June 4, 2013
Our Human Ancestors Had A Diet Rich In Grass
Lawrence LeBlond for redOrbit.com - Your Universe Online
Four new studies have taken a new look at the diets of our ancestors and have found their behavior was a “game changer” for early humans some 3.5 million years ago. An ape-like diet that included grasses and sedges paved the way for a diet rich in grains, meats and dairy from grazing animals.
“We don´t know exactly what happened,” said Sponheimer. “But we do know that after about 3.5 million years ago, some of these hominids started to eat things that they did not eat before, and it is quite possible that these changes in diet were an important step in becoming human.”
Sponheimer's paper has been published online this week in the Proceedings of the National Academy of Sciences (PNAS), along with the three other related papers.
Prior to this ground-breaking research, scientists analyzed teeth from 87 hominids. The new paper from CU presents information on an additional 88 specimens, including five previously unanalyzed hominid species.
Sponheimer, who specializes in stable isotope analysis, compared particular forms of the same chemical element in fossilized teeth. Carbon isotopes obtained from the ancient hominids can help researchers piece together the types of plants that were being eaten way back when, he noted.
The carbon signals from the ancient teeth are derived from two distinct photosynthetic pathways, he added. C3 signals are from plants like trees and bushes, and C4 signals are from grasses and sedges. The wear of the teeth also provided more information on the type of foods these hominid specimens were eating.
After evolving from Australopithecus, the genus Homo likely looked to broaden its food choices. During this time, one short, upright hominid known as Paranthropus boisei from eastern Africa was moving toward a C4 type of diet. Originally dubbed the “Nutcracker Man” because of its large, flat teeth and powerful jaws, this species was later redefined, with scientists theorizing that the back teeth were actually used for grinding grasses and sedges, explained Sponheimer.
“We now have the first direct evidence that as the cheek teeth on hominids got bigger, their consumption of plants like grasses and sedges increased,” he said. “We also see niche differentiation between Homo and Paranthropus -- it looks probable that Paranthropus boisei had a relatively restricted diet, while members of the genus Homo were eating a wider variety of things.”
“The genus Paranthropus went extinct about 1 million years ago, while the genus Homo that includes us obviously did not,” Sponheimer said.
Sponheimer noted that there still remain some puzzling differences in the evolutionary tree of hominids in eastern Africa and those of southern Africa. P. robustus of southern Africa was anatomically similar to its cousin P. boisei in eastern Africa, but the new analysis indicates the two species had very different carbon isotopic compositions in their teeth. P. robustus seems to have consumed a fair amount of C3 vegetation along with the evolved C4 diet.
“This has probably been one of the biggest surprises to us so far,” said Sponheimer. “We had generally assumed that the Paranthropus species were just variants on the same ecological theme, and that their diets would probably not differ more than those of two closely related monkeys in the same forest.”
“But we found that their diets differed as much isotopically as those of forest chimpanzees and savanna baboons, which could indicate their diets were about as different as primate diets can be,” he said. “Ancient fossils don´t always reveal what we think they will. The upside of this disconnect is that it can teach us a great deal, including the need for caution in making pronouncements about the diets of long-dead critters.”
Thure Cerling, a geochemist from University of Utah, and lead author of two of the four papers published online in PNAS, said: “At last, we have a look at 4 million years of the dietary evolution of humans and their ancestors.”
“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 said, when African savanna began expanding.
He noted that tropical grasses provided early hominids with a new food option and there is increasing evidence that our ancestors relied on this resource; oddly enough, most primates today still do not eat grasses.
Between six and seven million years ago grassy savannas and grassy woodlands in East Africa were abundant. But the question that remains is why our ancestors didn´t start exploiting this resource until less than four million years ago.
The isotopic method may paint a good picture of what types of vegetation was consumed, but it cannot distinguish which parts of these plants were eaten, such as the leaves, stems, seeds, or roots. It also cannot determine exactly when our ancestors began getting much of their grass through consuming grass-eating insects or from grazing animals.
Cerling said direct evidence of meat scavenging doesn´t occur until about 2.5 million years ago, and definitive evidence of hunting only exists 500,000 years ago. The new evidence does clear up some of the mystery as to what was on our ancestors´ plates, but there does still remain uncertainties, he added.
“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,” he said.
The four papers appear in the journal PNAS this week.
A paper on the teeth of hominids from Ethipoia´s Hadar-Dikika area was written by lead author Jonathan Wynn, program director at NSF´s Division of Earth Sciences, on leave from University of South Florida. Other lead authors are Arizona State University's William Kimbel and California Academy of Sciences scientist Zeresenay Alemseged.
One of Cerling´s papers is on the teeth from the Turkana Basin in Kenya, collaborating with lead author paleoanthropologist Meave Leakey of Turkana Basin Institute and geologist Frank Brown of the University of Utah. His other paper is on baboon diets.
Sponheimer´s research paper summarizes the other three studies.
Previous research has shown that an early relative of human, Ardipithecus ramidus (“Ardi”), from Ethipoia ate mainly C3 leaves and fruits.
Between 4.2 and 4 million years ago on the Kenyan side of the Turkana Basin, Cerling suggests Aus. anamensis subsisted on at least 90 percent leaves and fruits — the same diet that modern chimps have.
By 3.4 million years ago, Wynn describes Aus. afarensis of Ethiopia´s Awash Basin living on a rich diet (22 percent on average) of C4 grasses and sedges that extended anywhere from zero to 69 percent of their diet.
The switch to C4 vegetation “documents a transformational stage in our ecological history,” said Wynn.
Many scientists previously believed Aus. afarensis had an ape-like C3 diet. It remains a mystery why Aus. afarensis expanded its menu to C4 grasses when its likely ancestor, Aus. anamensis, did not, although both inhabited savanna habitats, Wynn says.
Also, at around 3.4 million years ago, the human relative Kenyanthropus platyops moved to a highly varied diet of both C3 and C4 vegetation. The average was 40 percent grasses and sedges, but individuals varied widely, eating anywhere from 5 to 65 percent, explained Cerling.
In Cerling´s baboon study, he presented findings that two extinct Kenyan baboon species represent the only primate genus that primarily ate grasses 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.”
Most modern baboons eat only C3 cool-season grasses.
The research was funded by the National Science Foundation (NSF), the National Research Foundation in South Africa, the Leakey Foundation, the Wenner-Gren Foundation, Arizona State University, the CU-Boulder Dean's Fund for Excellence, and George Washington University (GWU).