New fossils have provided a snapshot of proto-human diets during a critical evolutionary moment, when better fare helped our small-brained ancestors boost their cognitive capacity.

Two-million-year-old bones that belonged to fish, crocodiles and turtles — aquatic animals rich in brain-fueling fatty acids — were found together with stone tool fragments near Kenya’s Lake Turkana.

“We know that the hominin brain was growing at this time, but we’ve had little evidence that people were able to increase the quality of their diets,” said University of Cape Town archaeologist David Braun. “It may be that this was part of a broader hominin pattern.”

Preserved in sediments left by sudden flooding and described June 1 in Proceedings of the National Academy of Sciences, the fossil trove could have been left by any of several hominid species – Homo habilis, Homo rudolfiensis, Paranthropus boisei — who once lived around Lake Turkana.

One of these small-bodied, small-brained hominids evolved into the bigger-brained, bigger-bodied Homo erectus, a definite human ancestor that likely possessed language and lived in hunter-gatherer tribes. How that evolutionary jump happened is, however, a mystery.

The brain is an extremely energy-intensive organ. If chimpanzees are any indication, the early hominid diet consisted of fruits, plants and insects. Large brains couldn’t have evolved on such low-energy fare. “It looks like our diet would have had to go up a trophic level in order to support such an expensive organ,” Braun said.

As early hominids didn’t yet have the tools or social organization necessary for significant hunting or fishing, some anthropologists think they scavenged animal carcasses. According to Braun, that would have put them in direct competition with “a lot of large carnivores, which would have been dangerous.”

Instead, Braun thinks river and lake floodplains of the sort that preserved his fossils gave early hominids a low-risk hunting opportunity. “As lakes and rivers flooded and receded, animals could have been caught. The remains could be easily collected,” he said. Humanity’s ancestors “could have entered the higher trophic level without taking on the risks.”

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