Our human ancestors did not eat much fruit, but instead consumed a lot of root vegetables, nuts, insects and some meat, according to a new study.
The findings may help to explain how all humans emerged in the first place, the authors believe.
Early hominid ancestors may have left the trees to take advantage of ground-level foods, a behavioral shift that could have resulted in two of the major defining characteristics of humans: unique teeth and walking on two legs, a mode of locomotion known as bipedalism that is extremely rare elsewhere in the animal kingdom.
"It is no coincidence that the emergence of terrestrial bipedalism went hand in hand with changes in oro-facial morphology, tooth enamel thickness and tooth microstructure," said co-author Gabriele Macho, a professor of paleoanthropology at the University of Bradford.
Macho and colleague Daisuke Shimizu analyzed the teeth of Australopithecus anamensis, a hominid that lived in Africa 4.2 to 3.9 million years ago. The famous skeleton "Lucy" came from the same genus. And anthropologists now suspect that the recently found and even older Ardipithecus evolved into Australopithecus, which likely led to the modern human genus, Homo.
Based on actual tooth finds, Shimizu produced sophisticated computer models showing multiple external and internal details of the teeth. One determination was immediately clear: Unlike chimpanzees, which are fruit specialists, the hominid couldn't have been much of a fruit-lover.
"Soft fleshy fruits tend to be acidic and do not require high bite forces to be broken down," explained Macho. "The enamel microstructure of A. anamensis indicates that their teeth were not well equipped to cope with acid erosion, but were well adapted to masticate an abrasive and hard diet."
The researchers therefore believe this early human ate nuts, root vegetables, insects -- such as termites -- and some meat. While they think certain flowering plants known as sedges might have been in the diet, Lucy and her relatives were not properly equipped for frequent leaf-chewing.
For the study, accepted for publication in the Journal of Human Evolution, Shimizu also modeled chimpanzee and gorilla teeth as a comparison.
"Gorilla molars are relatively high crowned and have shearing crests, which are advantageous for the breakdown of leaves," Macho said.
In contrast, the shape of the hominid teeth and their internal enamel structure, suggests the early humans combined shearing with lateral lower jaw movement.
"Chew a toffee and feel the difference," Macho said, comparing this type of chewing with eating a brittle food, like a roasted peanut, which involves more up and down motion. Although he thinks the ancient humans ate certain nuts, including the peanut-like groundnut, their teeth suggest brittle edibles were mostly absent from their diet.
University of California at Berkeley paleontologist Tim White co-directed the recent project that brought to light the "Ardi" skeleton and Ardipithecus ramidus, which lived 4.4 million years ago.
White agrees that Australopithecus molars had thick enamel "more durable to heavy chewing of hard and tough foods with adhering grit," he told Discovery News. Ardi likely consumed a similar diet and "was probably omnivorous," according to White and his team, who add that mushrooms were also probably on the prehistoric menu.
Macho and Shimizu think the emergence of photosynthesizing plants at the Miocene/Pliocene boundary led to "major global and local environmental changes." These caused more seasonality, which meant fluctuating food supplies, greater predation risk at the forest edge and increased competition for resources.
To cope with these challenges, the scientists believe primates developed different strategies. Gorillas essentially became herbivores, chimps evolved into fruit specialists and hominids became omnivores, which was a wise path to follow.
Macho explained, "This subtle interplay between diet, social structure and life history ultimately led to the evolution of our large brains."