During their study, the researchers first examined the teeth of modern-day capuchins, howling monkeys, and other monkey species to discern the marks left by fruits, seeds, and other plants.
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Knowledge of the monkeys' diets helped researchers link certain foods to different tooth-wear patterns.
"We take living species with known diets and see how they differ in their microwear patterning," Ungar explained. "Once we have a relationship between pattern of wear and diet, we can infer diet from pattern of wear in the fossils."
The researchers then turned their attention to the fossil teeth of two extinct human species.
One of these extinct humans, A. africanus, lived between about 2.3 and 3 million years ago in the Sterkfontein Valley of South Africa. At the time, vegetation in the region was probably more closed-in than it is today, and A. africanus probably spent some time in the trees.
Paranthropus robustus lived laterbetween 1.8 and 1.5 million years agoin the same area but at a time when the landscape was more open.
The teeth and skulls of the two species differed dramatically: A. africanus had smaller cheek teeth; P. robustus had a heavier jawbone, larger teeth, and probably more powerful chewing muscles.
Scientists have long argued that the two species had markedly different diets.
The new study by Ungar, Brown, and colleagues suggests that, on average, A. africanus probably ate a greater share of soft and tough foods than P. robustus, which probably ate more hard and brittle foods.
The researchers found, however, that there was substantial overlap between the two species in their dental microwear, and presumably, in their diets.
"This was actually quite surprising to us at first," Ungar said. "This suggests that much of the time [the two species] ate similar foods."
Ungar said the observation gelled after further reflection: Both species would probably have preferred to eat easy-to-consume, energy-rich foods, such as fruits, when they were available.
A similar phenomenon can be seen in modern chimpanzees and gorillas that live in the same geographical area. These so-called sympatric animals share food resources much of the year, but differ mostly during times of food scarcity.
At these times, gorillas fall back on tougher foods, such as leaves and stems, because their teeth and guts allow them to do so.
Ungar says reconstructing the diet of early humans is important to understanding our evolutionary lineage.
"First and foremost, understanding the evolution of human diet gives us important insights into hominin ecology and evolution," he said.
"Diet is a direct link between an animal and its environment," he added. "It is the single most important factor underlying behavioral differences among living primates, and the same was probably true of early hominins. After all, you are what you eat."
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