Ancient families that lived together were unlikely to include cousins that mated with one another, new research has suggested. Scientists took another look at previously published genetic data from 1,785 humans that lived from about 10,000 to 45,000 years ago to determine how many pairs of parents were closely related. The work found that even in ancient times, it was unusual for humans to pick their cousins as mates. Only in three percent of cases were offspring found to show signs that their parents were cousins, in 54 of 1,785 ancient people analyzed. The findings have been reported in Nature Communications.
A new computational tool was used in this work, which can reveal whether a person's parents were closely related. It identifies long sequences of DNA that are identical in both copies of the sequence, one of which is inherited from each parent. As parents become more closely related, those long, identical stretches of DNA appear more frequently in their child's genome. Modern computational genomic tools can easily find those segments with high-quality genetic samples. But this becomes much more difficult when assessing samples of ancient DNA, which often have gaps in the sequence.
“By applying this new technique we could screen more than ten times as many ancient genomes than previously possible," sidled study author Harald Ringbauer from the Max Planck Institute for Evolutionary Anthropology.
In this study the researchers also analyzed relatedness in the ancestry of these individuals. There may be many distant, unknown relationships in small populations, and markers of that can be found in DNA. This analysis showed that as agricultural practices became more technologically advanced, populations expanded and parents became less and less closely related. This confirms previous studies that have suggested that as people shifted away from hunter-gatherer practices, societies became larger and began to farm.
The computational methods that were used in the study can be applied to other questions as well. Scientists are also discovering more samples of ancient human DNA, and it's likely that we will continue to learn a lot more about the migration, lives, habits, health, and societies of ancient people.