did inbreeding shape the course of human evolution?

newscientist |  TALK about an inauspicious beginning. For thousands of years our ancestors lived in small, isolated populations, leaving them severely inbred, according to a new genetic analysis. The inbreeding may have caused a host of health problems, and it is likely that small populations were a barrier to the development of complex technologies.

In recent years, geneticists have read the genomes of long-dead humans andextinct relatives like Neanderthals. David Reich of Harvard Medical School in Boston has now sequenced the Neanderthal genome and that of another extinct human, the Denisovan, to an unprecedented degree of accuracy. He presented his findings at a Royal Society meeting on ancient DNA in London on 18 November.

Describing the genomes as "nearly error-free", Reich says both species were severely inbred due to small populations. "Archaic populations had low genetic diversity, really extraordinarily low," he said. "It's among the lowest diversity of any organism in the animal kingdom."

One Neanderthal, whose DNA Reich obtained from a toe bone, had almost no diversity in about one-eighth of the genome: both copies of each gene were identical. That suggests the individual's parents were half-siblings.

That's in line with previous evidence of small populations, says Chris Stringerof the Natural History Museum in London. "In the distant past, human populations were probably only in the thousands or at best tens of thousands, and lived locally, exchanging mates only with their nearest neighbours."

Our genomes still carry traces of these small populations. A 2010 study concluded that our ancestors 1.2 million years ago had a population of just 18,500 individuals, spread over a vast area (PNAS, doi.org/dv75x8).

Fossils suggest the inbreeding took its toll, says Erik Trinkaus of Washington University in St Louis, Missouri. Those he has studied have a range of deformities, many of which are rare in modern humans. He thinks such deformities were once much more common (PLoS ONE, doi.org/p6r).

Despite the impact on health, it is unclear whether inbreeding could have killed off the Neanderthals and Denisovans. More likely is the effect of small populations on culture and technology, says Mark Thomas of University College London. Larger populations retain more knowledge and find ways to improve technologies. This "cumulative culture" is unique to humans, but it could only emerge in reasonably large populations. In small populations, knowledge is easily lost, which explains why skills like bone-working show up and then vanish, says Trinkaus.