The Origin of Speech
Like songbirds, dolphins, whales, bats, elephants, and–of course–humans, monkeys and apes can learn sounds and use them to communicate. For many decades, researchers have attempted to decode such animal messages. They have also tried to teach chimpanzees, bonobos, gorillas, and orangutans to use symbols, lexigrams, and sign language, and a few poster apes like Koko, Washoe, and Kanzi have no small measure of fame thanks to PBS documentaries, magazine cover stories, and books about their communication skills. Some have even shown what appears to be a remarkable ability to understand spoken words.
Nevertheless, an impassable border separates our speech and language abilities from theirs. The best-trained apes can learn only a few hundred words. Most any human three-year-old has a larger vocabulary, and the average high-school graduate has a mental lexicon of about 60,000 words. Linguists and psychologists who have studied “talking apes,” including researchers who have taught them to communicate, stress that the animals rarely combine even two words into a semantic whole and never utter the type of complex “recursive” sentence–like this one–that embeds one thought in another.
In the hope of beginning to explain this discrepancy, Geschwind investigated which genes are turned on in the brains of humans and in those of chimpanzees, our closest genetic relatives. He found hundreds of differences but had no way to determine which ones mattered–which were most significant in driving evolution and determining brain function. Overwhelmed, he turned to a mathematician friend at UCLA, Steve Horvath.
With Horvath’s guidance, Geschwind and his grad student Michael Oldham arrived at a new way to approach the problem. Rather than looking at differences between individual genes, they analyzed differences between networks of genes expressed at the same time. Specifically, they looked at autopsied slices of human and chimp brains and compared these “coexpressed” genes in specific “modules,” including the cerebral cortex, the cerebellum, and the primary visual cortex.
They found that within each module’s networks, some genes served as hubs, connecting to many other genes. Diagrams of the networks look much like maps of airline routes, and both the human and chimp maps have a ridiculous number of hubs and spokes. But the diagrams make it easy to see the most important genes–those at the hubs. And when the team took the human map of a module and removed all the chimp connections for the same module, only a few genes were left. It became startlingly clear not only which genes are uniquely human, but also which of those are most important.
This approach yielded insights that weren’t possible with older techniques; simply comparing human and chimp expression of individual genes misses the vast majority of variation that takes place between groups of genes. Though no new connections between genes and language have emerged yet, Geschwind and his colleagues did find that most of the differences occurred in the cerebral cortex–the very part of the brain that expanded the most in humans, and in which Broca’s and Wernicke’s areas reside. Geschwind is hopeful that taking a broader view of not only the genome but also the transcriptome–the set of genes that are turned on at any given time–will lead to more insights into the genetics of language. “We need to understand the transcriptome in the same way we understand the genome,” he says.
So far, however, the most intriguing and concrete genetic clues to the evolution of speech and language have emerged from simple, direct comparisons of animal and human versions of FOXP2. “FOXP2 is paradigmatic,” says Geschwind. “It’s this beacon, and the first proof that this area of research might lead to great insights about human beings and evolution.”
Soon after Fisher, Monaco, and their colleagues linked FOXP2 to human speech and language, they teamed up with a leading evolutionary-biology group headed by Svante Pääbo at the Max Planck Institute in Leipzig, Germany. They found that the protein made by the FOXP2 gene in chimps is virtually identical to that made in mice: just one amino acid differs between the two. Biologists believe that if proteins undergo little alteration over an evolutionary span of tens of millions of years, they must perform such essential functions that they simply cannot tolerate change. But two amino acids in human FOXP2 differ from those in the chimp protein–a total of three changes from the mouse version. That the gene withstood such dramatic change in such a short time span (evolutionarily speaking) suggests that the change helped us survive–as the development of language surely did.