HUMANS and chimpanzees did not gradually evolve into different species by living geographically apart, as textbooks suggest. Our isolation from chimps occurred within our own chromosomes, not across the plains of Africa, says a team who compared human and chimp DNA.
The study gives rise to the theory that the chromosomes of our common ancestors became accidentally rearranged during reproduction, and this gradually led to genetic 鈥渘o-go鈥� zones between proto-humans and proto-chimps. The two could still mate, and swap genes between compatible parts of their chromosomes, but any mutations within non-compatible regions that conferred an evolutionary advantage would have been retained for good by each lineage.
鈥淚t became utterly impossible to swap genes between those zones, so they became areas where you had increasingly separated gene pools,鈥� says Arcadi Navarro of the Pompeu Fabra University in Barcelona, who co-analysed the DNA with Nick Barton of the University of Edinburgh.
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The implications are huge. First, it means the parting of the ways between human and chimp was much more gradual than previously believed. Instead of the sharp division brought about by geographic separation, common ancestors destined to become either chimps or humans were able to interbreed with each other until their chromosomes were no longer compatible.
Secondly, and more controversially, it means that the common ancestors of both species gradually diverged from one another despite sharing the same habitat and territory. 鈥淚t might explain discrepancies in anthropological data about the time and location of divergence,鈥� says Christophe Soligo of the human origins research group at the Natural History Museum in London. 鈥淚t implies there was a longer period of hybridisation than thought.鈥�
Humans have 10 chromosomes that have undergone different rearrangements to those of chimps. In 9 of these there are pericentric inversions 鈥� essentially a swapping of two large pieces of the chromosome around the centromere. The 10th was created by the fusion of two smaller chromosomes, which remain separate in chimps (see Graphic).
Navarro and Barton compared DNA from 115 genes, half of which sit on regions of the chromosomes that are rearranged differently in humans and chimps, and half on 鈥渃olinear鈥� chromosomal regions common to the two species.
Although gene mutations occur at the same rate in both ordinary and rearranged chromosomes, the researchers found that mutations accumulated twice as often in the rearranged segments. In essence, they are a refuge within which useful mutations can be retained and evolve, backing the hunch that this is the genetic engine driving development of separate species (Science, vol 300, p 321). 鈥淭hey are probably the genes that made the two species incompatible, producing traits that eventually stopped them choosing each other as mates,鈥� says Navarro.
Other researchers have tried unsuccessfully to implicate chromosomal isolation as the driving force for the development of different species. Their ideas foundered, says Navarro, because chromosomal rearrangements were thought to either be lethal to the recipient, or to render them infertile. However, experiments in fruit flies and sunflowers over the past decade have shown organisms can survive with rearranged chromosomes, which are then passed down the generations.
The divergence eventually prevents the exchange of genes and they become separate species. 鈥淔or the first time it provides a plausible explanation for how speciation might have taken place without geographical or ecological separation,鈥� says Soligo.