Evidence supports theory that humans evolved in grasslands

Researchers from the Columbia University Lamont-Doherty Earth Observatory have discovered new evidence to support the theory that key human traits, including large brains and the ability to walk on two legs, evolved as our ancestors adapted to living in open grasslands.

Writing in a special human-evolution issue of the journal Proceedings of the National Academy of Sciences, postdoctoral research scientist Kevin Uno and his colleagues found a 24 million year old vegetation record buried deep within seabed sediments off the coast of eastern Africa.

This vegetation is the longest and most complete record of ancient plant life discovered to date in the purported birthplace of humanity, modern-day Kenya and Ethiopia, the researchers noted in a statement. It also indicates that between 24 million and 10 million years ago, well before the first human ancestors arose, the region was dominated by woodlands with few grasses.

That all changed due to a dramatic shift in climate, and within a few million years time, grasses became dominant – a trend that continued throughout the entire course of  human evolution, Uno and his colleagues said. As our ancestors adapted to these changes, they evolved physically, their diets became more flexible, and their social structures grew in complexity.

Life on the ancient grasslands made humans into what we are today.

Life on the ancient grasslands made humans into what we are today.

Plant remains obtained through core drilling key to new findings

Genetic evidence suggests that early hominids first split from other apes between six and seven million years ago, and many scientists believe that it was the shift from dense forests to savannas in eastern Africa that served as the catalyst for their eventual development into modern humans. The new study indicates that the rise of grasslands had a tremendous impact on hominins.

“The entire evolution of our lineage has involved us living and working in or near grasslands. This now gives us a timeline for the development of those grasses, and tells us they were part of our evolution from the very beginning,” Uno said, adding that those savannas likely popped up in small patches at first, and were only one of several factors – including the ability to hunt in a more open landscape – that resulted in the physical and social advancement of our species.

Unlike previous studies, which collected scattered evidence in the form of pollen and chemical isotopes that were at most four million years old, the new study analyzed a series of sediments obtained through core drilling by a research ship working in the waters near northeaster Africa. These sediment cores contained tens of millions of years world of chemicals from plants which grew on land but were later washed out to sea, where they collected and were preserved.

Filling in the details of grassland evolution in eastern Africa

By analyzing carbon-based chemicals called alkanes, which comprise the waxy outer parts of leaves and contain the fingerprints of different types of plants, the study authors determined that grasses started to arise roughly 10 million years ago, and their coverage area seemed to increase by seven to eight percent every one million years. By two to three million years ago, grasses had become the dominant form of vegetation in eastern Africa, and they remain so today.

The findings match previous chemical analyses from ancient herbivore teeth, which showed that the creatures started to switch to a more grass-rich diet approximately 10 million years ago, said Uno. Several million years later, the first hominins appeared, and by 3.8 million years ago, their tooth enamel indicates that they had developed a flexible diet that included food based on grasses – that is, the meat of creatures that ate grass, not grass itself.

“Lots of people have conjectured that grasslands had a central role in human evolution,” study co-author Peter deMenocal, a climate scientist at Lamont-Doherty, explained. “But everyone has been waffling about when those grasslands emerged and how widespread they were. This really helps answer the question.”

Smithsonian Institution anthropologist Richard Potts called the study “the very best examination and most compelling demonstration” of long-term grassland expansion, adding that “bipedality emerged as a way of combining walking on the ground and climbing trees; toolmaking expanded the adjustments to a much wider range of foods; brains are the quintessential organ of flexibility. Geographic expansion requires adaptability to change.”


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