The shift that our ancestors made from relying mainly on trees and shrubs for food, to searching for grub on the ground and eating more of a grass-based diet could have helped them fare better in a changing environment. New evidence points to this event occurring much earlier than was previously thought – about 400,000 years earlier, to be precise.
“A refined sense for when the dietary changes took place among early humans, in relation to changes in our ability to be bipedal and terrestrial, will help us understand our evolutionary story,” says Naomi Levin, from John Hopkins University, the lead author of the study published in Proceedings of the National Academy of Sciences. By creating a timeline and placing each event in order, they hope to understand how one change might lead to another. Now they can place the shift in diet to around 3.8 million years ago.
Analyzing fossil teeth, the team found that the move to a more grass-based diet, which includes not only grasses but also their roots, insects and animals that feed on them, came after the development of teeth and jaws to help process it. But the research also shows that this doesn’t necessarily always have to be the case. As well as looking at the diet of different human ancestor species found in Ethiopia, they also analyzed the diet of an extinct species of baboon.
“The results not only show an earlier start to grass-based food consumption among hominins and baboons, but also indicate that form does not always precede function,” explains Yohannes Haile-Selassie, one of the study's four coauthors. “In the earliest baboons, dietary shift toward grass occurred before its teeth were specialized for grazing.”
They were able to work out the general diet of the animals by analyzing certain plant-derived carbon-based molecules found in the enamel of the teeth. The researchers looked at 152 teeth from an array of creatures, from pigs to giraffes, but also human ancestors. Most plants, including trees and shrubs, are what are called C3 plants, whereas grasses and succulents are normally C4 plants, which are adapted to more arid and hot ecosystems thanks to a more efficient photosynthesis process. C3 plants convert carbon dioxide into a molecule containing three carbons, while C4 plants convert it into a molecule containing four. By looking at which of these two are present in the enamel, the researchers can work out what sort of plants the animals were eating.
Among the human ancestors, they found that earlier species were eating mostly C3 plants, but by the time Australopithecus afarensis, made famous by the skeleton called Lucy, came on the scene they had started to include more C4 plants as they moved out into the grasslands. The researchers expect that this move to a more generalist diet allowed the apes to be more mobile, able to live away from the forests, and more adaptable to a changing environment.
What caused this shift in diet is a little harder to pin down. It could, the authors suggest, be to do with a changing animal community – as new African primate species emerged, competition for food may have increased, making a more diverse diet an advantage.
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