A popular sarcastic explanation for human behavior is: “This is what happens when cousins marry.” It works so well that it may go back to our prehistoric ancestors. If they used it, they were wrong – new research finds that ancient humans rarely chose their cousins as mates. Did they know something we don’t about genetics … or was it just because royal families hadn’t been invented yet?
“Parental relatedness of present-day humans varies substantially across the globe, but little is known about the past. Here we analyze ancient DNA, leveraging that parental relatedness leaves genomic traces in the form of runs of homozygosity.”
In a new study published in the journal Nature Communications, researchers from the Max Planck Institute for Evolutionary Anthropology in Germany and the University of Chicago explain how they searched previously published DNA data from ancient humans that lived during the last 45,000 years for signs of “homozygosity” — possessing two identical forms of a particular gene, one from each parent, indicating the parents were closely related. Using DNA from 1,785 individuals, they applied new techniques to compensate for the DNA being so old. The results surprised them.
“In a global dataset of 1,785 individuals only 54, that is, about three percent, show the typical signs of their parents being cousins. Those 54 did not cluster in space or time, showing that cousin matings were sporadic events in the studied ancient populations. Notably, even for hunter-gatherers who lived more than 10,000 years ago, unions between cousins were the exception.”
A mere three percent showed signs of the parents being cousins, even among recent hunter-gatherers whose choices for mates were limited. The analysis also confirmed what had been assumed – switching from hunter-gatherer to an agricultural society caused populations to boom, resulting in more mating choices outside of the immediate family – a trend that happened worldwide.
So, is the “This is what happens when cousins marry” insult outdated? Not by a long shot. Anthropologists estimate more than ten percent of all global marriages occur among first or second cousins. The practice is most common in people of Pakistani, Bangladeshi and Middle Eastern origin. Some religions allow first cousin marriages (Protestantism, Islam, Judaism and some forms of Hinduism), while others ban it up to sixth cousins. Governments banning it include China, Taiwan, North Korea, South Korea, the Philippines and 24 of the 50 United States. (Before you call your cousin, check on your state here.) And, despite all of the jokes and suspicious behavior, inbreeding within royal families has been extremely rare for centuries –even they know about homozygosity.
Despite costing us a good insult, it’s good to lose homozygosity. Perhaps we should lose a few mores insults too … although a good one still beats fighting, shooting and murder. Here’s a classic from Mae West:
“His mother should have thrown him away and kept the stork.”