Reading the genetic instruction books of gorillas and chimpanzees has provided more insight into what sets humans apart from their closest primate relatives. The two new studies also provide details about how these primate species may have evolved.
Comparing a newly compiled genetic blueprint, or genome, of a western lowland gorilla named Kamilah with the genomes of humans and chimpanzees has revealed that the three species didn’t make a clean break when splitting from a common ancestor millions of years ago. Although humans are more closely related to chimps over about 70 percent of the human genome, about 15 percent of the human genome bears a closer relationship to gorillas. An international group of researchers reports the findings, which come from the first gorilla genome to be deciphered, in the March 8 Nature.
A separate study of western chimpanzees, published online March 15 in Science, also has implications for understanding the human-chimp split. The new work shows that humans and chimps have different strategies for shuffling their genetic decks before dealing genes out to their offspring. Neither humans nor chimps shuffle genetic material randomly across the genome. Instead, both species have what are called hot spots, locations in the genetic material where matching sets of chromosomes recombine most often, Gil McVean, a statistical geneticist at the University of Oxford in England and colleagues report.
Recombination is an important part of sexual reproduction. In the process of making eggs and sperm, chromosomes pairs are matched up and DNA is exchanged. Such shuffling of chromosome parts ensures that offspring will have different combinations of genetic variants than their parents. Changing recombination patterns may help keep newly separated species genetically distinct from each other.
McVean and his colleagues deciphered the genetic instructions of 10 unrelated western chimpanzees and constructed a fine-scale map of all recombination points, including hot spots. The map reveals that chimps tend to mix and match genetic material in regions where genes are most active. Humans tend to confine genetic shuffling to parts of the genome with inactive genes.
Chimpanzees’ shuffling pattern is similar to that seen in several other previously studied organisms, while the human pattern is unusual, McVean says. In humans, the location of recombination hot spots is determined by where a protein called PRDM9 latches onto DNA, but that protein doesn’t appear to be the driving force for recombination in chimps. “Everything points to humans being the odd ones out,” he says.
PRDM9 may play an important role in the development of separate species, McVean suggests. Previous work has shown that mouse species with different versions of the protein cannot breed, but giving different mouse species compatible versions of PRDM9 allows them to mate successfully. Humans are known to have a few different versions of PRDM9, but the new study shows that “in chimps, virtually every one has different versions of that protein,” McVean says.
The overlap seen with the human genome and Kamilah the gorilla’s genome seems to indicate that species don’t form in simple, rapid separations of gene pools, says Jeffrey Rogers, a primate geneticist at Baylor College of Medicine in Houston. “This example tells us it can be a little bit messy, a little complicated,” he says. Shared genes may reflect more intermingling between the ancestors of humans, chimps and gorillas than scientists previously thought.
But the mixed ancestry apparent in the genomes is not necessarily a product of interbreeding, says evolutionary geneticist Aylwyn Scally of the Wellcome Trust Sanger Institute in Hinxton, England. The degree of sharing seen among gorillas, humans and chimps may mean that the three species came from a large population of common ancestors or that the species split relatively recently. (With a smaller ancestor population or a longer time since species diverged, the human and chimp lineage would have accumulated even more DNA changes, obscuring the shared heritage with gorillas.)
Both humans and gorillas have undergone rapid evolution of some genes related to hearing, Scally and his colleagues discovered. Previously, scientists had speculated that the changes seen in human hearing genes were related to the development of speech, but the new gorilla data argue otherwise, Scally says. “The simple fact of the acceleration in humans isn’t enough to make connections to speech.”