Oldest known evidence of father-daughter incest found in 3,700-year-old bones in Italy

An excavation in Italy has unearthed the oldest and first known evidence of father-daughter incest in the archaeological record, a new genetic study reveals.

The team found genetic clues of this incest in the remains of a teenage boy who was buried in a Bronze Age cemetery in south Italy.

The cave site of Grotta della Monaca in Calabria — the "toe" of Italy — was used as a burial ground between 1780 and 1380 B.C. Archaeologists analyzed the DNA of the 23 people buried there in order to understand the genetic background of the group, but they did not anticipate finding such "extreme parental consanguinity."

In a study published Monday (Dec. 15) in the journal Communications Biology, a team of researchers outlined their genetic findings from prehistoric Grotta della Monaca.

Even though the skeletons were fragmented and mixed up, the researchers were able to identify the genetic sex for 10 females and eight males. They also found a variety of mitochondrial and Y-chromosome DNA haplotypes — genetic information that is passed along by a parent — which indicated that the group included a mixture of people from different backgrounds.

While investigating genetic relationships within the burial site, researchers found two cases of first-degree relatives, meaning parents and their offspring.

On the surface, this finding is not particularly noteworthy, as many cultures bury their dead with biological kin. Indeed, genetic analysis revealed that a mother and her daughter were buried near one another at Grotta della Monaca.

But the case of an adult male and a pre-adolescent male buried in the Grotta della Monaca cave was different. The researchers measured runs of homozygosity (ROH) segments in their DNA. ROH refers to chunks of similar genetic material that get passed down from parent to offspring. Typically, when humans mate outside their biological family, they mix their genes and end up with low ROH. A higher ROH, on the other hand, correlates with inbreeding.

Most of the people buried at Grotta della Monaca had ROH numbers that suggested their parents were distantly related — perhaps in the last six to 10 generations, the researchers wrote. But the pre-adolescent male had "the highest sum of long ROH segments ever reported in ancient genomic datasets to date."

Further investigation yielded "indisputable evidence that the young male was the offspring of a first-degree incestuous union," the researchers wrote, unambiguously showing that he was the son of an adult male buried at the site and his own daughter. The researchers did not, however, find the skeletal remains of the boy's mother.

Humans tend to avoid incestuous unions, perhaps due to biological instinct or to cultural taboos. But incest has been documented by archaeologists. For example, the Altai Neanderthal's genes suggested her parents were half-siblings. Brother-sister marriages occurred among royal families in ancient Egypt, and a Stone Age man found in Ireland also had parents who were likely brother and sister.

But those sibling examples are considered second-degree unions, whereas parent-child unions are first-degree — and tend to carry a higher probability of genetic disorders in offspring. The researchers investigated the teenager's genes to determine if he had any rare genetic disorders, but they did not find any.

The discovery that a father and daughter produced a son is "an exceptionally rare and remarkable finding," the researchers wrote, as well as "the earliest identified in the archaeological record."

It is currently unclear why people at Grotta della Monaca engaged in this unusual behavior. The community wasn't particularly small and did not appear to have a hierarchical or royal inheritance system where close-kin marriages would help consolidate wealth and power.

"The reproductive union between parent and offspring observed in our study may reflect a socially sanctioned behaviour," the researchers wrote. That may explain why the father was the only adult male buried in a cemetery otherwise full of the graves of women and kids.

But whether the union was acceptable to everyone, a one-off event, or the result of coercion or violence may never be known.

"This exceptional case may indicate culturally specific behaviours in this small community, but its significance ultimately remains uncertain," study co-author Alissa Mittnik, an archaeogeneticist at the Max Planck Institute for Evolutionary Anthropology in Germany, said in a statement.

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