Complimentary Studies Show Change in Hominin Diets Around 3.5 Million Years Ago

A series of scientific papers have been published in the Proceedings of the National Academy of Sciences (PNAS) that shed light on the changing dietary habits of prehistoric primates, including hominins in eastern and southern Africa.  From these studies, it seems that 3.5 million years ago (Pliocene Epoch), these creatures began to expand their diets, leaving behind a forest-based diet similar to that seen in extant Great Apes today and moving towards diets which indicate more time spent on the savannah.

The change in dietary habits may help explain why so many species of large primate were able to co-exist in Africa during the Late Pliocene, it seems that with more types of food on the menu and more habitats being called home, these hominins may not have directly competed with each other.

Fossilised Teeth Provide Clues to Ancient Eating Habits

You are what you eat!

You are what you eat!

Picture Credit: AFP

In four new research papers, scientists analysed the carbon isotopes found in the fossilised tooth enamel from eleven species of hominins and other primates.  The fossils spanned a period of geological time from approximately 4.1 million years ago to the beginning of the Holocene Epoch (10,000 years ago).  The researchers found that there was an increase in the consumption of grasses and sedges, plants from the savannah, or perhaps animals that fed on these plants were being consumed by these species.

University of Utah geochemist, Thure Cerling, the principal author of two of the new studies stated:

“At last, we have a look at four million years of dietary evolution of humans and their ancestors.  For a long time, primates stuck by the old restaurants – leaves and fruits – and 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.  Tropical grasses provided a new set of restaurants.  We see an increasing reliance on this new resource by human ancestors that most primates still don’t use today.”

This leaves the scientists with an intriguing question.  As the average global temperature on the planet fell to around 15 degrees Celsius as the Miocene Epoch progressed, so extensive grasslands began to form and forests shrank back.  However, if grasslands were widespread by 6 million years ago, why didn’t the primates exploit this new habitat earlier?

The diets were analysed from the chemical make up of their teeth, identifying the carbon isotopes within them.  Broken fossil teeth were used in the study, with tiny amounts of fossil material extracted to make up each sample.

Chewing Grass and Sedges – Our Hominin Ancestors

At home on the plains.

At home on the plains.

The ratios of different types of carbon atoms, or isotopes, in fossils can give clues to what a fossil creature ate because different foods have different carbon isotope signatures.  In simple terms, when it comes to carbon isotopes “you are what you eat”.

Commenting on the study, Dr. Zeresenay Alemseged (California Academy of Sciences), a co-author on two of the papers stated:

“What we have is chemical information on what our ancestors ate, which in simpler terms is like a piece of food item stuck between their teeth and preserved for millions of years.  Because feeding is the most important factor determining an organism’s physiology, behaviour and its interaction with the environment, these finds will give us new insight into the evolutionary mechanisms that shaped our evolution.”

The isotope analysis methodology cannot determine what parts of grasses and sedges hominins ate – leaves, stems, seeds and-or underground storage organs such as roots or rhizomes.  This technique is also unable to determine when human ancestors began getting much of their grass by eating grass-eating insects or meat from grazing animals.  Direct evidence of human ancestors scavenging meat does not appear until approximately 2.5 million years ago, and definitive evidence of hunting dates to only about half a million years ago.

Professor Cerling explained that although this study provides some useful information there were still many puzzles to solve, he pointed out that:

“We don’t know exactly what they ate.  We don’t know if they were pure herbivores or carnivores, if they were eating fish [which leave a tooth carbon isotope signature that looks like grass-eating], if they were eating insects or if they were eating mixes of all of these.”

The scientists have concluded that species such as Australopithecus afarensis and Kenyanthropus platyops around 3.5 million years ago began to have a diet which was more grass based, they also tended to live in more open habitats.  The new studies show that these species not only lived on the savannah, but began to consume progressively more foods from their grassland homes.

Professor Cerling’s second study shows that while human ancestors ate more grasses and other apes stuck with trees and shrubs, two extinct Kenyan baboons represent the only primate genus that ate primarily grasses and perhaps sedges throughout its history.  Theropithecus brumpti ate a sixty-five percent tropical grass-and-sedge diet when the baboons lived between 4 million and 2.5 million years ago, contradicting previous claims that they ate forest foods.  Later, Theropithecus oswaldi ate a seventy-five percent grass diet by 2 million years ago and a 100 percent grass diet by 1 million years ago.  Both species became extinct, possibly as a result of increased competition from ungulates which were more efficient at processing grasses in their large guts.

The professor noted that primate grass-eaters such as the Theropithecus and the robust Australopithecines, also referred to as the Paranthropus, went extinct while modern human ancestors ate an increasingly grass-based diet.  Why?

He added:

“We now have good evidence that some early hominins began using plant foods that are not used in abundance by living African apes today, and this probably led to a major change in the way they used the landscape.  One consequence could be that the dietary expansion led to a habitat expansion, as they could travel to more open habitats more efficiently.”

The professor went on to add:

“We know that many early hominins lived in areas that would not have readily supported chimpanzees with their strong preference for forest fruits.  It could also be argued that this dietary expansion was a key element in hominin diversification.”

The study has also provided a possible answer to why there were so many different large primates able to co-exist in Africa during the Pliocene and early Pleistocene Epochs – they were not competing for the same food stuffs.

The researchers hope to be able to build on their isotope studies and gain access to new hominin discoveries so that evidence from new specimens can be added to their research programme.

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