Human Ancestors Ate Grass Earlier than Known

Key Dietary Shift in Human Evolution

Posted: September 14, 2015

Millions of years ago, our primate ancestors abandoned life on the trees in search of food on the ground. This made all the difference to their evolutionary success.
 
The change to living on the ground marked a significant step toward the diverse eating habits that became a key human characteristic, and would have made these early human ancestors more mobile and adaptable to different environments.
 
Curator of Physical Anthropology Dr. Yohannes Haile-Selassie co-authored research indicating that this significant dietary shift took place about 400,000 years earlier than experts previously thought, providing a clearer picture of a time of rapid change in conditions that shaped human evolution.

New evidence published in the Proceedings of the National Academy of Sciences outlines the dietary shift as one of an array of changes that took place across many animals around the same time during the Pliocene era—2.6 to 5.3 million years ago—when the fossil record indicates the presence of multiple species of human ancestors who were starting to spend more time walking on two feet. 
 
The study involved an analysis of fossil teeth of early human ancestors found in the Woranso-Mille study area of Ethiopia. The results show that in early hominins the shift from a diet based on trees and shrubs to one that included grass-based foods took place about 3.8 million years ago. Previous research indicated that this shift occurred approximately 3.4 million years ago.
 
Diversifying your dietary habits can aid in evolutionary success. “You can then range wider,” lead author Dr. Naomi Levin of Johns Hopkins University said of the human precursors that would have included several species including Australopithecus afarensis, extinct some 3 million years ago and represented most famously in the fossil informally known as “Lucy.” “You can be in more places, more resilient to habitat change.”
 
“This research reveals surprising insights into the interactions between morphology and behavior among Pliocene primates,” said Haile-Selassie. “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. In the earliest baboons, dietary shift toward grass occurred before their teeth were specialized for grazing. On the other hand, hominins have acquired dental morphology that would allow them to eat grass-based food resources as early as 4.2 million years ago. However, they did not start exploiting these food resources effectively until after 3.8 million years ago. ” Researchers analyzed 152 fossil teeth from an array of animals including pigs, antelopes, giraffes and other big mammals gathered from a roughly 100 square-mile are of what is now the Woranso-Mille study area of the Afar region of Ethiopia. Among the samples were teeth from hominins—a group that includes contemporary humans and our extinct ancestors—believed to represent 16 different individuals. The teeth were examined for their carbon isotope composition. These isotopes record the type of foods that the animals were eating. The data showed that both human ancestors and members of a now-extinct, large species of baboon were eating large amounts of grass-based foods as early as 3.76 million years ago. Previous research dated the earliest evidence for grass-based foods in early human diets to about 3.4 million years ago.
 
The researchers could not firmly establish a link between external environmental change and the diet of hominins and baboons, but instead attribute the dietary expansion to changes in relations among members of the African primate communities, such as the appearance of new species of primates. Co-authors of the paper were Stephen R. Frost of the University of Oregon and Beverly Z. Saylor of Case Western Reserve University.