DNA methylation is the term for a modification to certain DNA bases that involves the addition of a methyl group. This slows down gene expression, which may or may not be advantageous in context – though if you remove it entirely in mice you kill them outright. To cut to the chase, in late August a paper – Divergent Whole-Genome Methylation Maps of Human and Chimpanzee Brains Reveal Epigenetic Basis of Human Regulatory Evolution (pdf, supplemental data) – came out comparing levels of methylation, concentrating on the brain, between humans and chimpanzees. They found differences, particularly in regions apparently associated with diseases that we suffer from more than chimps do. Here’s a figure stolen from their supplemental information:

"The mean fractional methylation levels (± S.E.) of different genomic regions." (From page two)

“NS” stands for “not significant,” but the ones with stars are. Something to note with that graph is that there is a hidden 20% below the axis, so the differences are not quite as large as they would appear. Continue reading