At some point in human evolution our ancestors began to catch increasing quantities of marine animals which provided them with nutrients which in turn promoted brain development and ultimately helped create modern homo sapiens.

It is not known exactly how this process occurred, but a research team from Kyoto University in Japan has revealed the first ever evidence of wild chimps habitually catching and consuming freshwater crabs.

The findings may provide scientists with new insight into the behaviours which drove our own progress, the scientists suggest.

Writing in the Journal of Human Evolution, the team describes year-round, freshwater crab-fishing behaviour – primarily among female and infant chimpanzees – living in the rainforest of the Nimba Mountains in Guinea, west Africa.

“The aquatic fauna our ancestors consumed likely provided essential long-chain polyunsaturated fatty acids, required for optimal brain growth and function,” said lead author Kathelijne Koops from the University of Zurich and Kyoto University’s leading graduate programme in primatology and wildlife science.

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“Further, our findings suggest that aquatic fauna may have been a regular part of hominins’ diets and not just a seasonal fallback food,” she said.

The study began in 2012 when the researchers first saw the chimpanzees fishing for crabs.

For two years, they documented the demographics and behaviour of the group of chimps, while also analysing and comparing the nutritional value of the crabs to other foods in the chimpanzees’ diet.

Crabbing, they learned, not only took place year-round – without regard to season or fruit availability – but intriguingly was negatively correlated with the chimps’ consumption of ants, another diet staple. Mature males were the least likely to consume aquatic fauna.

“Energy and sodium levels in large crabs are comparative with ants,” Dr Koops said, “leading us to hypothesise that crabs may be an important year-round source of protein and salts for females – especially when pregnant or nursing – and for growing juveniles.”

The study sheds light on our own evolution by illustrating that fishing behaviours may not be restricted by habitat as could be assumed.

“This isn’t the first case of non-human primates eating crabs,” said senior co-author Tetsuro Matsuzawa, “but it is the first evidence of apes other than humans doing so.”

He said notable previous observations of primates eating crabs came from “monkey species in locations consistent with aquatic faunivory – lakes, rivers, or coastlines – and not in closed rainforest.”