VOA标准英语2009年-Study Provides New Evidence for Geneti（在线收听）

Véronique LaCapra | Washington 02 December 2009

 
A very small genetic difference between humans and chimpanzees may help explain why chimpanzees can't speak, but humans can

The ability to communicate using complex spoken language is a uniquely human characteristic, but surprisingly little is known about how we developed language when our close primate relatives, like chimpanzees, did not. A single gene may hold much of the answer.

Researchers investigate the genetic basis of language

Dan Geschwind is a professor of neurology, psychiatry and human genetics at the University of California in Los Angeles. He says we still do not know much about how language evolved in humans. "But it clearly is going to reside in changes in genes: either new genes, or changes in old genes that gives them new functions."

Genes carry hereditary biological information that determines much about who we are, from our height to our hair color to which hand we use to write with. Geschwind has been studying the function of one particular gene, called FOXP2. "It's one of the few genes that's been very clearly tied to the capacity for human speech and language."

FOXP2 works in an interesting way, Geschwind says. "It turns other things on and off."  Specifically, it regulates other genes. In a cell, FOXP2 acts like a master switch, producing a protein that binds to other genes and increases or decreases their activity.

FOXP2 protein almost identical in humans and chimps

In spite of FOXP2's apparent role in human speech, it turns out that our FOXP2 protein is almost identical to the version found in our closest primate relative, the chimpanzee. "A protein like FOXP2 is made up of hundreds of amino acids," says Geschwind. "And just two of them are different between human and chimpanzee."

Previous work by other researchers had suggested that the amino acid composition of the human FOXP2 protein may have changed at about the same time in evolutionary history that humans started to speak. Could a difference in just two amino acids trigger enough changes in downstream gene function to create the capacity for language?

In humans and chimps, FOXP2 behaves differently

Genevieve Konopka, a postdoctoral fellow in the Geschwind lab, led a study to look into this question. She says the purpose of the study was to determine whether the difference between the human version of FOXP2 and the chimpanzee version of FOXP2 would have any functional consequence in neurons.

Brain cells expressing human FOXP2. Cells that are orange/red have high FOXP2 expression

Konopka looked at the effects of FOXP2 on human neurons, or brain cells, in cell cultures in the lab. She manipulated the cells to express - or produce - either the human version of the FOXP2 protein, or the chimpanzee version. "And then we used something called a microarray, which allows you to examine the expression of every gene in the genome."

Konopka found that the human and chimpanzee versions of FOXP2 did function differently in human brain cells, targeting different genes and triggering different levels of gene activity.

Dan Geschwind says similar differences in gene expression were observed in brain tissue samples taken from humans and chimps that died of natural causes. The samples were frozen soon after death to retain the characteristics of living tissue.

The researchers saw a very big overlap in what the genes were doing in the cell culture, and in the brains. "And this gave us a lot of confidence that what we were actually seeing was relevant to brain function."

Genes regulated by FOXP2 likely to play role in language and speech

Geschwind says that many of the genes and proteins affected by FOXP2 are known to have functions in the brain, but that others may be involved in the development of the physical apparatus of speech, like the larynx and vocal chords.

Because to have spoken language, two things had to happen, says Geschwind. The first was a change in vocal anatomy. "Our vocal chords, tongue, and all of that changed to allow us to speak. But of course the major thing that happened was the changes in the brain that gave us the capacity for language."

Geschwind emphasizes that speech and language are very connected, and that at least one connection may be via FOXP2.

Geschwind and Konopka say the next step in their research will be to look more closely at the genes FOXP2 regulates, and to investigate their possible role in disorders affecting speech and language. Their current study is published in the journal Nature.