A Grassy Trend in Human Ancestors' Diets
Tooth enamel shows surprising change in our ancient buffet
Most apes eat leaves and fruits from trees and shrubs.
But new studies show that human ancestors expanded their menu 3.5 million years ago, adding tropical grasses and sedges to an ape-like diet. The change set the stage for consuming more modern fare: grains, grasses, and meat and dairy from grazing animals.
In four studies of carbon isotopes in fossilized tooth enamel from scores of human ancestors and baboons in Africa from 4 million to 10,000 years ago, researchers found a surprise increase in the consumption of grasses and sedges--plants that resemble grasses and rushes but have stems with triangular cross sections.
"At last, we have a look at 4 million years of the dietary evolution of humans and their ancestors," says University of Utah geochemist Thure Cerling, lead author of two of four papers published online today in the journal Proceedings of the National Academy of Sciences (PNAS).
Funding was primarily from the National Science Foundation's (NSF) Divisions of Behavioral and Cognitive Sciences, Earth Sciences and Integrative Organismal Systems.
"For a long time, primates stuck by the old restaurants--leaves and fruits--but by 3.5 million years ago, they started exploring new diet possibilities--tropical grasses and sedges--that grazing animals discovered a long time before, about 10 million years ago," Cerling says, when African savanna began expanding.
"Tropical grasses provided a new set of restaurants. We see an increasing reliance on this resource by human ancestors, one that most primates still don't use today."
Grassy savannas and grassy woodlands in East Africa were widespread by 6 million to 7 million years ago. A major question is why human ancestors didn't start exploiting savanna grasses until less than 4 million years ago.
The isotope method cannot distinguish what parts of grasses and sedges human ancestors ate--leaves, stems, seeds and/or underground storage organs such as roots or rhizomes.
The method also can't help with determining when human ancestors began getting much of their grass through eating grass-eating insects or meat from grazing animals.
Direct evidence of human ancestors scavenging meat doesn't appear until 2.5 million years ago, and definitive evidence of hunting dates to only about 500,000 years ago.
With the new findings, "we know much better what they were eating, but mystery does remain," says Cerling.
"We don't know if they were pure herbivores or carnivores, if they were eating fish [which leave a tooth signal that looks like grass-eating], if they were eating insects, or if they were eating mixes of all these."
Why our ancestors' diets matter
The earliest human ancestor to consume substantial amounts of grassy foods from dry, more open savannas "may signal a major and ecological and adaptive divergence from the last common ancestor we shared with African great apes, which occupy closed, wooded habitats," writes geologist Jonathan Wynn, lead author of one of the papers.
Wynn is currently a program director in NSF's Division of Earth Sciences, on leave from the University of South Florida.
"Diet has long been implicated as a driving force in human evolution," says Matt Sponheimer, an anthropologist at the University of Colorado, Boulder.
He notes that changes in diet have been linked to larger brain size and the advent of upright walking in human ancestors roughly 4 million years ago.
Human brains were larger than those of other primates by the time our genus, Homo, evolved 2 million years ago. (Our species, Homo sapiens, arose 200,000 years ago.)
"If diet has anything to do with the evolution of larger brain size and intelligence, then we are considering a diet that is very different than we were thinking about 15 years ago," says Cerling. At the time, it was believed that human ancestors ate mostly leaves and fruits.
How the studies were performed: you are what you eat
The new studies analyze carbon isotopes in 173 teeth in 11 species of hominins.
Hominins are humans, our ancestors and extinct relatives that split from other apes roughly 6 million years ago.
Some of the analyses were done in previous research, but the new studies include new carbon-isotope results for 104 teeth from 91 individuals of eight hominin species.
Those teeth are in African museums and were studied by two groups of scientists working at separate early human sites in East Africa.
Wynn wrote the paper about teeth from Ethiopia's Hadar-Dikika area, where research is led by Arizona State University's William Kimbel and California Academy of Sciences scientist Zeresenay Alemseged.
Cerling wrote the paper about teeth from the Turkana Basin in Kenya, where the research team is led by Turkana Basin Institute paleoanthropologist Meave Leakey, Cerling and geologist Frank Brown of the University of Utah. Cerling also wrote a paper about baboon diets. Sponheimer wrote a fourth paper, summarizing the other three.
The method of determining ancient creatures' diets from carbon isotope data is less than 20 years old, and is based on the idea that "you are what you eat," Sponheimer says.
Tiny amounts of tooth enamel were drilled from already broken fossil teeth of museum specimens of human ancestors and relatives.
The powder was placed in a mass spectrometer to learn ratios of carbon isotopes incorporated into tooth enamel via diet.
Ratios of rare carbon-13 to common carbon-12 reveal whether an animal ate plants that used so-called C3, C4 or CAM photosynthesis to convert sunlight to energy.
Animals eating C4 and CAM plants have enriched amounts of carbon-13.
C3 plants include trees, bushes and shrubs and their leaves and fruits; most vegetables; cool-season grasses and grains such as timothy, alfalfa, wheat, oats, barley and rice; soybeans; non-grassy herbs and forbs.
C4 plants are warm-season or tropical grasses and sedges and their seeds, leaves or storage organs like roots and tubers. C4 plants are common in African savannas and deserts.
C4 grasses include Bermuda grass and sorghum. C4 grains include corn and millet.
CAM plants include tropical succulent plants such as cactus, salt bush and agave.
Today, North Americans eat about half C3 plants, including vegetables, fruits and grains such as wheat, oats, rye and barley, and about half C4 plants, which largely come from corn, sorghum and meat animals fed on C4 grasses and grains.
The highest human C3 diets are in northern Europe, where only C3 cool-season grasses grow, so meat animals there graze on them rather than on C4 tropical grasses.
The highest C4 diets likely are in Central America because of the heavily corn-based diet.
If early humans ate grass-eating insects or large grazing animals like zebras, wildebeest and buffalo, it also would appear they ate C4 grasses.
If they ate fish that ate algae, it would give a false appearance of grass-eating because of the way algae takes up carbonate from water, Cerling says.
If they ate small antelope and rhinos that browsed on C3 leaves, it would appear they ate C3 trees-shrubs.
Small mammals such as hyrax, rabbits and rodents would have added C3 and C4 signals to the teeth of human ancestors.
The findings: a dietary history of human ancestors and relatives
Humans: the only surviving primates with a C4 grass diet
Cerling's results show that while human ancestors ate more grasses and other apes stuck with trees and shrubs, two extinct Kenyan baboons represent the only primate genus that ate primarily grasses and perhaps sedges throughout its history.
Theropithecus brumpti ate a 65 percent tropical grass-and-sedge diet when the baboons lived between four million and 2.5 million years ago, contradicting previous claims that they ate forest foods.
Theropithecus oswaldi ate a 75 percent grass diet by two million years ago and a 100 percent grass diet by one million years ago. Both species went extinct, perhaps due to competition from hooved grazing animals.
Modern Theropithecus gelada baboons live in Ethiopia's highlands, where they eat only C3 cool-season grasses.
Cerling notes that primate tropical grass-eaters--Theropithecus baboonsand Paranthropus human relatives--went extinct while human ancestors ate an increasingly grass-based diet.
Why is an open question.
Additional support for the research came from many other organizations, including the National Geographic Society and the Leakey Foundation.
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