The close relationship between man and chimp has just got cosier, according to a study which suggests that ancestors of the two species interbred at some point in the distant past to form fertile hybrids.
It is well established that chimpanzees are the closest living relative of humans but this is the first time that scientists have found evidence for hybridisation through interbreeding.
The astonishing conclusion comes from an exhaustive analysis of the genomes of humans, chimps, gorillas and monkeys published in the journal Nature. The researchers were particularly interested in the point at which the last common ancestor of man and chimp split into two separate species - the process of speciation that gave rise to the chimp and human lineages.
"The study gave unexpected results, about how we separated from our closest relatives, the chimpanzees," David Reich, of the Broad Institute of Harvard and the Massachusetts Institute of Technology, said. "We found that the population structure that existed around the time of human-chimpanzee speciation was unlike any modern ape population," he said. "Something very unusual happened at the time of speciation."
A comparison of the entire DNA of chimps and humans suggests that they split apart no more than 6.3 million years ago,far more recently than previously thought. The study found that an original split was probably followed by a phase of interbreeding and hybridisation, possibly with fertile female hybrids cross-breeding with male chimps, before the final split occurred.
Hybridisation is a common feature of how plants split into different species but scientists have not seen it as being so important in the speciation of animals.
The suggestion could explain why an early human-like fossil known as Toumai or Chad Man appears to date back to seven million years ago, before the split between chimps and man, Nick Patterson, a member of the research team, said. "The fact that the Toumai fossil has human-like features suggests human-chimp speciation may have occurred over a long period with episodes of hybridisation between the emerging species," he said.
The human genome carries the genetic information for making a human being but it also acts as a source of evolutionary history. Some parts of the genome are known to be older than others, meaning that they could date back to the common ancestor of chimps and man.
Dr Reich and his colleagues looked at the variation in evolutionary history across the genomes of humans and chimps. They found that the time from the beginning to the completion of divergence between the two species ranges over more than four million years, much longer than expected. In particular, the study found that the X chromosome - which occurs twice in women and once in men - is the youngest part of the genome and was still being modified just before the final split occurred.
"A hybridisation event between human and chimpanzee ancestors could help to explain both the wide range of divergence times seen across our genomes as well as their relatively similar X chromosomes," Dr Reich said. "That such evolutionary events have not been seen more often in animal species may simply be due to the fact that we have not been looking for them."
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