Sex, spines and human evolution

Scientists from Stanford University, California, have compared human and chimpanzee genomes, revealing intriguing differences that may underlie the evolution of our species.

The researchers found 510 DNA sequences that were conserved in our closest relative, the chimpanzee, but lost in humans. 509 of these sequences were located in non-protein coding regions of the genome. Typically, these regions regulate when and where other genes are turned on.

'The goal of the project was to find molecular lesions that underlie human evolutionary traits, with the examples illustrating different aspects of the principle', said Professor David Kingsley who led the study alongside Dr Gill Bejerano.

One of the deleted DNA sequences was next to the androgen receptor (AR) gene, which mediates the body's response to male sex hormones such as testosterone. By inserting the chimp sequence into mouse embryos, the scientists found that it was responsible for the growth of penile spines. These structures are found in most male mammals and are thought to increase sensitivity for the male, thus reducing copulation time. In some species they are thought to cause damage to the female.

It has been suggested that the loss of penile spines may be associated with stronger pair bonding in humans and the development of a predominantly monogamous society. Dr Bejerano said: 'It is a small but fascinating part of a bigger picture about the evolution of human-specific traits. We add a molecular perspective to a discussion that has been going on for several decades at least'.

A second human-specific deletion, next to the tumour suppressor gene GADD45G, has been linked to the evolution of the larger human brain. The deleted sequence puts a brake on the growth of specific brain regions, and its absence may have enabled humans to develop larger brains. Human brains are three times the size of chimpanzees' and the cognitive processes that this enabled are thought to be fundamental to the evolution of humans as a species

Further investigation into the other lost DNA sequences is expected to unearth more associations with human-specific characteristics. Professor Kingsley noted that we are now 'beginning to tease out some of the molecular differences that make us human'.

The study was published in the journal Nature.