If you’ve ever wondered why people have varying skin tones, it’s because about 1.2 million years ago, we all migrated from Africa, where dark skin protected us from the sun’s damaging ultraviolet rays, to other areas of the world like Europe, where sunlight isn’t so strong. Over time, skin became lighter to allow for the sun’s absorption during winter months. When it comes to genetic diversity, those who left Africa brought with them only a small sample of the diversity that remained in Africa. This concept is known as a “bottleneck,” and a recent study finds it happened again more recently, but only in men.

Conducted by researchers at Arizona State University, the study found that between 4,000 and 8,000 years ago, a bottleneck among only men caused genetic diversity to decline. Women’s genetic diversity, on the other hand, thrived. The reason for this: Men began farming. And with that sedentary lifestyle came wealth, which allowed these few men to spend less time trying to survive, and more time reproducing.

“Instead of ‘survival of the fittest’ in 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,” said the study’s lead author Melissa Wilson Sayres, an assistant professor at the university’s School of Life Sciences, in a press release. Speaking to the Pacific Standard, she said that for every 17 women who reproduced at the time, there was only one man doing the same.

The findings are important because they offer insight into how our evolution may not have been pushed along solely by natural selection — which stipulates that as man evolved, so did genetic traits that benefited his survival in a particular environment. But while this has been the common evolutionary rule for years, the new research suggests a cultural phenomenon may have inspired a genetic revolution as well.

The researchers discovered this by analyzing DNA samples from the saliva or blood of 456 men living in seven regions of five continents, including Africa, the Andes in South America, South Asia, near East and Central Asia, Europe, and Oceania — islands in the middle of the Pacific Ocean. They specifically looked at these men’s Y chromosomes and mitochondrial DNA. By comparing these two, which are inherited exclusively from male and female ancestors, respectively, they’re able to determine the number of female and male ancestors the populations had.

Wilson Sayres said the results can help inform researchers on not only genetic diversity but also disease on a global scale. “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,” she said. “If we want to understand human health on a global scale, we need to know our global genetic history; that is what we are studying here.” The team’s next goal is to further its research with a larger amount of DNA samples.

Source: Karmin M, Saag L, Vicente M, et al. A recent bottleneck of Y chromosome diversity coincides with a global change in culture. Genome Research. 2015.