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According to the study authors, the addition of water-based prey into early-human diets may have been what boosted brain size in certain hominins—humans plus human ancestral species and their close evolutionary relatives.

That's because reptiles and fish are particularly rich in long-chain polyunsaturated fatty acids. Some experts think this so-called good fat was "part of the package" of human brain evolution, said study leader David Braun, an archaeologist at the University of Cape Town in South Africa.

Discovering evidence for "brain food" in the late Pliocene (about 3 to 1.8 million years ago) may explain how bigger brains—for instance in our likely direct ancestor Homo erectus—arose in humans and their relatives about 1.8 million years ago, Braun said.

Remains of about 48 animal species were found at the Kenyan site, once a delta crisscrossed with small rivers.

In additon to the aquatic animals, there's evidence early humans feasted on mammals such as ancient rhinoceroses, hippopotamuses, and antelope, the researchers report in a paper published online this week in the journal Proceedings of the National Academy of Sciences.