After the invention of farming, for every 17 women who passed their genes on to the next generation, only 1 man did the same. These lucky few were the world’s first wealthy and powerful elites.
Previous DNA studies have identified mankind’s first major genetic bottleneck around 60,000 years ago, when a small tribe of humans left Africa and began to populate the rest of the world.
A second, unknown bottleneck, affecting only males across the world from 4,000 to 8,000 years ago, has now been identified by scientists from Arizona State University, the University of Cambridge, and the University of Tartu in Estonia.
The study, published this week in the journal Genome Research, analysed samples of Y-chromosome DNA (passed down the male line) and mitochondrial DNA (passed down the female line) from almost 500 volunteers from Africa, the Andes, South-Asia, the near East, Central Asia, Europe, and Oceania.
Across all of these 7 regions of the world, modern populations represented by the samples have many more female ancestors than male ancestors.
The team believes that this male bottleneck, which occurred during a period of global population growth, was caused by the rise of an elite few males who accumulated lots of wealth, power and access to females, at the expense of other males. The elite then passed on their wealth, and reproductive success, to successive generations of sons.
Farming was the cause of this inequality. Nomadic hunter-gatherer societies had a very flat hierarchy – men hunted for meat while women gathered edible plants, and the welfare of the society depended on all its members sharing its resources equally. With the emergence of sedentary farming societies, more reliable sources of food from cultivated plants and domesticated animals meant the production of more resources than were needed for mere survival. This left some members of society free to pursue occupations other than agricultural labour. A prestigious few, such as warriors and priests, accumulated great power and wealth. They concentrated it within their immediate bloodlines in the permanent communities which rapidly grew in fertile river valleys.
Only after thousands of years did male genetic diversity begin to rise again. In more recent history, a global average of around 4 women have reproduced for every 1 man.
One of the study’s lead authors, Melissa Wilson Sayres of Arizona State University, said that it seems human culture, not biology, kept the earliest male farmers from reproducing: “Instead of ‘survival of the fittest’ in a biological sense, the accumulation of wealth and power may have increased the reproductive success of a limited number of ‘socially fit’ males and their sons.”