Studying Ancient Diets

New studies involving USF researchers shows early humans like Lucy fed from the Savanna, not the forest.

 

Mandibles of Australopithecus anamensis, 4.0 Ma from Kanapoi, Kenya (left), and A. afarensis, 3.3 Ma from Hadar, Ethiopia (right), are putative ancestor and descendant species that had dramatically different diets based on stable isotope evidence. Photos: Bill Kimbel, Mike Hettwer

 

By Vickie Chachere

USF News

 

TAMPA, Fla. (June 3, 2013) – More than 3 million years ago in an area of Ethiopia where some of the earliest human ancestors took their first steps on open grasslands, one species also began to develop a taste for foods that come from these grasslands — an event that may still influence our diets today, shows a University of South Florida study unveiled Monday as part of an international effort to unlock the mysteries of human evolution.

 

In the new edition of the Proceedings of the National Academies of Sciences, USF Geologist Jonathan Wynn, working with a prominent team of scientists, reports that man’s earliest ancestors made striking choices in food sources as they ventured from the tropical forests of Africa into new habitats that would shape their future abilities. The studies shed new light on how different branches of man’s family tree survived in difficult habitats and found new food sources that played an important role in their future development.

 

Field crew sieves for fossils, with geologist Wynn. Photo: Jonathan Wynn

The four new studies in PNAS examined carbon isotopes in fossilized tooth enamel from scores of fossil human ancestors and baboons in Africa from 4 million to 10,000 years ago. The team of two dozen researchers - including USF graduate student Jessica Norman, and PhD alum Zelalem Bedaso - found a surprising increase in the consumption of grasses and sedges – plants that photosynthesize using a similar mechanism and which results in distinct carbon isotope “signatures” in an animal’s teeth when they eat such plants.

 

The early humans’ choice of food was a sharp divergence from forests fruits and leaves which are the main sustenance for modern chimpanzees, and presumably that of the last common ancestor which chimpanzees and humans shared some 6 million years ago. Even groups of savanna chimpanzees, which traverse open grasslands frequently, do not consume significant foods from these open areas, but instead see them as barriers to cross before getting to their preferred foods back in the forests.

 

Studying the fossils from the same species as famed hominin Lucy, who lived about 3.4 million years ago, Wynn found that the behavior of Australoipthecus afarensis was quite different from that of chimpanzees, in that they ate significant amounts of foods that come from grasses and sedges, the latter being rush-like plants that include water chestnut, papyrus and sawgrass. Scientists had previously believed A. afarensis had a chimpanzee-like diet of tropical fruits and leaves similar to earlier species that lived about 4 million years ago, but who had also largely lived amongst savanna habitats.

 

The new research builds on ground-breaking work from 2011 when Wynn, an associate professor of geology, and his colleagues discovered scientific evidence that when early humans had learned to walk on two feet and developed more advanced survival skills, these evolutionary milestones happened in the savannas of East Africa. In that study, the scientists also used chemical signatures recorded in ancient soils where the fossils were found in Ethiopia and Kenya.

 

This new analysis of  what those beings ate was conducted from tiny samples drilled from fossilized teeth of specimens housed in the National Museum of Ethiopia, and involved collaboration with researchers from the Institute of Human Origins at Arizona State University, California Academy of Sciences, and University of Colorado.

 

“This ‘big picture’ understanding of dietary change would not have been possible without multi-project and international collaboration,” Wynn said. “We are no longer simply asking questions about what this or that species ate, but can now look at long-term trends in behavior and ecology in this new light.”

 

View of the sediments of the Hadar Formation at Dikika (foreground) and Hadar (background), which produced the fossils of Australopithecus afarensis analyzed by this study. Photo: Jonathan Wynn

What still remains a mystery, Wynn said, is what came first: the move to the savannas or the quest for new food sources. Grassy savannas and grassy woodlands in East Africa were widespread by 6 million to 7 million years ago, but scientists now know it was not until 3.5 million years ago that the early humans began to eat foods from these environments.

 

“Although we can say hominins went into grasslands and ate foods there, we can’t say whether they went there to specifically eat or just came across food for some other reason,” Wynn said.

 

The analysis also opens the door for new questions to narrow the range of potential other food sources that would tell scientists much about how the early ancestors lived, said University of Utah Geochemist Thure Cerling, who led the joint publications of these research projects.

 

 If early humans ate grass-eating insects or large grazing animals like zebras, wildebeest and buffalo – which eat grasses – the same chemical signature would show up in early humans’ teeth, he said. The same would go for those who ate fish, which had eaten algae. With the present analysis, scientists cannot yet distinguish between these possibilities, but can clearly say that their diets differed from forest fruits and leaves.

 

Scientists have only found direct evidence that early humans began scavenging animal flesh about 2.5 million years ago, with some indirect evidence about 3.4 million years ago. Meanwhile, the complex behavior of hunting didn’t show up until about 500,000 years ago.

 

The researchers also believe that changes in diet have been linked to both larger brain size and the advent of upright walking in human ancestors roughly 6 million to 4 million years ago. Human brains were larger than those of other primates by the time our genus, Homo, evolved 2.5 million years ago. Our current species, Homo sapiens, arose 200,000 years ago.

 

To read more on the complete set of studies, visit http://www.pnas.org/.

 

Vickie Chachere can be reached at 813-974-6251.