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The paper argues that our early ancestors acquired a taste for fat by eating marrow scavenged from the skeletal remains of large animals that had been killed and eaten by other predators. The argument challenges the widely held view among anthropologists that eating meat was the critical factor in setting the stage for the evolution of humans.

“Our ancestors likely began acquiring a taste for fat 4 million years ago, which explains why we crave it today,” says Jessica Thompson, the paper’s lead author and an anthropologist at Yale University. “The reservoirs of fat in the long bones of carcasses were a huge calorie package on a calorie-poor landscape. That could have been what gave an ancestral population the advantage it needed to set off the chain of human evolution.”

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While focusing on fat over meat may seem like a subtle distinction, the difference is significant, Thompson says. The nutrients of meat and fat are different, as are the technologies required to access them. Meat eating is traditionally paired with the manufacture of sharp, flaked-stone tools, while obtaining fat-rich marrow only required smashing bones with a rock, Thompson notes.

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“Our human ancestors were likely awkward creatures,” Thompson says. “They weren’t good in trees, like chimpanzees are, but they weren’t necessarily all that good on the ground either. So, what did the first upright walking apes in our lineage do to make them so successful? At this stage, there was already a small increase in the size of the brains. How were they feeding that?”