Today it is considered the closest accepted relationship for parents: a cousin-cousin relationship. How often such partnerships occurred among prehistoric people around the world is now highlighted by an extensive analysis of ancient DNA. According to this, our ancestors surprisingly rarely chose their cousins as partners, despite their often smaller population sizes. In addition, the study provides other interesting insights into the population genetics of the past.
They are the children of uncles or aunts and thus have common grandparents: People with this relationship are referred to as first cousins. How often partnerships between people with this relationship occur varies greatly from region to region in the world today. With us they are allowed, but rarely. In other cultures, on the other hand, they often clearly exceed the ten percent mark – many children there come from close relationships. Against this background, researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig and the University of Chicago have now devoted themselves to investigating the relationships between parents in prehistoric humans.
On the trail of parental relatives
To do this, they analyzed existing DNA information from a total of 1785 prehistoric people who have lived in different regions of the world over the past 45,000 years. The data came from finds of remains, from which the genetic material of the people could still be obtained. In order to enable conclusions to be drawn about the parental kinship, the scientists used typical similarities in the genome: the closer a person’s parents were related to each other, the more similar certain DNA segments in his genome and similarities also occur more frequently.
Proof using this method can be carried out comparatively easily with modern DNA. However, the genetic quality of specimens from ancient bones is often too poor to use standard methods. Therefore, the researchers developed an analysis method that is specially adapted to the study of ancient genomes. It now enables an improved detection of identical DNA segments in the often incomplete sequences. “With the help of this new technology, we were able to examine more than ten times as many old genomes as before,” says first author Harald Ringbauer from the Max Planck Institute for Evolutionary Anthropology.
The application of the method to the genetic data of the 1785 prehistoric humans showed: Only in 54 cases did the team find typical signs that the parents of the respective person were cousins. “We come to the conclusion that in our sample, significantly less than ten percent of all parental associations took place at the level of first-degree cousins,” the scientists sum up. These 54 cases did not occur either geographically or chronologically in the prehistoric populations examined. Even among the hunters and gatherers who lived more than 10,000 years ago, partnerships between cousins were the exception. Apparently, pairings with a relatively close relationship were probably rather untypical for people of the past.
Background kinship in view
As the researchers further report, the results also opened up broader insights into prehistoric population genetics: They provided information on the development of the so-called background relationship. It arises from connections between people with more distant, but unknowingly, comparatively close relatives. These networks of relationships are typical for relatively small populations. The results of the study reflected an effect of the transition to agriculture in human cultural development: accordingly, there was a decline in parental background kinship. The researchers were able to supplement earlier findings in different regions of the world and confirm that the respective population sizes have increased due to the transition from hunter-gatherer societies to agricultural cultures.
The scientists now see considerable potential for future research in their process. Because the branch of science “old DNA” is developing rapidly: year after year, more genomes are sequenced from finds and are available for analysis. Harald and his colleagues hope that your method could contribute to gaining more precise insights into our ancestors’ choice of partners and the development of prehistoric populations.
Source: Max Planck Institute for Evolutionary Anthropology, specialist article: Nature Communications, doi: 10.1038 / s41467-021-25289-w