Our brain size has shrunk by about 200 ccs. It peaked around 150,000 years ago at about 1500 ccs and now modern homo-sapiens are about 1300 ccs.
That likely coincides with the fact that some of our food supply ran out. Some people make the argument that we were eating fattier sources of meat at some point particularly megafauna animals. If we look at the work of Lisa Smith out of the university of New Mexico, she shows the average mammal size at the beginning of the pliocene age (about 125,000 years ago) was about 500 kilograms. Now the average mammal size is approximately 9 kilograms.
So we had a tremendous loss of animal mass available to us to eat, so as that occurred…fortunately we were smart enough to figure out how to harness some calories from grains and other foods, and that’s sustained us and kept us alive as a population and it continues to do so today. However, it doesn’t help us to thrive optimally.
Grains and plant based foods are a sub-optimal source of nutrients, it can give us calories, yes, but they’re not very nutrient rich or bio-available.
This is a good news / bad news thing for the human body which is so resilient! We’re able to go long periods of time without eating our optimal diet, we can survive but we can’t thrive.
For example, during the Irish Potato Famine, people lived for six months at a time on shoe leather and seaweed. It wasn’t an ideal source of nutrients in any way shape or form, but it kept people alive.
So to our credit we evolve these different systems by which we can extract energy from different substrates, but that does not mean that it was either what we evolved and what our genes expect today or that it’s an ideal situation for us today.
Source: Baker, Shawn, guest. Saladino, Paul, guest. “Part 47 – The Great Meat Debate w/ Mark Sisson, Dr. Paul Saladino, & Dr. Shawn Baker” Peak Human, 18 July 2019. https://www.peak-human.com/home/the-great-meat-debate-with-mark-sisson-dr-paul-saladino-and-dr-shawn-baker