“Wrangham research states he believed that cooking tubers is what directly lead to the bigger brain, smaller stomachs in H.eretus.
"Cooked tubers, says Wrangham, arguing that these starchy roots would have been quite abundant on the plains of Africa 2 million years ago, even when drier climates made fruits, nuts, and perhaps animal prey scarce. Today, there are 40,000 kilograms of tubers per square kilometer in Tanzania's savanna woodlands, for example. Other tuber-eating animals, such as wild pigs, thrived in Africa during this time, and Wrangham notes that fossil mole rats, which subsist almost entirely on tubers, have been found among hominid remains from 2 million years ago."
He also goes on to say if tubers made up 60% of diet, then the other 40 were most likely foraged, like nuts, berries perhaps animal remains but those would be a very small portion and much like the tribes Wrangham studied, they only get usually a kill once a month, so meat would have been a very unlikely staple cooked or not-- not enough calories. First, the academic community were all old skeptics and said "but there were no fired around that time" In the past 5 years they've found what do you know? They found hearths and ash that was used up to 1.6 million years ago. Recent evidence makes this theory more likely, but they still had to include the meat bit to please the industry which pays off a lot of academics.”