Human ancestors started eating grasses about 3.5 million years ago, a critical evolutionary turning point that opened the way for new habitats and possibly bigger brains, according to new research spearheaded by the University of Utah.
The results published in four studies lay out for the first time a 4-million-year timeline of dietary evolution. The consumption of tropical grasses and rush-like sedges as well as prey who ate grasses allowed early humans to move away from the trees that had provided their food source, said U. geochemist Thure Cerling.
"As the landscape was opening up in geological time, the dense forests were really receding in many places in East Africa," said Cerling. Eating grasses rather than fruits from trees and shrubs, "was an opportunity to take advantage of the openness of the landscape."
The reports were published online Monday in the journal Proceedings of the National Academy of Sciences, and were primarily funded by the National Science Foundation.
Humans are the only surviving primates that eat a large portion of grass-based foods. Nutrition has also been linked with upright walking and larger brains, said Matt Sponheimer, a University of Colorado-Boulder anthropologist and author of one of the studies.
"Diet has long been implicated as a driving force in human evolution," Sponheimer said in a statement.
Three of the papers concentrate on specific geographic areas in Africa, and the fourth ties the data together. While some of the samples had been used in earlier work, researchers analyzed more than 100 previously unexamined teeth from fossils of early African humans, their ancestors and extinct relatives, obtained from museums across Africa.
They took samples by carefully scraping already broken teeth using a Dremel tool and diamond-tipped dental bit, then put the powdered sample in a mass spectrometer.
Because leaves and grasses use different methods of photosynthesis, they leave behind different chemical signatures. A sample with more of the isotope carbon-12 means the individual ate more leaves and fruit, a category that also includes most vegetables and cool-season grains such as oats. More of the rare carbon-13, on the other hand, indicates a diet rich in grasses. Other carbon-13 foods included water chestnut, corn and sawgrass.
Until this study, most knowledge of dietary evolution came from studying the size and shape of teeth, Cerling said. "We really had no idea" when humans started eating grasses, he said.
The new results, though, don't differentiate whether the ancient hominins chowed down on grass directly or ate the meat of grass-fed animals.
"That's the next problem we have to figure out how to get at," Cerling said. The carbon isotope method is less than 20 years old, and scientists are still determining exactly how to read meat's atomic signature.
"My inkling is with the humans there was certainly a meat component involved," Cerling said perhaps as recently as 3 million years ago.
As it stands now, scientists don't have direct evidence of early man scavenging meat until 2.5 million years ago. Hunting appears about 500,000 years ago.
Also still a puzzle is why it took primates so long to eat grasses. The African continent had begun shifting to savannah millions of years before they gave the food source a try. Grazing animals started on grass much earlier, about 10 million years ago.
As Cerling said: "We know much better what they were eating, but mystery does remain."
Highlights: 4 million years of human ancestral dietary history
4 million years ago • Human ancestors ate at least 90 percent leaves and fruits, like modern chimpanzees.
3.4 million years ago • Two species in two different locations ate an average of 22 and 40 percent grasses and sedges, though some individuals ate much more.
2 million to 1.7 million years ago • Early humans ate a 35 percent grass-based diet that likely included meat or grass-eating insects.
1.4 million years ago • Human ancestors increase the proportion of grass-based food in their diets to 55 percent.
200,000 years ago • Our own species, homo sapiens, arose.
10,000 years ago • Homo sapiens' diet split 50-50 (half trees and shrubs, half grasses and meat) almost identical to the ratio in modern North Americans.
Source: University of Utah