By Kerri Smith
Two tiny changes in the sequence of one gene could have helped install the mechanisms of speech and language in humans.
In 2001, a gene called FOXP2 was found to underlie a rare inherited speech and language disorder. It encodes a transcription factor called FOXP2, a protein 'dimmer-switch' that binds to DNA and helps to determine to what extent other genes are expressed as proteins.
Experiments have now revealed that the human version of FOXP2, which has two different amino acids compared with the version carried by chimps, has differing effects on genes in the brains of the two species. These differences could affect how the brain develops, and so explain why only humans are capable of language.
To find out whether these changes in FOXP2 had a biological function, a team led by Daniel Geschwind of the University of California, Los Angeles, inserted the two versions into human brain cells and looked at expression of the genes that the protein regulates. They found that the human version increased the expression of 61 genes and decreased the expression of 51 genes compared with the chimp version of the protein. To double-check that the same was happening in real brains, they looked at the expression of these genes in human and chimp brain tissue and found similar expression levels as in the cells. Their study is published in Nature.
Many of the genes looked at by the team are known to have roles in brain development and function, firming up the central place of FOXP2 in the brain's language and speech networks. They also affect soft-tissue formation and development, linking FOXP2 to the physical side of speech and articulation.
"I'm not a person who necessarily believes that one gene is going to tell us everything, but this was really quite remarkable and does place FOXP2 in a relatively central position," says Geschwind.
The study also lends weight to the idea that language didn't evolve from scratch. It "depended on the retuning of genetic pathways present in non-verbal ancestors, rather than the appearance of completely novel mechanisms", says Simon Fisher of the Wellcome Trust Centre for Human Genetics in Oxford, UK, and part of the team that discovered the gene and first linked it to language.
Whether FOXP2 is the main driver of the evolution of language in humans, or just a cog in the wheel, remains unclear, says Fisher. "It is worth remembering that a large number of genetic differences distinguish the brains of these two species, not just the substitutions in FOXP2," he adds.
Differences in the cells or even in the animals used for the analysis could also skew the picture. "The results could depend on particular cell-line samples or particular humans or chimps," says Wolfgang Enard, who studies the evolutionary history of FOXP2 at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Geschwind and his team now plan to dig deeper into the genes that FOXP2 regulates and find out whether these too are different in humans from in chimps. "It's plausible that since language is so important, that not only FOXP2 but many of its targets might be under selection," he says.
The team would like to know where these genes are expressed in the brain, and what kind of brain cells they are most active in. Their results could even throw up new candidates for genetic screening programmes aimed at identifying language impairments, Geshwind says.