Kamila |
By Elizabeth Pennisi, Science Now, March 7, 2012
Ever since the human genome was sequenced a decade ago,
researchers have dreamed about deciphering DNA from our three great ape cousins
as well. Now the final remaining genome, that of the gorilla, is in hand, and
it reveals interesting connections between us and them. Surprisingly, parts of
our genome are more similar to the gorilla's than they are to the chimp's, and
a few of the same genes previously thought key to our unique evolution are key
to theirs, too.
Today there are four groups of great apes: chimps and
bonobos, humans, gorillas, and orangutans. The genome of the chimp—our closest
relative—was published in 2005; the orangutan sequence came out in early 2011. Now
researchers have analyzed the DNA of a western lowland gorilla named Kamilah,
who lives at the San Diego Zoo. In addition, they sequenced DNA from three
other gorillas, including one eastern lowland gorilla, a rare species estimated
at only 20,000 individuals. "It's essential to have all of the great ape
genomes in order to understand the features of our own genome that make humans
unique," says Gregory Wray, an evolutionary biologist at Duke University
in Durham, North Carolina, who was not involved in the study. Adds
paleoanthropologist David Begun of the University of Toronto in Canada:
"It will allow us to begin to identify genetic changes specific to humans
since our divergence from chimps."
Humans and apes are nearly identical in the vast
majority of base pairs, or letters of the genetic code: The human genome is
1.37% different from the chimp's; 1.75% different from the gorilla's; and 3.4%
different from the orangutan's, researchers from the Wellcome Trust Sanger
Institute in Hinxton, U.K., and their colleagues report today in Nature.
Although chimps and humans are indeed closest kin, 15% of the human genome more closely matches the gorilla's. Those genes' activity patterns are
similar too, says Sanger evolutionary genomicist and lead author of the study
Aylwyn Scally: "Some of our functional biology is more gorillalike than
chimplike."
Moreover, with all the great ape genomes to compare,
researchers are better able to assess when gorillas, chimps, and humans evolved—a
matter of current debate. The molecular data indicate that humans and chimps
went their separate ways only about 4.5 million years ago. But fossils that old
and older look either ape or protohuman, so some paleontologists argue for a
split as far back as 7 million years ago. Scally's group comes up with a date
of about 6 million years ago, adjusting what would have been a more recent
estimate by assuming that the mutation rate slowed over time in ape evolution.
Another possible complication is that interbreeding may have occurred in the
incipient species, slowing the actual separation of the DNA into distinct
genomes, Scally points out. The authors suggest that ancestors of the gorilla
separated from the human-chimp line about 10 million years ago, consistent with
previous estimates.
"I am very happy to see the authors conclude [with]
divergence dates that are consistent with both the fossil and genetic
records," Begun says. "Usually one line of evidence is used to
discredit the other."
The comparison with other sequenced genomes revealed
that over the course of great ape evolution, about 90% of the genome has been
influenced by natural selection. In each of the chimp, human, and gorilla, more
than 500 genes have been evolving faster than expected, suggesting that they
have changed in a way that confers some advantage. In the gorilla, one of the
faster-evolving genes is involved in the hardening of skin, as happens in the
knuckle pads for knuckle walking. Hearing genes that have evolved rapidly in
humans also show accelerated evolution in gorillas. Some researchers had
thought that those human genes might partly underlie language evolution, but
that idea will need some rethinking now, Scally says. The researchers also saw
parallel acceleration in the evolution of genes for brain development in
gorillas and humans.
As part of the study, Scally and colleagues compared the
genomes of eastern and western lowland gorillas and concluded that the species
split up 0.5 million years ago but continued to interbreed a little after that.
Moreover, many millennia ago, for some unknown reason, the eastern lowland
gorilla population severely shrank, causing low genetic diversity and low
numbers. Some researchers have suggested that the small population is due to
recent human activity, but the new analysis suggests "it's a long-term
thing," Scally says. "That's not to say that current human activity
hasn't been a big threat."
Much more work remains to be done comparing the gorilla genome
with other great apes. But already "it has provided a lot of important
information regarding mechanisms and timing of speciation, gene evolution,
[and] gene flow between [species and] subspecies," says gylcobiologist and
great ape expert Ajit Varki of the University of California, San Diego, who was
not involved in the study. "This information is, of course, of great
interest to many investigators."
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