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| Bonobos: all you need is love. |
By Ann Gibson, Science Now, June 13, 2012
Chimpanzees now have to share the distinction of being
our closest living relative in the animal kingdom. An international team of
researchers has sequenced the genome of the bonobo for the first time,
confirming that it shares the same percentage of its DNA with us as chimps do.
The team also found some small but tantalizing differences in the genomes of
the three species—differences that may explain how bonobos and chimpanzees
don't look or act like us even though we share about 99% of our DNA.
"We're so closely related genetically, yet our
behavior is so different," says team member and computational biologist
Janet Kelso of the Max Planck Institute for Evolutionary Anthropology in
Leipzig, Germany. "This will allow us to look for the genetic basis of
what makes modern humans different from both bonobos and chimpanzees."
Ever since researchers sequenced the chimp genome in 2005, they have known that humans
share about 99% of our DNA with chimpanzees, making them our closest living
relatives. But there are actually two species of apes that are this closely
related to humans: bonobos (Pan paniscus) and the common chimpanzee (Pan
troglodytes). This has prompted researchers to speculate whether the ancestor
of humans, chimpanzees, and bonobos looked and acted more like a bonobo, a
chimpanzee, or something else—and how all three species have evolved
differently since the ancestor of humans split with the common ancestor of
bonobos and chimps between 4 million and 7 million years ago in Africa.
The international sequencing effort led from Max Planck
chose a bonobo named Ulindi from the Leipzig Zoo as its subject, partly because
she was a female (the chimp genome was of a male). The analysis of Ulindi's
complete genome, reported online today in Nature, reveals that bonobos and
chimpanzees share 99.6% of their DNA. This confirms that these two species of
African apes are still highly similar to each other genetically, even though
their populations split apart in Africa about 1 million years ago, perhaps
after the Congo River formed and divided an ancestral population into two
groups. Today, bonobos are found in only the Democratic Republic of Congo and
there is no evidence that they have interbred with chimpanzees in equatorial
Africa since they diverged, perhaps because the Congo River acted as a barrier
to prevent the groups from mixing. The researchers also found that bonobos share
about 98.7% of their DNA with humans—about the same amount that chimps share
with us.
When the Max Planck scientists compared the bonobo
genome directly with that of chimps and humans, however, they found that a
small bit of our DNA, about 1.6%, is shared with only the bonobo, but not
chimpanzees. And we share about the same amount of our DNA with only chimps,
but not bonobos. These differences suggest that the ancestral population of
apes that gave rise to humans, chimps, and bonobos was quite large and diverse
genetically—numbering about 27,000 breeding individuals. Once the ancestors of
humans split from the ancestor of bonobos and chimps more than 4 million years
ago, the common ancestor of bonobos and chimps retained this diversity until
their population completely split into two groups 1 million years ago. The
groups that evolved into bonobos, chimps, and humans all retained slightly
different subsets of this ancestral population's diverse gene pool—and those
differences now offer clues today to the size and range of diversity in that
ancestral group.
While the function of the small differences in DNA in
the three lineages today is not yet known, the Max Planck team sees clues that
some may be involved in parts of the genome that regulate immune responses,
tumor suppression genes, and perception of social cues. The common chimpanzee,
for example, shows selection for a version of a gene that may be involved in
fighting retroviruses, such as HIV—a genetic variant not found in humans or
bonobos, which may explain why chimps get a milder strain of HIV (called simian
immunodeficiency virus) than humans do. Another difference is that bonobos and
humans, but not chimps, have a version of a protein found in urine that may
have similar function in apes as it does in mice, which detect differences in
scent to pick up social cues.
"This paper is a significant benchmark achievement
that lays the groundwork for other types of investigations into Homo-Pan
differences," says molecular anthropologist Maryellen Ruvolo of Harvard
University, who was not involved in the work. As researchers study the genome
in more depth, they hope to find the genetic differences that make bonobos more
playful than chimps, for example, or humans more cerebral. The bonobo genome
also should put to rest arguments that humans are more closely related to
chimps, says primatologist Frans de Waal of Emory University in Atlanta. "The
story that the bonobo can be safely ignored or marginalized from debates about
human origins is now off the table," says de Waal.
This item has been updated to reflect that chimps and bonobos
are two species of chimpanzees that are close enough to humans to share 99.6%
of their DNA. The international sequencing effort was led by Max Planck
composed of multiple teams including 454 Life Sciences in Branford,
Connecticut. The researchers also found that the ancestors of humans split from
the ancestor of bonobos and chimps more than 4 million years ago, not more than
5 million years ago as originally reported.
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