Inbreeding shaped the course of human evolution

[Jean M]

New Scientist Magazine issue 2945 (28 November 2013)


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. 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." ...

That's in line with previous evidence of small populations, says Chris Stringer of 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." ...

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.

http://www.newscientist.com/article/mg22029453.500-inbreeding-shaped-the-course-of-human-evolution.html

[Jean M]

This theme harks back to a question that came up on the now defunct DNA Forums in May 2009 on why certain cultures are inventive. My reply:

It's largely a question of numbers. The larger the number of people within a society, the more likely it is that among them will be an inventive type who thinks up something new. An important factor is the size of the effective communicating group, so that inventions can be passed on and built on. The type of economy dictates the size of communities.

* Hunter-gatherer economy = low levels of human density in the landscape; small communications groups = slowest pace of technological change.
* Agricultural economy = higher population density; wider communications = faster pace of technological change.
* Industrial economy = very high population density; global communications = fastest pace of technological change yet known.

A physical environment which creates a cost/benefit ratio conducive to change seems to be the key to moving from one economic base to the next. An astonishing range of inventions were thought up by the Ancient Greeks, but the time/place just wasn't right for the Industrial Revolution it seems.

[Jean M]

That was just before Adam Powell et al., Late Pleistocene Demography and the Appearance of Modern Human behavior, Science 5 June 2009: Vol. 324. no. 5932, pp. 1298 - 1301:


The origins of modern human behavior are marked by increased symbolic and technological complexity in the archaeological record. In western Eurasia this transition, the Upper Paleolithic, occurred about 45,000 years ago, but many of its features appear transiently in southern Africa about 45,000 years earlier. We show that demography is a major determinant in the maintenance of cultural complexity and that variation in regional subpopulation density and/or migratory activity results in spatial structuring of cultural skill accumulation. Genetic estimates of regional population size over time show that densities in early Upper Paleolithic Europe were similar to those in sub-Saharan Africa when modern behavior first appeared. Demographic factors can thus explain geographic variation in the timing of the first appearance of modern behavior without invoking increased cognitive capacity.

[geebee]

I've always thought that that our ancestors of 90,000 years ago were probably more or less as intelligent as we are today, and that it is not increased intellect that explains the huge cultural and technological changes of especially the past 10,000 years or so.

But in many ways this is not anything new or surprising. I can remember even in high school -- which was moons of moons ago for me -- being taught that the rise of agriculture was associated with increased cultural and technological development.

Nor do I think that this was solely because larger communities are more likely to have "inventive types". I don't doubt they are, but in addition to having folks who invent, you need folks who can understand and pass on what has been invented. "Teachers" don't have to be researchers, they just have to be able to understand (mostly) whatever they're teaching [edit: and be able to communicate to others]. Many populations who may not have invented a particular technology, have nevertheless shown considerable ability to make use of it, once introduced.

One thing that is new, of course, is the ability to unravel ancient DNA, and to recognize the genetic evidence of inbreeding. If this is seen in widely scattered remains, it's a fairly logical inference that it likely reflects small, scattered populations.

It would be interesting to see whether ancient remains of H. sapiens show a similar pattern, or whether from the get-go Sapiens showed less inbreeding. Perhaps we gained ascendancy over our cousins not chiefly through superior intellect, but simply somewhat better numbers? Even a slight edge in that regard might make a significant difference over time.