Tooth Finding Shakes Human Family Tree

Diet didn't doom one of Homo's early competitors, study suggests

THURSDAY, Nov. 9, 2006 (HealthDay News) -- Vaporized gas from million-year old tooth enamel is stirring debate on how modern humans' ancient kin won the evolutionary race.

The research focuses on Paranthropus robustis, a genus of small, upright-walking hominids that once coexisted with the early Homo genus in Africa but died out about 1 million years ago.

Based on their relatively huge jaws and teeth, anthropologists have long assumed that Paranthropus dwindled away because they only ate grasses, which began to disappear as the continent's climate got drier. Homo, on the other hand, had a more varied diet, so it adapted and flourished.

But high-tech research is taking a bite out of that theory.

Re-examination of Paranthropus tooth enamel suggests this creature ate not only grasses, but fruits, nuts and even meat or animal products -- much like its neighbor, Homo. It also hints at a level of intelligence and adaptability in Paranthropus that scientists had never guessed at before.

Instead of passively hoping to find one type of food, "Paranthropus is now looking at the same landscape, but they are looking at it differently -- looking at what kinds of things are available for eating," explained lead researcher Matt Sponheimer, a professor of anthropology at the University of Colorado, in Boulder.

His team published its findings in the Nov. 10 issue of Science.

Both modern Homo sapiens and Paranthropus arose from the ancient hominid line known as Australopithecus, made famous by "Lucy," a 3 million-year-old female fossil unearthed in Ethiopia 30 years ago.

"Her line gave birth to two other lines, one of which is Homo and the other of which is Paranthropus," Sponheimer explained.

Beginning about 1.8 million years ago, the two lines coexisted in Africa for more than a million years. In fact, remains of both Homo and Paranthropus have been found in the same cave in Swartkrans, South Africa.

According to Sponheimer, Paranthropus was small -- shorter than today's chimpanzees, in fact. These creatures walked upright, had large jaws and teeth, plus small brains relative to their Homo cousins.

For decades, anthropologists "have treated Paranthropus as if they were our unloved relatives," said Jeffrey Laitman, a specialist in hominid craniofacial anatomy at Mount Sinai School of Medicine in New York City. "They're often portrayed as a group of big-molared, not very bright vegivores, munching away in a corner of the forest, while our Homo ancestors rapidly and proudly climb the evolutionary ladder."

Paranthropus did have big, grinding teeth reminiscent of herbivores such as sheep or horses. So anthropologists assumed they were trapped into a diet based solely on low-lying grasses. According to the prevailing theory, when those grasses got scarcer during drought, so too did Paranthropus -- until they gradually became extinct.

That theory has been hard to challenge because scientists traditionally have had to drill away at fossilized tooth enamel to test it for chemical clues to diet. Most museums have barred this type of work on rare fossils.

However, in recent years, a new noninvasive technique has emerged called laser ablation stable isotope analysis. The technique involves "a very short burst of energy from a laser, which vaporizes a small portion of the tooth and turns it into carbon dioxide that we can analyze," said study co-researcher Thure Cerling, a professor of biology, geology and geophysics at the University of Utah.

Chewed food -- either animal or vegetable -- leaves distinct chemical isotope "signatures" in tooth enamel that are released in the carbon dioxide vapor, Cerling noted. Analyzing these signatures, "we are able to reconstruct the diets of animals back millions of years," he said.

Because this method is much less destructive to artifacts, museums generally allow this type of research.

In its study, the Sponheimer team performed this type of laser analysis on four 1.8-million-year-old Paranthropus teeth found in the Swartkrans cave.

Sponheimer said he was "greatly surprised" by the results, with "tremendous changes" in diet for just this one individual. Based on the isotope tests, this ancient ancestor appeared to move from a fruits-and-nuts-type regimen in one season to a wholly grass-based diet in another.

The researchers even uncovered hints that Paranthropus dined on grass-eating animals, although Sponheimer stressed that the animal could be anything from large mammals to grass-munching termites (which chimpanzees eat today).

"This study certainly gives us plenty to chew on," said Laitman, who has done his own work at the Swartkrans site and is director of the department of anatomy at Mount Sinai.

"If Paranthropus were dietary generalists with the ability to adapt season-to-season, then our simple picture of them no longer holds," Laitman said. The notion that they were scavengers or even hunters also suggests they may have been "more complex-brained creatures," he said, since these activities require more brainpower than simple grazing.

So, why did Paranthropus die out, if the blame can no longer be place on a restricted diet?

According to Sponheimer, theories abound. Homo's use of tools might have helped -- "perhaps Homo was a better technological innovator," he said.

Homo may also have "outbred" Paranthropus because it had a shorter weaning time for offspring than its competitor. Anthropologists estimate that, like modern humans, early Homo females weaned their young in less than one year, leaving them ready to reproduce again. In contrast, it's thought that Paranthropus reproduced only once every five or so years, as happens with today's great apes.

But weaning time leaves its own traces in teeth, so Sponheimer said he hopes his group can test that theory, too.

"We thought we knew the answer to all these questions," he said. "But there are a lot of possibilities."

More information

Find out more about human evolution at Minnesota State University.

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