It’s around this time that we see in the fossil record (based mainly on rib and pelvis fossils) a reduction of the size of the gut areas in Homo erectus, the early human species credited with consistently incorporating animal foods into its diet. This species evolved a smaller, more efficient digestive tract, which likely released a constraint on energy and permitted larger brain growth, as predicted by the expensive tissue hypothesis. Yet the increase in brain size we see in the fossil record at about 2 million years ago is basically tracking body size; while absolute brain size was increasing, relative brain size was not. Maybe meat was not completely responsible—so what was?
Perhaps it was the shift from eating antelope steak tartare to barbecuing it. There are hints of human-controlled fires at a few sites dating back to between one and two million years ago in eastern and southern Africa, but the first solid evidence comes from a one-million-year-old site called Wonderwerk Cave, in South Africa. In 2012, Francesco Berna, then of Boston University, and his colleagues reported bits of ash from burnt grass, leaves, brush, and bone fragments inside the cave. Microscopic study showed that the small ash fragments are well preserved and have jagged edges, indicating that they were not first burned outside the cave and blown or washed in, as those jagged edges would have been worn away. Also, this evidence comes from about 30 meters inside the cave, where lightning could not have ignited the fire.
Soon after that, the 790,000 year old site of Gesher-Benot Ya’aqov in Israel yielded evidence of debris from ancient stone tools that had been burned by fire. Nearby to the burnt tools were concentrations of scorched seeds and six kinds of wood, including three edible plants (olive, wild barley, and wild grape), from more than a dozen early hearths. This marks the first time that early humans came back to the same location repeatedly to cook over these early campfires. Hearths are more than just primitive stoves; they can provide safety from predators, be a warm and comforting location, and serve as places to exchange information.
Cooking was unquestionably a revolution in our dietary history. Cooking makes food both physically and chemically easier to chew and digest, enabling the extraction of more energy from the same amount of food. It can also release more of some nutrients than the same foods eaten raw and can render poisonous plants palatable. Cooking would have inevitably decreased the amount of time necessary to forage for the same number of calories. In his 2009 book Catching Fire, primatologist Richard Wrangham postulates that cooking was what allowed our brains to get big. It turns out that using fossil skulls to measure brain size, we see the biggest increase in brain size in our evolutionary history right after we see the earliest evidence for cooking in the archaeological record, so he may be on to something. Modern human bodies are so adapted to cooked foods that we have difficulty reproducing while on an exclusive diet of raw foods. For example, a 1999 study found that about 30 percent of reproductive-age women on a long-term raw-food diet had partial to complete amenorrhea, which was probably related to their low body weight.
–Briana Pobiner, Meat-Eating Among the Earliest Humans