A Bigger Brain
The history of human beings has long included a mystery. At two points in our ancient past, the brains in our evolving predecessors suddenly increased significantly in size. Sometime between one and two million years ago, the brain size of our forebears doubled over the short evolutionary time span of about 200,000 years. Then, between 500,000 and 100,000 years ago, another brain expansion occurred within our modern human form, Homo sapiens. How much did our brains grow? A very early human ancestor, Australopithecus, who lived 4.4 to 1.4 million years ago, had a brain volume of about 400 cubic centimeters, not much larger than that of the great apes. After the two significant periods of brain enlargement, the modern human brain of today is about 1350 cubic centimeters in volume. Our brains increased more than three-fold in a relatively short amount of time. (1,2,3)
Many hypotheses have been proposed to explain the need for a bigger brain. Here are some examples (1);
- Once humans began to stand erect, freeing their hands for other activities, they would benefit from a brain large enough to multitask.
- Larger brains would help in remembering important details such as where to find tasty and nutritious plant foods and/or developing the ability to plan ahead and predict where the desired animals to be hunted for food might be found.
- More sophisticated and precise motor nerve cells were needed for the making of tools.
- Increases in complexity of language would require more brain space.
- As societies became more complex, a larger brain would help in the organization of communities.
However, most of these developments occurred millions of years before the expansions of the brain.
Beyond the reason for an enlarged brain is the means by which it was accomplished. This is what has puzzled anthropologists over many decades. Human (Homo) species differ from the great apes in their much larger brain along with a smaller digestive system while maintaining the same basal metabolic rate as their close primate relatives. This seems to be a paradox. On one hand, the bigger brain, which demands about 20 to 25% of all the energy required by the human body; on the other, a reduction in gut size, a condition that doesn’t fit with the need for more nutrients to support the larger brain (2).
In 1995, anthropologists Aiello and Wheeler offered an answer. Their Expensive-Tissue Hypothesis stated that the increased metabolic requirements of larger brains were actually offset by the corresponding reduction in gut size. Aiello and Wheeler proposed that the human species living during the time of the brain enlargements began to eat increasingly greater amounts of animal products. Smaller intestinal systems are only compatible with higher quality food that is both more energy-dense and easier to digest. Evidence supporting this theory was mixed (4). In the early 2000s, a modification of the Expensive-Tissue Hypothesis was put forward. The Energy Trade-Off Hypothesis suggested that the cost of increased brain size could be better met by trade-offs with other metabolically expensive aspects of the human body such as body maintenance, locomotion and reproduction. For instance, the energy requirements of an increase in brain mass could be offset by more efficacious reproduction. This advantage would not involve an increase in energy needs for the parents but would take place through better care of their children (5,6).
Whatever method evolution utilized to achieve a larger brain, such an alteration could not happen without an obtainable energy-dense diet. At first it was theorized that an increase in hunting and scavenging meat or a transition to a more fat-rich diet (through eating such items as bone marrow and brain tissue) provided the additional calories. Tools and cut marks on fossilized animal bones certainly indicated that early humans processed and consumed animal-derived food. But there was another significant change in the lives of our predecessors. This was the mastering of more efficient food preparation skills such as cooking. Cooking dramatically enhances the availability of nutrients from both animal and plant products. Indications of the regular use of fire for cooking as far back as 1.5 million years ago exist and certainly there is copious evidence for the controlled use of fire from 400,000 years ago on. This timeline encompasses the period in which both brain expansions took place. (2)
Archaeologists generally agree that ancestors from our earliest beginnings existed on mostly plant matter such as leaves, fruits, nuts and underground storage organs (roots, rhizomes, tubers and bulbs). Such foods were a dependable source of starch-rich, energy-dense food. More recent archaeological sites involving archaic human species and early Homo sapiens continue to show that wild plant foraging and cooking, especially of starchy plants, was a widespread feature of their lifestyle. (2)
In 2007, a report published in the journal “Nature” suggested that starch might well have been the catalyst that stimulated human evolution and its rapid brain growth. Humans have up to fifteen times more copies of the gene responsible for producing the enzyme, amylase, in their saliva as their close relatives such as chimpanzees, a condition that has likely existed for at least the last million years of human evolution (7). Amylase is responsible for the breakdown of starch into its smallest components, digestible carbohydrates (simple sugars from plants such as glucose). Studies reveal that more copies of the amylase gene translate into higher levels of amylase in the saliva. In fact, the most abundant protein in human saliva is amylase. The brains of humans, both ancient and modern, run most proficiently on glucose. Researchers noted that the ability to process starch into glucose quickly and efficiently by beginning its digestion in the mouth would be a huge nutritional advantage in the support of a larger brain. In addition, fruits were seasonal, meat was difficult to obtain but starches from plants would offer a reliable energy source for the development and maintenance of the brain. (8)
Research from 2015 proposes that only a diet high in digestible carbohydrates would have been up to the task of providing the energy needed for the expansion of brain size that occurred during late human evolution. The human brain uses up to 25% of the body’s total energy needs and up to 60% of the glucose presented in the blood supply in order to function. The onset of cooking starchy foods was likely the key that increased availability of the energy required for the human tissues with high glucose demands, the brain, red blood vessels, and for the production of offspring. The enzyme, amylase, is inefficient in the digestion of raw starch, but cooking starch greatly facilitates the process. (7). Additionally, recent experimental work in modern humans suggests that important human goals such as successful reproduction is difficult on a diet completely made up of raw foods (2,9).
Science may never agree on any particular event that was the spark that triggered the expansion of the human brain or the exact process by which it was accomplished. But it is clear that plant-derived foods have played a major part in the diet of humans throughout their existence.
2 Yates, J.A., Velsko, I.M. et al. The Evolution and Changing Ecology of the African hominid oral microbiome. PNAS, May 18, 2021; 118(20): e2021655118.
4 Aiello, L.C., Wheeler, P. The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution. Current Anthropology. April,1995; 36(2): 199-221.
5 Tsuboi, M., Husby, A., Kotrschal, A., Hayward, A., Buechel, S.D., Zidar, J., Løvlie, H., Kolm, N. Comparative support for the expensive tissue hypothesis: Big brains are correlated with smaller gut and greater parental investment in Lake Tanganyika cichlids. Evolution. 2015; 69: 190-200. Doi.org/10.1111/evo.12556.
6 Isler, K., van Schaik, C. Costs of encephalization: the energy trade-off hypothesis tested on birds. Journal of Human Evolution. September 2006; 51(3): 228-224. Doi.org/10.1016/j.jhevol.2006.03.006.
7 Hardy, K., Brand-Miller, J., Brown, K.D., Thomas, M.G., Copeland, L. The Importance of Dietary Carbohydrate in Human Evolution. The Quarterly Review of Biology, 2015; 90 (3): 251. Doi:10.1086/682587.
8 Perry, G.H., Dominy, N.J., Claw, K.G., Lee, A.S., Fiegler, H., Redon, R., Werner, J., et al. Diet and the evolution of human amylase gene copy number variation. Nat Genet. 2007 Oct; 39(10): 1256–1260. Doi: 10.1038/ng2123.
9 Koebnick, C., Strassner, C., Hoffmann, I., Leitzmann, C. Consequences of a Long-Term Raw Food Diet on Body Weight and Menstruation: Results of a Questionnaire Survey. Ann Nutr Metab 1999; 43: 69–79. Doi.org/10.1159/000012770.