Much has been written in the scientific literature on our brain size increase, which has doubled in the timespan of about 3 million years. It is assumed that our brains became bigger so we could become smarter. However, recent data shows that the amount of blood our brains use dramatically increased over the course of human evolution—the amount of blood our brains use increased some 600 percent over the course of human evolution, substantially more than our brain size increase (350 percent).
Seymour, Bosiocic, and Snelling (2016) showed that while there was a 3.5-fold increase in brain size while there was a 6-fold increase in total cerebral blood flow rate. This is due to increased interneuron connectivity, synaptic activity and cognitive function which all depend on the cerebral metabolic rate. This is yet another reason why cooking was so important during our brain evolution. If the brain has a higher metabolic rate, only a high-quality diet will allow it to function. This can only occur if and only if there is a high-quality diet in the first place.
The metabolic intensity of cerebral tissue in our lineage could only be satisfied by a high-quality cooked diet. Clearly, the evolution of the human brain most always goes back to nutrition and the quality of the human diet. Without erectus’ control of fire around 1.5 mya, our brains wouldn’t have been able to grow this big, nor would we have the cerebral blood flow we eventually had. The below picture is figure 1 from the paper. The left slide is Australopithecus Afarensis, the middle is a Neanderthal, and the right is archaic Homo Sapiens.
They measured the lumen radius of the internal carotid arteries and were able to deduce that there were large changes in cerebral blood flow in hominin evolution due to the increasing size of the ICAs. Arterial size, blood flow rate and metabolic rate are tightly related. So if there are bigger ICAs, then that hominin had more blood flow to feed a bigger brain. This is clear evidence that as our brain size increased that we needed more blood to feed our growing brain.
Kilroy et al (2013) hypothesize that due to widespread anatomical differences in the anterior cingulate cortex (ACC), PFC and insula and subcortical cortices, those regions must be a “central node of the brain’s network underlying individual differences in intellectual development throughout childhood and adolescence.” Cerebral blood flow in the subgenual/ACC correlates the highest with IQ. They also showed that it’s possible to delineate “where CBF is modulated by IQ.” More blood flow in these regions means a higher IQ. Since the ICAs grew larger over the course of hominin brain evolution to increase intelligence, it’s no surprise that more blood flow to certain parts of the brain is related to higher intelligence in children and adolescents.
Even CBF at rest is correlated with higher intelligence and creativity (Takeuchi et al, 2011). They showed that gray and white matter in the brain is correlated with CBF at rest and significantly and positively with psychometric intelligence. Further, the Raven’s Advanced Progressive Matrices (RAPM) and scores on the creativity test that were administered to the cohort correlated positively with white matter and cerebral blood flow. They also noticed that there was an association between negative mood and increased cerebral blood flow. Grey and white matter CBF at rest were both correlated with the RAPM and the creativity test administered. This is yet more evidence that blood flow to certain parts of the brain dictates intelligence (and most likely individual differences in intelligence as well).
The more vampiric a brain is (especially in certain regions), the higher one’s intelligence will be, on average. By looking back at the fossil skulls of our hominin ancestors and the radius of the ICA, we can infer that as hominin evolution ‘progressed’ through time, the ICA radius increased which meant increased blood flow to the brain. This is directly related to brain metabolism and could only be afforded with a high-quality diet which started with the advent of tool-making and the use of fire to cook by erectus. Cerebral blood flood in the anterior cingulate cortex is significantly and positively correlated with IQ. CBF at rest is also correlated with IQ and certain regions of the brain. This shows that a brain with a higher metabolic rate will be, on average, more intelligent than a brain that has a lower one. The current data on intelligence and CBF points to increased blood flow in certain parts of the brain is related to higher levels of intelligence. This does make sense, as our blood flow to the brain increased by 600 percent over the course of human evolution. So, in a way, we can say that along with our brain size increasing for expertise capacity (which was most definitely needed over the course of hominin evolution) (Skoyles, 2009) along with more cerebral blood flow due to larger arteries and a higher metabolic rate.
This does make sense, as our blood flow to the brain increased by 600 percent over the course of human evolution. So, in a way, we can say that along with our brain size increasing for expertise capacity (which was most definitely needed over the course of hominin evolution) (Skoyles, 2009) along with the need for more blood to the brain to increase intelligence (as blood will also shuttle oxygen to the brain). This is yet another reason why our not-so-special brains are remarkable compared to the rest of the animal kingdom—the one variable that gives us our cognitive superiority over other animals is the ability to cook and use fire. A lot of our physiologic, anatomic and brain evolution can be explained simply as: no cooking, fire, and meat, no big brains (and as a consequence, everything you see around you today would not be here), and the only thing that can drive such a metabolically demanding brain is cooking and eating high-quality foods. The outstanding number of neurons crowded into our cerebral cortex along with much blood our vampiric brain guzzles explains our cognitive superiority over other animals.
Kilroy, E., Yan, L., Wang, D. J., Dapretto, M., Mendez, M. F., Liu, C. Y., & Kim, Y. C. (2011). Relationships between Cerebral Blood Flow and IQ in Typically Developing Children and Adolescents. Journal of Cognitive Science,12(2), 151-170. doi:10.17791/jcs.2011.12.2.151
Seymour, R. S., Bosiocic, V., & Snelling, E. P. (2016). Fossil skulls reveal that blood flow rate to the brain increased faster than brain volume during human evolution. Royal Society Open Science,3(8), 160305. doi:10.1098/rsos.160305
Dr. John R. Skoyles (1999) HUMAN EVOLUTION EXPANDED BRAINS TO INCREASE EXPERTISE CAPACITY, NOT IQ. Psycoloquy: 10(002)
Takeuchi, H., Taki, Y., Hashizume, H., Sassa, Y., Nagase, T., Nouchi, R., & Kawashima, R. (2011). Cerebral Blood Flow during Rest Associates with General Intelligence and Creativity. PLoS ONE,6(9). doi:10.1371/journal.pone.0025532