Originally posted by Darfius
The product doesn't explain the process, Thousand. Our "common ancestor" (what apes and us supposedly branced off from) had ridiculously small brains.
"Ridiculously small"? Chimps, our closest relatives, have pretty big brains.
The process probably involved the ASPM gene. This gene and homologous genes in other organisms has a repetitive structure and there is a correlation between how many times it repeats and how large the nervous system is between species as well as within the Homo sapiens species. When this gene gets shortened, microencephaly is the result.
There is also a phenomenon called allometry in which mutations in regulatory genes can increase or decrease the rate of growth of part of the body relative to the whole. This explains the differences between the skull shapes of humans, chimps, and baboons; the human's cranium grows much faster than the facial bones compared to the chimp, while the baboon is the opposite. The result is three skulls which are very similar except for the proportions.
Now, let's hypothesize a "common ancestor" or CA for chimps and humans. This ancestor would be a tree dwelling ape of some sort. Mutations would provide genetic variability which would give variability to the cranium size and the brain size.
Some of these apes would continue to live in the forest and would evolve into chimpanzees. Others might be forced out of the forest, or might simply find resources available in nearby plains that there was no ape competition for. These latter apes would succeed if they were adaptable and had the other benefits a larger brain would provide. Natural selection would select for larger brains up to the limit of the skull's volume, and then mutation would provide more genetic variability in cranium size, and those with larger craniums would be able to handle larger brains that would later evolve, etc. Mutation would allow the cranium and brain to increase and decrease in size, and natural selection would allow those with larger brains to thrive, because larger brains provide the benefits that they do.