Only 3% of prehistoric humans mated with their cousins — it’s 10% today!

LEIPZIG, Germany — Inbreeding is practice people often link to the distance past and prehistoric humans. However, a new genetic study finds only three percent of prehistoric people were the offspring of cousins. For comparison, researchers say that number is actually ten percent today.

The findings come from a genomic analysis of 1,785 individuals who lived over the last 45,000 years. Sex between cousins or second cousins has become over three times more common, according to an international team working on the project.

“Parental relatedness of present-day humans varies substantially across the globe, but little is known about the past,” writes lead author Dr. Harald Ringbauer of the Max Planck Institute for Evolutionary Anthropology and his team in the journal Nature Communications.

Where did prehistoric humans mate with their cousins?

Previous studies have pointed to modern humans being the descendants of “extreme inbreeding” between close relatives over the years. However, the German team found our ancient ancestors rarely chose their cousins as mates.

Only 54 individuals from the sample showed typical signs of their parents being cousins. Dr. Ringbauer says the results are “surprising.”

Moreover, there were no clusters of cousins mating, indicating that these incidents were sporadic events. Notably, even for hunter-gatherers who lived more than 10,000 years ago, unions between cousins appear to have been the exception.

A scanner screened for identical stretches of DNA, one inherited from the mother and the other from the father. The closer the parents are related, the longer and more abundant the segments become. Scientists call this phenomenon “long ROH” or Runs of Homozygosity — a sign parents are cousins.

A fifth of the cases occurred on islands, including Stone Age Britain. In modern populations, computational methods can detect these DNA markers with ease. However, the quality of DNA from very old bones is too low.

“By applying this new technique we could screen more than ten times as many ancient genomes than previously possible,” Dr. Ringbauer says in a university release.

After conducting the same analysis with 1,941 modern individuals, the team discovered 176 people who had ROH.

When did mating trends change among humans?

Researchers found a single “archaeological cluster” with more than two individuals on the archaic populations, in Iron Age Republican Rome. They also found a substantial demographic impact of the technological innovation of agriculture.

This was always followed by a marked decay in “background parental relatedness,” a sign of increasing population sizes. This was due to societies practicing farming compared to hunter-gatherer subsistence strategies.

“Here we analyze ancient DNA, leveraging that parental relatedness leaves genomic traces in the form of runs of homozygosity,” study authors write in their report. “Analyzing genomic data from 1,785 ancient humans who lived in the last 45,000 years, we detect low rates of first cousin or closer unions across most ancient populations.”

Ringbauer’s team adds that they found a “marked decay in background parental relatedness co-occurring with or shortly after the advent of sedentary agriculture.”

A 2019 analysis of DNA of more than 450,000 people in the UK Biobank study revealed 125 individuals with genes suggesting they were the offspring of first or second-degree relatives. This includes uncles, grandparents, and half-siblings and is known as “extreme inbreeding.”

Extrapolated to the whole population, this would work out at about 20,000 individuals. Cousins are at least third and fourth degree relatives, depending on generations.

South West News Service writer Mark Waghorn contributed to this report.

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