Modern humans - Homo sapiens sapiens - have notably big heads - with large brains inside. When lined up next to chimpanzees, gorillas, baboons and other primates humans have proportionally larger skulls and brains, and smaller rib cages and jaws. Compared to other primates and mammals, humans and our earlier hominid ancestors devote less body space and energy to digesting food - and far more to energetically supporting our big brains. This is called “encephalization” by primatologists, that is we have relatively large brain mass relative to our body mass.
Why and how did we evolve in this way? Although the science isn't settled there are intriguing theories and they involve food as fuel for those big brains.
Humans and our hominin bipedal ancestors such as Homo heidelbergensis and Homo neanderthalensis have been transforming the natural environment in order to more efficiently seize and process nutrients for millennia.
According to some anthropologists humans could develop intellectually (and therefore become more socially and culturally complex) by shifting energy away from our guts and toward our brains. The “expensive-tissue hypothesis” is the idea that big brains are energetically demanding tissues and in order to support them food has to be easily digested. Brains demand quality glucose as fuel. Glucose is the only fuel that our brains use.
Humans cannot eat cellulosically dense grasses, as do ruminants and other big-gut animals whose digestive systems expend energy to break down bulk. Rather humans eat easily digested grains, seeds and nuts and meats. Further, milling, crushing, cooking and fermentation and other preparation processes unlock nutrients before eating and aid digestion. So our refined-fuel diets and our intellectual capacity are directly connected according to this hypothesis. We use tools and fire to harvest and process food. Over time we evolved small teeth and slimmer bodies because we didn't need big digestive factories like cows and gorillas because we start to process our food outside of our bodies.
The earliest use of stone tools is traced to Homo habilis some 2.8 million years ago. Homo erectus and Homo ergaster were the first hominids to leave Africa and are believed to be the first to use fire and complex tools. Capturing energy by using our brains has been going on a long time.
References and additional reading:
Aiello, Leslie C., and Peter Wheeler. 1995. “The Expensive-tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution”. Current Anthropology 36 (2). [University of Chicago Press, Wenner-Gren Foundation for Ant