PEOPLE who have trouble accepting that we are descended from apes, and even some who are fine with the concept, will not be happy with this week鈥檚 news about human origins. Not only were our ancestors related to chimpanzees, they carried on mating with them long after our family tree branched from theirs.
It鈥檚 not as bad as it sounds. First, this startling idea is still only a hypothesis 鈥� albeit the one that best explains the unexpectedly limited differences between our own genome and those of our nearest ape cousins, the chimpanzee and gorilla. To know for certain will take much more genomic detective work.
Even if true, though, this is no sordid tale of scandal and perversion. After all, at the time any hybridisation would have happened, our ancestors had barely begun to walk upright and probably looked very much like the proto-chimps they interbred with (see 鈥淒id humans and chimps once merge?鈥�). Instead, this new glimpse of our history serves as a lesson in how fuzzy the boundaries of a species can be, and in how evolution bumbles along without any grand plan.
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Most people schooled in biology can trot out the textbook definition of a species: a group of interbreeding organisms that are unable to interbreed with any other groups. But every evolutionary biologist knows that the real world is much messier than this. Some species reproduce without sex and so cannot possibly interbreed. Others hybridise with closely related species in some parts of their range and under some conditions but not in other places and circumstances. Some species begin to split in two, only to collapse back together again (see 鈥淲hen evolution runs backwards鈥�). Out in the forest or on the savannah, it is often difficult to draw a clear line around a species.
Our family tree is messy, too. The process by which that knuckle-walking ape gave rise to two lineages 鈥� one that became chimps and one that became us 鈥� was drawn-out and shambolic. At times it would have been hard to decide if there was one species or two: evolution just selected what worked. Hybridisation between the two nascent species might have provided traits that helped save our ancestors from extinction.
Other speciation events could be equally complex, and the burgeoning of genomic information should soon let us find their traces in daughter species alive today. That should give us deeper insight into one of the key steps in evolution, the origin of species. It also reinforces a point that conservation biologists have made for a long time: every bit of biodiversity is valuable because you never know which holds the seeds of the next evolutionary innovation.
