The theory used to be meat (though sometimes starch) but now maybe it’s fat:
The prevailing view, supported by a confluence of fossil evidence from sites in Ethiopia, is that the emergence of flaked tool use and meat consumption led to the cerebral expansion that kickstarted human evolution more than 2 million years ago. Thompson and her colleagues disagree: Rather than using sharpened stones to hunt and scrape meat from animals, they suggest, earlier hominins may have first bashed bones to harvest fatty nutrients from marrow and brains…
The new paper goes further: Harvesting outer-bone meat would have come at significant costs, the authors argue. The chance of encountering predators is high when scraping raw flesh from a carcass. Chewing raw meat without specialized teeth doesn’t give much energetic benefit, studies have shown. In addition, meat exposed to the elements will quickly rot.
Marrow and brains, meanwhile, are locked inside bones and stay fresh longer. These highly nutritional parts are also a precursor to the fatty acids involved with brain and eye development. And more easily than flesh-meat, bones could be carried away from carcass sites, safe from predators. Richard Kemeny, “Fat, Not Meat, May Have Led to Bigger Hominin Brains” at Sapiens
Paper. (paywall)
Theories about the evolution of the human brain are basically a war of trivial explanations. It’s like blaming World War II on indigestion, only monstrously bigger.
Never mind that the human brain exceeds the most powerful computers in efficiency or that people can lose large hunks of brain and still think. Noise about fat, meat, or starch distracts us from seeing what it is we are trying to explain.
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See also: Earlier discussion of the fat theory.
Starchy food may have aided human brain development
Do big brains matter to human intelligence?
Human evolution: The war of trivial explanations
and
What great physicists have said about immateriality and consciousness