Lasers vaporize ancient diet mysteries

Groundbreaking dental work on 1.8 million-year-old teeth has brought new nuance to our understanding of the dining habits of ancient human relatives. The new technique shows variation in a diet assumed to have been static and promises to shed light on the diets and behavior of a variety of human forebears.

In a study that took nearly five years, University of Colorado anthropologist Matt Sponheimer and colleagues vaporized tiny grooves along the growth-lines of teeth. The puffs of smoke carried clues as to the type of carbon deposited as the tooth grew.

The technique, called laser ablation, was done on four tooth fragments from a rugged beast called Paranthropus robustus, which split from the human branch about 2.5 million years ago. Paranthropus coexisted with similar creatures known now as the genus Homo - our direct ancestors.

Paranthropus's large and flat cheek teeth, thick enamel and buttressed facial features led scientists to assume the creatures fed off grass and sedges available in the African savanna. Such a limited diet brought about Paranthropus's extinction as Africa dried out, some scientists have assumed.

Sponheimer's work, published in today's journal Science, challenges that belief.

The laser work was performed on Paranthropus tooth fragments from Swartkrans Cave in South Africa. Until late adolescence, teeth grow in layers - the equivalent of tree rings - less than one-tenth of a millimeter thick. The layers grow in nine days in humans and an estimated seven days in Paranthropus.

Tooth smoke from different layers showed varying concentrations of carbon 13. Sponheimer says it means Paranthropus was eating different foods at different times of the year, and thus had a varied diet.

"The idea that Paranthropus went extinct because it was so specialized is at the very least overly simplistic," Sponheimer said.

Unlike bones, highly crystalline tooth enamel is almost "pre-fossilized," Sponheimer said, and less prone to chemical change over time. The laser ablation technique works because plants from different environments convert the carbon dioxide and the sun's rays into carbon-rich plant matter in different ways. Savanna-based grasses and sedges, so-called C4 plants, contain less carbon 13 than bananas or other forest plants, which go by C3.

The growth-rings of Paranthropus's teeth showed dietary swings between one type of vegetation and the other. Sponheimer speculates that the animals moved seasonally between forested areas and the savanna.

He said the technique could eventually help explain why ancient Homo survived while Paranthropus's genes fossilized. Studying the teeth of infant Homo could help determine the age babies were weaned, perhaps demonstrating speedier breeding than Paranthropus. He said laser ablation could also help nail down just how mobile our ancient ancestors were, providing proof otherwise lacking that bipeds moved more efficiently than those relying on feet and knuckles.

Frederick Grine, professor of anthropology at Stony Brook University in New York, called Sponheimer's effort "some of the most exciting scientific work in this field that I have seen in the last decade."

He said the results indicate the "robustness" of Paranthropus may have evolved because it was needed in hard times.

"It probably would have preferred peaches, but when these weren't available, it could have fallen back to small, hard objects," Grine said.

In a Science essay accompanying Sponheimer's findings, University of Illinois anthropologist Stanley Ambrose called on museum curators to avail their fossilized teeth to Sponheimer's lasers . He called the approach a "powerful and versatile technique" for reconstructing short-term variation in diet , climate and the structure in animal communities.