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Women Were Out Reproducing Men 4,000 Years Ago

1335 Women Were Out Reproducing Men 4,000 Years Ago
Ancient rock drawings including one showing a farmer using oxen. A decrease in genetic diversity occurred during a time period when humans in different parts of the world had become sedentary farmers / Olof Ekström, 2003, Wikimedia Commons

Researchers have discovered a massive decline in the genetic diversity of male lineages some 4,000 to 8,000 years ago: Only a limited number of males were reproducing, likely the wealthiest and the most powerful. Meanwhile, female genetic diversity was on the rise in comparison, according to findings published in Genome Research last week. 

High levels of genetic diversity benefits a group in many ways. When individuals carry genes that are different than others, the population has a better chance of surviving diseases, for example, and it also helps reduce the chances of passing on unfavorable traits. “When a doctor tries to provide a diagnosis when you are sick, you’ll be asked about your environment, what’s going on in your life and your genetic history based on your family’s health,” Arizona State’s Melissa Wilson Sayres says in a news release. “If we want to understand human health on a global scale, we need to know our global genetic history.”


For example, about 50,000 years ago, there was a major decrease in genetic diversity—called a bottleneck—that occurred as a group of our ancestors left Africa to explore the rest of the world. Signs of this bottleneck can be found in most genomes of modern non-African populations. 

A huge international collaboration led by University of Tartu and University of Cambridge researchers studied DNA samples from the saliva or blood of 456 people living in seven regions across five continents: Africa, Europe, Oceania, the Andes, and South Asia, the Near East, and Central Asia. They examined both the Y chromosome DNA (passed down through males) and mitochondrial DNA (inherited from mothers). 

Their modeling revealed another bottleneck 4,000 to 8,000 years ago. It occurred during what was supposed to be a period of global growth as hunter-gatherers gave up the nomadic life to become sedentary farmers—and it was specific to males. In other words, there was an extreme reduction in the number of males who reproduced, but not in the number of females. 

“The striking difference in the number of reproductive males and females in that time window certainly affected the diversity of genes on the male genetic line,” lead author Monika Karmin from the University of Tartu says in a statement. Across the planet, for every 17 women who were reproducing, only one man was doing the same, Pacific Standard reports. For the full infographic click here


So why this strange dip? Unless there was a fatal event that affected only males across multiple continents, the team thinks it was something cultural. “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,” Wilson Sayres adds.

Next, the team is gathering more DNA to increase the diversity of their samples. 

Images: Wikimedia (top), Sabine Deviche (middle) via ASU 


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