Human brain size has been shrinking over the last 20,000 to 30,000 years, potentially correlating with a reduced consumption of animal organs and meat.

“the association between the diet quality and the size of the brain so you can say that at the end of the stone age the diet quality started to decline so when it starts to decline actually individuals that have a smaller brain they don't demand that much energy because brain is a is a energy hungry organ they don't demand so much energy but still can manage to get plants they actually have a higher fitness and this is why you see an evolution towards brain size decline at the end of the stone age”