East African grassy woodlands which propelled evolution of bipedalism in human ancestors emerged 10 million years earlier than thought

Chemical analysis of East African fossils and ancient soil samples from 21 million years ago show that hominoids were eating drier leaves as opposed to juicy fruits, and probably were not restricted to the canopies of trees as has long been thought.

Two related studies reveal that savannah grasses emerged in East Africa more than 10 million years earlier than previously thought. This challenges the longstanding palaeoecological theory that almost all of East Africa was covered in dense forest 21 million years ago.

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It has long been theorised that a drying climate which saw the reduction of East Africa’s forests in favour of grassy woodland and savannah, drove skeletal adaptations in hominoids toward bipedalism – an essential biomechanical step on the road to becoming human.

Walking upright on two legs allowed early ancestors of humans to see over long grass and freed up their hands for the development of tools, accelerating human evolution.

Now, it appears that this transformation began taking place in the early Miocene between 21 million and 17 million years ago.

A paper published in Science reveals that ancient soil samples from nine sites in what is now Kenya and Uganda, bear high chemical signatures of plants that have evolved to withstand drier and hotter conditions.

The biochemical signature shows an abundance in that time period of grasses that photosynthesise via the C­₄ carbon fixation pathway.

Carbon fixation refers to the process by which carbon dioxide is used to synthesise sugars such as glucose. C₄ plants transform CO₂ into a four-carbon molecule as the first step in producing sugar. C₃ plants first transform CO₂ into a three-carbon molecule.

C₄ grasses withstand direct sunlight better than their C₃ predecessors which prefer shadier environments.

Past research indicated that C₄ grasses appeared in East Africa, heralding the transformation of dense jungle into open plains, less than 10 million years ago. The new research pushes their emergence back over 10 million years.

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Another paper published in Science analysed the dental and skeletal fossils of Morotopithecus – an ancient ape that lived in Uganda 21 million years ago during the early Miocene. The study allowed researchers to determine what the animals were eating.

Dental patterns showed the apes were eating leaves with lower water content, such as the drier stalks of C₄ grasses, rather than juicier forest fruits.

Skeletal structure, including femurs, vertebrae and torso of the prehistoric apes, also resembled those of modern non-human primates that inhabit savannahs and other open ecosystems today.