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 and extinct relatives like Neanderthals. 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 in London on 18 November.
Describing the genomes as 鈥渘early error-free鈥�, Reich says both species were severely inbred due to small populations. 鈥淎rchaic populations had low genetic diversity, really extraordinarily low,鈥� he said. 鈥淚t鈥檚 among the lowest diversity of any organism in the animal kingdom.鈥�
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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鈥檚 parents were half-siblings.
That鈥檚 in line with previous , says of the Natural History Museum in London. 鈥淚n 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.鈥�
Our genomes still carry . 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 ().
Fossils suggest the inbreeding took its toll, says 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 ().
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 of University College London. Larger populations retain more knowledge and find ways to improve technologies. This 鈥�cumulative culture鈥� is unique to humans, but it could only emerge in . In small populations, knowledge is easily lost, which explains why skills like bone-working show up and then vanish, says Trinkaus.
Tiny populations may have prevented Neanderthals and Denisovans from developing cumulative culture. 鈥淚t would place some limits on their cultural complexity,鈥� says Thomas. The same thing held our species back, until the population reached a critical density, unleashing the power of culture 鈥� at which point there was no stopping us.
鈥淭iny populations may have prevented Neanderthals from developing more advanced technologies鈥�
The genomes also show that early human species interbred with other hominins (see 鈥�We鈥檙e all Homo, aren鈥檛 we?鈥�). Many of us carry genes from Neanderthals, or from the mysterious Denisovans, who are known only from a single cave in Siberia.
We鈥檙e all Homo, aren鈥檛 we?
Early hominins weren鈥檛 picky about their sexual partners. We already knew that our species, Homo sapiens, interbred with two other hominin species, the Neanderthals and Denisovans.
Now it looks like the Denisovans did some interbreeding of their own. Some stretches of the Denisovan genome look much older than the rest, says of Harvard Medical School. The most likely explanation, he says, is that the Denisovans interbred with an unidentified species and picked up some of their DNA. The question is, who?
It could be evidence of a new species of hominin, as yet unknown to science. Alternatively, it could be the first genetic record of one of the many known species. of the University of T眉bingen in Germany suspects it was the latter, since many hominin species identified from their fossils have never been genetically analysed.
The most likely candidate is Homo heidelbergensis, says of London鈥檚 Natural History Museum. This species lived between 600,000 and 250,000 years ago, and spread from Africa into Europe and western Asia. That means Denisovans, whose ancestors followed a similar path, could well have met them.
This article appeared in print under the headline 鈥淚nbreeding shaped human evolution鈥�
