Fossil Teeth Offer Something to Chew On

Humans' extended adolescence may be 'recent' evolutionary development

WEDNESDAY, Jan. 2, 2002 (HealthDayNews) -- A new study out of Great Britain gives fresh resonance to the phrase "Grow up!"

Humans have long distinguished themselves from other creatures -- modern and prehistoric -- by how long they take to reach adulthood: 18 to 20 years versus only 12 for great apes. Supposedly, those extra years of physical maturation give humans more time for learning and development.

The question for evolutionary scholars has been: When in history did this prolonged adolescence start?

A research team led by Christopher Dean of University College London examined 13 fossil tooth fragments from hominins (that's a subgroup of the family of primates that are able to walk upright) and made a surprising discovery: Homo erectus, our 1.5-million-year-old ancestor who we thought had reveled in an extended childhood much like our own, actually grew up quicker, more like an ape.

Scientists had originally thought that Homo erectus, which had body proportions similar to modern humans, would have a similarly extended adolescence.

If the new finding is correct, it would mean that the prolonged adolescence, considered a hallmark of human existence, actually arose much later than thought -- maybe half a million years ago, Dean says.

"Humans are distinguished by having this long growth period, and [the researchers'] conclusion is if you look at so-called Homo erectus and Neanderthals, the origins of human growth rates are relatively recent," says Jeffrey H. Schwartz, an anthropology professor at the University of Pittsburgh.

There's no direct evidence in the fossil records to explain when such characteristics as longer life spans, delayed reproduction and extended adolescence for humans began. There is, however, indirect evidence in the form of tooth remains. Markings on different tooth tissues can basically tell how fast the tissue grew and, by extension, how fast the hominin grew.

"One level of development is the sequence or pattern in which teeth grow or erupt into the jaws. They don't come in from front to back but helter skelter, and you can recognize patterns for species or group of species," Schwartz says. "When you look at the speed of growth as reflected in the deposition of enamel, it appears that, while at one level Homo erectus may have looked like humans in terms of sequence, the rate was different."

Based on the tooth formation information, Dean and his researchers speculate that members of Homo erectus developed their first permanent molar somewhere between 4 and 4½ years of age. Modern humans get their first molars when they're about 6 years old; apes get theirs earlier, when they're at 3 ½.

Much of this is really a classification issue and begs the age-old existential questions: Who are we and where do we come from?

"One of the issues that comes out of this is what constitutes Homo erectus? What should we place in Homo erectus?" says Schwartz.

And those questions won't be easy to answer. So much of what is "known" in the field of evolutionary history is really various people's conclusions, extrapolations and interpretations.

All we know, it seems, is that modern humans take a comparatively long time to grow up.

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For more information on anthropology research at the University of Pittsburgh, click here.

For the last 10 years, Schwartz and Ian Tattersall from the American Museum of Natural History have been conducting the first-ever study of the entire human fossil record. "We're hoping it will open up the realm of conversation to a larger audience," says Schwartz.

SOURCES: Interviews with Jeffrey H. Schwartz, Ph.D., professor, departments of anthropology and history and philosophy, University of Pittsburgh; Christopher Dean, Ph.D., Evolutionary Anatomy Unit, department of anatomy and developmental biology, University College London
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