You Are How You Eat

Blood was rushing through my veins as I gasped at the violent duel between the magnificent Tyrannosaurus Rex and a genetically enhanced super dinosaur (don’t worry, no spoilers!). This was a few weeks ago, and while the taste of the buttery movie theater popcorn no longer lasts on my tongue, the uneasiness of my physical insignificance in comparison to the Mesozoic Era giants does. I left Jurassic World feeling thankful for the ice age that starved the dinosaurs to death and paved the way for mammals to grow and flourish.

Maybe I’m self-congratulating when I say this, but I find it kind of amazing that a mere 210 million years later, we (as in mammals) went from the meek, insect eating scum of the planet to not only large and diverse creatures like lions, giraffes, wolves and what have you, but, and almost more importantly, highly intelligent creatures like humans. What happened, in the evolution of our species, that led us to diverge from the rest of life and transcend into a richer and more intelligent (or not, depending on how you look at human actions) existence?

One thing is for sure: in order for our brains to grow so large and complex (the human brain is three times larger than that of our closest living relative, the chimpanzee), we would have needed a lot of energy. Since energy is derived from food, you would think that the mammal that ate the most would be the most intelligent. On the contrary, most mammals that consume large amounts of food (such as big cats, grazing animals, and more) spend most of their time simply digesting their food. Indeed, these animals have grown extensively complicated digestive tracts, teeth, and hunting and gathering strategies. Their brains, however, have not kept up with ours.

This led researchers to posit that, in fact, it wasn’t how much we ate that led to our intelligence. Rather, it was how we ate. Paleoanthropologist Leslie Aiello’s expensive tissue hypothesis (ETH) argues that about 1.5 million years ago, primates and early humans began to eat meat, not only a food source with a high number of calories, but also one that is compact and easy to digest without a large intestinal system. Historians studying parasites using isotopic ecology have known to find similar tapeworm species in early hominids as in hyenas, alluding that we may have shared similar carrion with these known scavengers. Furthermore, our extensive use of tools and cooking techniques also aided in making foods easy to digest. In fact, a recent study with chimpanzees, our closest primate, showed that they necessarily prefer to eat cut and cooked food over their regular wild life diet!

Eating high caloric foods also had social implications. While in most primate species, there is no concept of food sharing between females and their offspring, difficulty in attaining these high energy foods probably led to cooperativity among early humans, thereby strengthening the bond between females and their offspring. It also could have led to the division of labor between sexes as the males would hunt and provide food, whereas the females would raise the young and face intense motherhoods.

The ETH, as its name foreshadows, is based on the fact that creating brain tissue is a highly expensive endeavor. In fact, a gram of brain tissue takes about 20 times more energy to grow and maintain than a gram of kidney, heart, or liver tissue. Although more recent studies have shown that there may actually be no tradeoffs in organ tissue development and brain tissue development, there may still be energy tradeoffs with other tissue that are less expensive than visceral organs but are very abundant.

Indeed, Aiello and her colleagues don’t argue that eating meat gives you a larger brain; instead, their claim is more on the evolutionary history of how our larger brain could have occurred. Our divergent eating habits from our primate cousins is just one piece of a larger puzzle. Large scale correlation studies on brain mass in comparison to the mass of other organs and fat deposits in a variety of different species have revealed that there is definitely more the our brain size than just that fact that we started eating meat and cooked food.

The study concludes that starting with early hominids, there could have been three sources for the increased brain size. First, as the ETH claims, improved diet quality (meat consumption, tool-assisted food processing, and, later on, cooking). Second, they tried to eat regularly and therefore diminish large volumes of fat deposits, even when living in seasonal habitats. Finally, the social aspect of food sharing and cooperative breeding and living. Furthermore, brain enlargement could also be caused by allocating energy to the brain by decreasing costs of locomotion; that is, bipedalism, or walking on two feet, is a much more cost effective way that quadrupedal locomotion. All of these paths to energy effectiveness are seen in some way or form in many different species. However, it was a unique mix of all of them that caused the extraordinarily large brain size of our particular lineage.