Tooth of Paranthropus, Image: Science

You are what you eat—or, if you’re a 2 million-year-old hominid fossil, what you ate.

By analyzing the tooth enamel of Paranthropus robustus, anthropologists have discovered that these big-jawed bipeds—who shared the South African savannas with Homo erectus about two million years ago—snacked on a much wider variety of foods than researchers previously suspected. The new study, which is published in the Nov. 10 issue of the journal Science, challenges the long-held belief that Paranthropus went extinct because of its picky eating habits.

Both Homo and Paranthropus descended from Australopithecus, the genus that includes the famous 3 million-year-old fossil “Lucy.”

“Lucy, or something like Lucy, probably gave birth to the Homo genus on one hand, and the Paranthropus on the other,” said Matt Sponheimer, an anthropologist at the University of Colorado at Boulder and lead author of the new study. “So the question is why, in this story of two offspring, did Homo ultimately thrive while Paranthropus went extinct?”

Anthropologists had assumed that Paranthropus—with its enormous molars and premolars, thick tooth enamel, and robust, muscular jaw—was a “chewing machine” specialized for eating tough, high-fiber plants, and that this specialized diet caused the demise of the species. When the weather became dryer and more seasonal, they figured, a varied diet would have benefited a species, while organisms with a highly specialized diet would have struggled.

The study analyzed four Paranthropus teeth found at the famous Swartkrans cave site, just northwest of Johannesburg, South Africa. Discovered in 1948, Swartkrans has proved a goldmine for anthropologists, who have found remains of both Paranthropus and Homo species there, as well as evidence of stone tools and controlled fires.

In 2001, Sponheimer and an international team began to study the enamel of individual Paranthropus teeth from the site using a non-invasive technique called “laser ablation.” The two million-year-old enamel still contained layers of carbon isotopes from the food that had passed through the teeth. During Paranthropus tooth development, a new layer of enamel grew about once a week. In the samples used for this study, Sponheimer says they saw as many as 90 layers in one tooth.

A reconstruction of a generalist Paranthropus. Artwork by Walter Voigt, provided by Lee Berger and Brett Hilton-Barber

Almost all trees, bushes, shrubs and forbs—which produce high-nutrient, easy-to-chew fruit—use a “C3” chemical pathway that produces a low Carbon-13-to-Carbon-12 ratio. In contrast, grasses and sedges—which need to be chewed with strong teeth—use a “C4” pathway and produce a high Carbon-13-to-Carbon-12 ratio. So by analyzing the ratio of carbon isotopes found in the different layers of the Paranthropus teeth, the team could piece together not only a general dietary palette for the species, but also a picture of how an individual’s diet changed within his own lifetime.

Sponheimer was “definitely surprised” by the results: The four Pararthropus teeth showed at least a 40 percent variation in proportions of C4 and C3 pathways, meaning that the “chewing machines” often ate high-quality foods that actually didn’t need all that much chewing. Sponheimer said one individual, in fact, went from eating a primarily C4-based diet to a purely C3-based diet.

“It was shockingly dramatic,” he said. “The differences were so big that at the beginning, we didn’t necessarily believe the data. We spent a long time making sure everything was right.”

From previous work, Sponheimer’s team knew that the Paranthropus species as a whole had a varied diet. “But we’d only get one number per tooth, and each tooth could be from thousands of years apart, so it was hard to tell what that meant,” Sponheimer said.

So now that we know Paranthropus ate a variety of foods, why, then, did Homo‘s cousins go extinct?

Nobody knows for sure. Some researchers propose Paranthropus had slower reproduction rates or didn’t take advantage of tools like Homo did. Sponheimer said that the physical differences between Paranthropus and Homo may have been “two different [evolutionary] solutions to the same problem. In order to best utilize available food, it seems that Homo used technological innovations, while Paranthropus used anatomical innovations.”

Indeed, similar situations occur today between competing gorillas and chimpanzees or warthogs and bush pigs, according to Stanley Ambrose, a paleoanthropologist at the University of Illinois at Urbana-Champaign. When two closely related omnivorous species live in the same area, he said, one will feed primarily on the higher quality foods, such as fruits, while the other will evolve to specialize on the lower quality foods, like fibrous plants.

During times of food abundance, both species will feed on the same kinds of food. But in times of scarcity, their ecological niches separate.

If Paranthropus faced a seasonal shortage in its niche food and the species had a technological disadvantage compared to Homo, it would have been unable to compete for the higher-quality food.

“It only takes one season to starve to death,” Ambrose said. “It’s a competitive place out there; it’s no Garden of Eden.”

Originally published November 16, 2006

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