In comparison with other animals, human beings have big brains. So, what happened? How the human brain grew is one of the most intriguing questions in human evolutionary studies. A few recent studies have identified some pieces of what is likely to be a very complicated puzzle. (Luckily, we have big brains to figure it out.)
Copies and Size
The first one looked at one specific protein domain, called DUF1220, which is encoded in a chromosome region that has previously been indicated in brain disorders such as micro- and macrocephaly (unusually small or large brains respectively). It turns out that the amount of gene copies coding for this protein domain has a significant influence on brain size: relatively few copies = small brain; a lot of copies = big brain? Interestingly, this holds not just in human beings, but also across the tree of life, so to speak. Where human beings have about 270 copies on average, chimpanzees have 152 copies, gorillas 99, marmosets 30 and mice 1. All this leads to the suggestions that the copy number of DUF1220 has been, and still is, a relevant factor in brain size.
Regulation and Activity
The second study focuses on the wiring rather than the size. By studying the gene activity in the brain tissue of humans, chimpanzees and rhesus macaques, the researchers went looking for differences between the three species. The region where most differences were found, was the frontal lobe. Here, the human brain shows significantly more complex gene regulation. When digging a little deeper, they were able to identify some key genes in these complicated regulatory networks. One was CLOCK, previously known for being one of the main genes involved in the regulation of the circadian rhythm (our roughly daily biological clock). Another one was FOXP2, aka one of the genes involved in speech and understanding language. So, it’s not just size. Complexity matters as well.
Epigenetics and Disease
And finally, the third study, which investigated epigenetic differences. More specifically, the researchers looked at differences in DNA methylation in between human beings and their close evolutionary cousins, chimpanzees. DNA methylation is the attachment of a methyl-group to a base in the DNA, which can change gene activity. The researchers found a substantial amount of genes shared between humans and chimpanzees that showed a different level of methylation. A high number of these genes are involved in diseases, which, according to the scientists, might be a relevant clue to our susceptibility to certain diseases. So, different methylation patterns can have serious functional consequences. Maybe our big brains come with a price?
Of course, there are many more factors that influence brain size and the evolution of the brain of this weird kind of naked, big-brained primate. The origin and workings of our complex brains are, unsurprisingly, very complex themselves.
Dumas, L.J., O’Bleness, M.S., Davis, J.M., Dickens, M.C., Anderson, N., Keeney, J.G., Jackson, J., Sikela, M., Raznahan, A., Giedd, J., Rapoport, J., Nagami, S.S.C., Erez, A., Brunetti-Pierri, N., Sugalski, R., Lupski, J.R., Fingerlin, T., Cheung, S.W., & Sikela, J.M. (2012). DUF1220-Domain Copy Number Implicated in Human Brain-Size Pathology and Evolution. The American Journal of Human Genetics DOI: 10.1016/j.ajhg.2012.07.016
Konopka, G., Friedrich, T., Davis-Turak, J., Winden, K., Oldham, M.C., Gao, F., Chen, L., Wang, G.-Z., Luo, R., Preuss, T.M., & Geschwind, D.H. (2012). Human-Specific Transcriptional Networks in the Brain Neuron DOI: 10.1016/j.neuron.2012.05.034
Zeng, J., Konopka, G., Hunt, B.G., Preuss, T.M., Geschwind, D.H., & Yi, S.V. (2012). Divergent Whole-Genome Methylation Maps of Human and Chimpanzee Brains Reveal Epigenetic Basis of Human Regulatory Evolution. The American Journal of Human Genetics DOI: 10.1016/j.ajhg.2012.07.024