Study published in Science identifies Australopithecus as a plant eater, narrowing the scope on when regular animal consumption increased and brains grew.

 An international team of researchers including Dominic Stratford, PhD, of Stony Brook University and the University of the Witwatersrand in South Africa, have discovered that an ancient human ancestor found in deposits at the Sterkfontein Caves, Australopithecus, which lived more than three million years ago in South Africa, primarily ate plant-based foods. The finding, published in the journal Science, stems from an analysis of tooth enamel from seven Australopithecus fossils and is significant because the emergence of meat eating is thought to be a key driver of a large increase in brain size seen in later hominins.

Every human behavior, from abstract thought to the development of complex technology, is a result of the evolution of the brain. According to evolutionary scientists, meat consumption is a primary driver of many aspects of the evolution of our own genus, Homo, including brain size. When hominins started to exploit and consume highly nutritious animal products is a major question in human evolution studies because it represents a turning point in our evolution. However, direct evidence of when meat eating emerged among our earliest ancestors, and how its consumption developed through time, has remained elusive to scientists.

The research team included investigators from the Max Planck Institute for Chemistry (MPIC) in Germany and the University of Witwatersrand. They analyzed stable nitrogen isotope data (¹⁵N/¹⁴N) from tooth enamel of Australopithecus fossils found in the caves, an area known for its rich collection of early hominin fossils.

The ratio of stable nitrogen isotopes accumulated in animals’ tissues has been used to understand its trophic position – place in the food chain – for many years. An enrichment of ¹⁵N is generally indicative of a higher position in the food chain and consumption of animal tissue. Conventionally, bone collagen or dentin are sampled to attain enough nitrogen isotopes for analysis. But these tissues typically decay relatively rapidly, limiting the application of nitrogen isotope analysis to about 300,000 years.

The recent development of more sensitive analytical techniques that can measure less nitrogen provided the opportunity to sample enamel, the hardest tissue of the mammalian body that also traps Nitrogen stable isotopes while it is forming. Enamel can potentially preserve the isotopic fingerprint of an animal’s diet for millions of years.

According to Stratford, an Adjunct Lecturer in the Department of Anthropology in the College of Arts and Sciences at Stony Brook University, and Director of Research at the Sterkfontein Caves, and his colleagues, this advancement in nitrogen isotope analysis enabled the researchers to obtain the first direct evidence of the diet of ancient hominin fossils and explore when meat eating started, the behavior that set hominins on a new evolutionary path.

They compared the isotopic data from those fossils with tooth samples of other coexisting animals at the time, such as monkeys, antelopes, hyenas, jackals and big cats. The comparison revealed that while its possible Australopithecus occasionally consumed meat, its primary diet was plant-based.

In fact, the isotopic data showed the hominin ate more like a herbivore than a carnivore. One interpretation of this result, explains Stratford, is that changes in behavior known to occur in Australopithecus may not be a result of an increase in meat consumption. It may also suggest that regular meat eating had not yet emerged as a behavior in a hominin this old, implying that it occurred only later in time, or in a different geographic area.

“Overall, this work provides clear evidence that Australopithecus in South Africa did not eat significant amounts of meat three million years ago, and it represents a huge step in extending our ability to better understand diets and trophic level of all animals back into the scale of millions of years,” adds Stratford.