Close cousins
We are genetically very similar to chimps - we share nearly all of our DNA sequence with them. That in itself does not tell us much - a single base pair difference can be hugely important or irrelevant, depending where it is found.
Of particular interest, therefore, are genes (more correctly, alleles) that show signs of positive selection - of variants being favoured by natural selection. To start with, analysis was done on a gene-by-gene basis, with a search for the alleles underlying known biological differences. Changes in lactase (linked to diary farming) and the sickle cell genes (associated with malaria resistance) were found in this way.
More recently, genome sequence analysis has created new tools to identify positive selection. In essence, this involves searching for particular signatures in the genome, such as regions that seem to be evolving faster than the expected natural 'background' rate of change.
This statistical analysis picks up possible positively selected alleles. The case for positive selection can be made stronger if the function of the gene can be identified. Typically, such genes affect reproductive biology, brain function or defence against infections.
Several alleles have been identified that may underlie key differences between humans and chimps. A number seem to affect the development and division of neurons, and hence brain size.
Possibly crucial is a gene known as FOXP2. This was discovered in a family with a speech and language disorder, who had a non-functional form of the gene. Chimp FOXP2 differs by only two amino acids. Although not the only gene needed for language development, the human version of FOXP2 could be one of the crucial differences between chimps and humans.
Sometimes, a gene is the same in chimps and humans, but it is controlled in different ways. As a result, although a protein has not changed, it is made at different times or in different places.
Finally, the differences between chimps and humans can be quite unexpected. By chance, researchers discovered that humans have a different form of myosin, a muscle protein. The human form makes weaker muscle. Chimps have very strong jaws and jaw muscle, which need to be firmly attached to the skull. Human jaw muscles are much weaker, which may have given the human cranium more freedom to expand to accommodate bigger brains. The human myosin gene is sometimes known as RFT, for 'room for thought'.