Online dating might have opened up the pool of partners to an unprecedented extent, but worldwide one in 10 marriages are between first and second cousins. This carries a major risk of inbreeding, but it might be thought humanity is used to much worse. Astonishingly, however, a study of DNA from prehistoric humans indicates mating with close cousins used to be much rarer than today.
Inbreeding poses such a threat to species' success that many animals go to great lengths to avoid it. Elephants are just one of the many species that kick young males from the herd so they don't mate with their sisters or cousins. Female banded mongooses go to war so they can meet partners they're not related to.
In times when few people journeyed more than a day's walk from their village, it's not surprising people often married close relatives, since that was all they met. It's intuitive to extrapolate back to a time before modern transport, or even horses, and think that our ancestors probably had children with members of the small bands in which they live. It's certainly how fiction such as Clan of the Cave Bear represents the paleolithic. Yet a new study in Nature Communications reveals the exact opposite to be the case.
Sifting through data on 1,785 people, dating from 45,000 to a few hundred years ago, just 53 (3 percent) have DNA indicating their parents were first cousins, while one was probably the result of brother-sister or parent-offspring incest. The 53 people were randomly strewn through history and the populated continents, rather than concentrated in any one era, aside from including three of 11 people from Iron Age Republican Rome.
How our ancestors managed this is a mystery. Perhaps incest taboos extended to cousins, perhaps festivals brought distant people together and acted as a sort of primeval Tinder.
The paper also notes, “Parents can also be more distantly related to each other, often via many deeper connections in their pedigree, as a common consequence of small population sizes, or as a consequence of founder effects in tight-knit groups.”
This is known as “background relatedness”, and the authors investigated it as well. For most of human history, they found background relatedness was high, but it fell dramatically as agriculture was adopted, increasing population density.
On the other hand, not everyone took advantage of the larger mating pool, as demonstrated by Egyptian Pharaohs frequently marrying their siblings. European royal families looked at this with disdain but had such high background relatedness that genetic diseases became common.
The research was possible because people with highly related parents have stretches of DNA with little genetic variation, called “runs of homozygosity” or ROH. “The more recent the genealogical relationship of the two parents, the more frequent and longer the resulting ROH tends to be,” the authors write.