Researchers have assembled the complete genome of the bonobo, an African ape that is one of humans' closest relatives.
The achievement, reported Wednesday in the journal Nature, marks a milestone. Adding the bonobo genome to the already-sequenced human, chimpanzee, gorilla and orangutan genomes gives scientists a complete catalog of the DNA of all of the so-called great apes.
That should help researchers better understand how humans evolved, scientists said.
"There's a common ancestor that we and these apes were derived from. We want to know what that ancestor looked like," said Wes Warren, a geneticist at Washington University in St. Louis, who was not involved in the research. "By adding the bonobo to the mix, we have a better idea."
Now, with all the great ape sequences complete, scientists can better use genetics to help determine if a particular trait cropped up for the first time in humans, said Kay Pruefer, a postdoctoral researcher at the Max Planck Institute in Leipzig, Germany.
Pruefer, who was first author of the bonobo genome study, worked with an international team to sequence the DNA of Ulindi, a female bonobo who lives at the Leipzig Zoo.
The work revealed new details about bonobos' early history, he said.
Bonobos and chimps -- which are very closely related but behave in strikingly different ways, with bonobos relying on sex and chimpanzees relying on aggression to resolve conflict -- split from each other about 1 million years ago.
Comparing the genome of Ulindi and a few other bonobos with those of chimps from different parts of Africa, Pruefer and his colleagues found that bonobos share similar amounts of DNA with all of them. That suggested that the split between chimps and bonobos was rapid and complete, with little if any mating between neighboring groups of chimps and bonobos, he said. Otherwise, one would have found that the chimps closest to the bonobos' territory would be more genetically similar.
Such a clean break is unusual among apes, he added: Breeding between early humans and Neanderthals, for example, is still evident in the DNA of people living today.
The clean split may have resulted from the formation of the Congo River, which bisected the territory of the species' shared ancestor, the team wrote.
Ancient humans split away from bonobos and chimps about 4.5 million years ago.
Examining Ulindi's DNA alongside the human genome, the team calculated that about 3% of the human genome is more closely related to bonobos or to chimpanzees than those animals are to each other.
That was surprising, Pruefer said, because previous studies of the bonobo genome had suggested that only about 1% of human DNA would be so similar to the apes' DNA.
John Hawks, an anthropologist at the University of Wisconsin who was not involved in the research, said it would be interesting to learn how these genetic differences influenced behavior.
Perhaps the three species' shared ancestor was more like a chimp -- and humans and chimps share genes, say, for aggression. Or perhaps the shared ancestor was more like a bonobo, with some other consequence for human traits.
"What branch do humans come from? Is it 'Make love, not war,' or 'Make war, not love'?" Hawks wondered.
But for now, Pruefer said, it remains unclear what genetic differences between humans, chimps and bonobos have any bearing on human traits.
"The genome is a resource for further study. You have to go and test the genes," he said.
He said he expected scientists would soon begin doing just that.