Around 3.5 million years ago, our primate ancestors made a shift in their diet that some scientists believe set them on course for human evolution. The findings were reported in a series of studies published this week in the Proceedings for the National Academy of Sciences.

It was previously assumed that our human ancestors' diet was largely the same as today's modern chimps, notes Thure Cerling, a geology and biology professor at the University of Utah. He was lead author on two of the three studies, which brought together dozens of researchers from across the world. Cerling describes the work, "We have a look at 4 million years of the dietary evolution of humans and their ancestors."

To determine what our ancestors were eating, the researchers exploited a unique property of carbon atoms found in plants, the main component of our ancestors' diet. Plants use carbon dioxide, water and light to make sugar and oxygen in a process called photosynthesis. However, different plants carry out photosynthesis slightly differently. As a result, sugars from different plants have slightly different carbon compositions. One group, C3 plants, has a distinctly lower ratio of carbon-13, a unique isotope of carbon, compared to common carbon-12. Two other groups, C4 and CAM plants, have a relatively higher ratio of carbon-13.

The researchers found the composition of the primates' diet by examining their teeth. As primates age, their tooth enamel becomes fuller and harder. During this process, tooth enamel absorbs the molecules, including carbon, from the food being eaten. 173 fossilized teeth samples, representing a period of roughly 4 million years of evolution, were analyzed for carbon signatures that revealed the relative amounts of the two plant groups within them.

They discovered that from roughly 4.2 to 1.5 million years ago, our ancestors moved from a diet of almost exclusively C3 plants - leaves, fruits, vegetables, wheat, barley and herbs - to a diet of mostly C4/CAM plants - grasses, sedges, seeds, millet and roots. The expanded diet more closely resembles the modern human diet and correlates with a shift from living in forested areas to living on the savannah.

While these studies go much farther in advancing our knowledge of ancient hominid diets, many details remain unknown. The carbon-13 method cannot be used to determine which specific plants were eaten, nor can it distinguish whether our ancestors ate these plants directly or ate the meat of animals that had eaten these plants.

Evolutionary biologists believe the dietary expansion may have contributed to the evolution of our ancestors, partially by enabling them to move out of the forests where most apes still live today. "Diet has long been implicated as a driving force in human evolution," says Matt Sponheimer, anthropologist at the University of Colorado, Boulder, and lead author of the third study.

This new information about diet may lead to a shift in thinking by evolutionary biologists about how humans evolved. "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, when it was widely assumed our human ancestors ate mostly leaves and fruits," Cerling says. By learning more about what our ancestors were eating, we are learning about our own evolutionary history and ultimately what makes us human.

A set of new studies from the University of Utah and elsewhere found that human ancestors and relatives started eating an increasingly grassy diet 3.5 million years ago. The studies included analysis of tooth enamel from fossils of several early African humans, their ancestors and extinct relatives, some of which are shown here. Top left: Paranthropus bosei, 1.7 million years ago. Top right: Homo sapiens, 10,000 years ago. Center left: Paranthropus aethiopicus, 2.3 million years ago. Center right: Homo ergaster, 1.6 million years ago. Bottom left: Kenyanthropus platyops, 3.3 million years ago. Bottom center: lower jaw from Australopithecus anamensis, 4 million years ago. Bottom right: Homo rudolfensis, 1.9 million years ago.

Credit: Copyright National Museums of Kenya. Photos by Mike Hettwer, except Homo sapiens by Yang Deming.