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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 psycholo-
gists 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 di erences 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 mathe-
matician 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 di erences between individual genes, they
analyzed di erences 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 expres-
sion of individual genes misses the vast majority of variation that
takes place between groups of genes. Though no new connec-
tions between genes and language have emerged yet, Geschwind
and his colleagues did find that most of the di erences 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 pro-
tein made by the FOXP2 gene in chimps is virtually identical to
that made in mice: just one amino acid di ers 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 di er 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.
Then, in October 2007, Pääbo and coworkers published a jaw-
dropping paper about FOXP2 in Neanderthals, evolutionary
relatives of modern humans that died out 30,000 years ago. The
researchers isolated parts of the FOXP2 gene from the bones of
two Neanderthals. Although they have yet to sequence the entire
gene, they found that Neanderthals and modern humans matched
at the two critical spots that separate humans and chimpanzees.
Though often depicted as knuckleheads, our closest hominid rela-
tives may have shared at least some of our capacity for speech and
language. "There is no reason to think that Neanderthals did not
have language as we do," says Pääbo. But he adds that the many
unknown genes involved in language will eventually have to be
found and looked at in Neanderthals.
Geschwind is continuing his hunt for those unknown genes,
applying to his behavioral-genetics work the technique he devel-
oped to compare human and chimp gene expression. His lab is
now doing the same sort of coexpression studies on brains from
healthy humans and schizophrenics, which he hopes will uncover
connections that are broken in schizophrenia and perhaps lead
to still more genetic pathways related to speech and language. He
hopes eventually to do similar analyses with autopsied brains from
people who had autism-spectrum disorders.
So far, Geschwind and his colleagues have found what amount to
some interesting genetic words that they've been able to string into
a few sentences to explain the roots of speech and language. They
can't yet tell a coherent story. Still, confidence is building that in the
not-too-distant future, scientists will be able to write a lengthy book
about how we evolved our phenomenal gift of gab, highlighting the
critical suites of genes that make it possible. If they do, they could
also find ways to correct disruptions to this network---disruptions
that can leave people at a serious loss for words.
JON COHEN, A SAN DIEGO BASED FREELANCE WRITER, IS WORKING ON A BOOK
THAT REËXAMINES THE DIFFERENCES BETWEEN HUMANS AND CHIMPANZEES.
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