Scientists followed a genetic trail stretching millennia to better understand our reproductive patterns. (bigstock)

Roughly 12,000 years ago, humans started farming commercially. Those with fruitful harvests discovered what it meant to be wealthy. And wealthy men apparently fathered way more babies, according to a new study.

Survival of the fittest might have actually been survival of the richest.

Researchers recently uncovered a sharp decline in genetic diversity in male lineages across the world during the Stone Age. The study’s authors hypothesized that material gains made through early agricultural success — a proxy for wealth — gave smaller groups of related men the reproductive upper hand for generations.

“Men who had more wealth and power might have had more to offer to women,” said co-author Melissa Wilson Sayres, an Arizona State University professor who studies sex-biased biology. “Their sons and grandsons could have been more successful in the same way.”

The semblance of prosperity, therefore, could have been a bigger genetic factor than natural selection, the researchers found. This difference in reproductive rates probably shrank the pool of genetic traits that are passed down from men, Wilson Sayres said, while boosting the mix of female-inherited characteristics.

Ancient mating patterns can show us how social priorities have changed over millennia. Rich people, for example, tend to reproduce less these days. America’s birth rate has been declining for years. That could have public health implications: The more genetic diversity  population has, the healthier it is.

“When I think about wealth and culture in modern times, I don't see a lot of wealthy people having large families anymore,” she said. “We're affecting ourselves in ways that have never happened before.”

The study opens up a number of policy questions about wealth and inequality and how those factors affect which genetic traits are handed down.  The advent of birth control and who can and cannot access contraceptives likely plays a role in this process as well.

Scientists examined DNA samples from blood and saliva samples of 456 men living in seven regions across Africa, Asia and Europe. (“Each of our genomes is a composite of our parents, and our parents’ parents, and so on,” Wilson Sayres said.) They focused on the Y chromosome, which is passed down through the male lineage, and the mitochondria, cell matter from an offspring’s genetic mother.

Computer and statistical modeling showed throughout history two ancient “bottlenecks," or significant decreases in genetic diversity, according to the study, published last week in Genome Research. Migration from Africa drove the first shift, Wilson Sayres said. The rise of agriculture — and a new way for prospective mates to gauge male desirability — likely drove the second. For every 17 women who passed on their DNA, researchers could find genetic evidence of only one male whose lineage stretched to modern times.

The takeaway: Today’s baby-making habits shape DNA way down the road