Studying the varying genetic diversity of different population groups is one method for piecing together the migration history of our species. Many analyses find a bottleneck in non-African populations dating back to around 50,000 to 100,000 years ago. This coincides neatly with the current estimate of the first wave of anatomically modern humans out of Africa. When humans first migrated out of Africa, they created a genetic bottleneck. Because a minority of people migrated, they took a minority of total human genetic diversity with them to the new colonies in Europe and Oceania.
What has not been noticed before, however, is a bottleneck of specifically male genetic diversity around 8,000 years ago. A recent paper in Genome Research suggests that, for every 17 female humans who reproduced at this time, only one male human managed to pass along his DNA. What's lacking is an obvious explanation for this pattern.
The analysis was based on data from 456 people from Africa, Europe, Siberia, Oceania, the Andes, and various regions across Asia. The researchers analyzed Y chromosome DNA, the genetic material passed down the male line, which can be used to glean information about historical male populations. They compared it to analyses of mitochondrial DNA, which is the genetic material passed from every mother to all her offspring, used to reconstruct information on historical female populations.
This comparison allowed the researchers to establish the number of males and females who passed on their genetic material to the next generation. What they found was an unexpected, drastic drop in male reproduction around 8,000 years ago—a period when human populations were generally increasing.
The reasons for this drop are unknown at this stage. Was the bottleneck caused by fewer men reproducing or fewer men surviving to reproductive age? Cultural practices can often create a preference for children of one sex or another, resulting in sometimes drastic changes in demographics, so this could be the cause. But if that were the case, we’d expect the bottleneck to be limited to only certain population groups, and it wasn’t.
What we do know is that this was a time of significant cultural change for humans. The bottleneck coincides with the Neolithic Revolution, a period of rapid technological innovation that gave rise to agriculture, the domestication of animals, permanent settlements, and a surge in transport technology. The changes in human culture might somehow have created new reproductive pressures that disproportionately impacted on males. Rather than needing pure biological fitness, the data could suggest that cultural fitness was now important, especially for men.
The pattern seen in the data is actually consistent with increased competition, the researchers write. In certain cases, this kind of fierce competition could be limited to one or the other gender. It’s likely, they say, that these cultural changes made a more competitive environment for men, possibly due to distribution of resources—some men had significantly more wealth than others. Better transport, brought about by the wheel, horses, and camels, could also have led to male-driven conquests increasing reproductive competition among males in a given region.
Right now all of these ideas are just speculation. More research is needed to try to figure out whether there are other signals in the DNA that suggest cultural fitness was involved in the bottleneck. But, if the researchers are correct that human culture was the cause, this would be one of the more extreme examples found so far of gene-culture coevolution—human culture shaping human genetics.
Genome Research, 2015. DOI: 10.1101/gr.186684.114 (About DOIs).
http://arstechnica.com/science/2015/03/neolithic-culture-may-have-kept-most-men-from-mating/
