doesn't look like we can pin that on their maturation rate," Gautelli-Steinberg said.

She said theories abound as to why Neanderthals lost the evolutionary race. "At the end of their reign, about 38,000 to 30,000 years ago, they overlapped in Europe with anatomically modern humans, arriving in Europe from Africa," she said. "They competed with us for resources, and there's evidence that modern humans did have superior technology."

Neanderthal bones also show signs of serious skeletal trauma, fracture and arthritis, Gautelli-Steinberg said. "They were a population that was suffering in some ways."

Laitman, who specializes in reconstructing the Neanderthal upper respiratory tract, believes differences there may have also played a role. He pointed out that Neanderthal noses, inner and outer ears, throats and voice boxes were all markedly different than those of modern humans -- enough to suggest that they could have been a different species.

"And as one who works in areas of respiratory biology, I've wondered whether they ran into respiratory problems when they met different groups of humans -- disease," Laitman said.

Whatever the reasons for their demise, Laitman believes the time has come to discard old notions of Neanderthals as simply being "bumpy, ugly versions of us."

"They were often portrayed as being some kind of brutish, ape-ish creature that has nothing to do with our own kind," he said. "But people should see the beauty of the Neanderthals. They were distinct in their own right with a wonderful history, and we have to deal with the fact that different doesn't always mean inferior. Just how different is what we are trying to figure out."

More information

To learn more about Neanderthals, head to the Smithsonian Institution.