Most humans are born with 46 chromosomes. There are 22 pairs of autosomes plus a pair of XX or XY sex chromosomes. However, there are times when some individuals are born with an alteration in this sequence. People with Down syndrome, for example, have an extra copy of chromosome 21
Currently, this rare genetic alteration affects one in every 1,000 births worldwide. In just 100 years, a great leap in both medical and cultural quality has been made to improve the quality of life of people with this syndrome. At the beginning of the 20th century, their life expectancy barely reached 10 years, when it now exceeds 50.
What no one knew exactly is the presence of this condition thousands of years ago, in the middle of prehistory. That is why an international team led by researchers at the Max Planck Institute analyzed almost 10,000 DNA sequences from ancient humans from between 5,000 and 2,500 years ago.
The results obtained, as explained in an article published in the journal Nature Communications, reveal the presence of up to six children with Down syndrome, two of whom were found in Navarra. Five of them were buried more than 2,000 years ago and lived, at most, one year old.
Although their lives were short, all of these babies received a burial—often with special grave goods including colorful bead necklaces, bronze rings, or seashells—showing that they were valued as members of their communities.
The five oldest burials, in addition, were located within settlements, a privileged position for the deceased, especially considering that the funerary practices of these societies included cremation. “Their burials seem to show us that these individuals were cared for and appreciated as part of their society,” says Adam “Ben” Rohrlach, lead author of the study.
Rohrlach and his colleagues were surprised to identify six people with an unusually high number of chromosome 21 DNA sequences that could only be explained by an extra copy of the chromosome. One of the cases was found in the cemetery of a church in Finland and dates back to between the 17th and 18th centuries. The remaining five were much older and were found in sites from the Bronze Age in Greece and Bulgaria and another two from the Iron Age in Spain (Alto de la Cruz and Las Eretas).
Although today people with Down syndrome can live long lives, often with the help of modern medicine, this was not the case in the past. Age estimates made from the skeletal remains showed that all six individuals died at a very young age, and only one child lived to be 12 months old.
During the study, researchers also discovered an individual with a different condition. In this case, which was also found in one of the Spanish cemeteries of the early Iron Age (800-400 BC), the fetus had an unexpectedly high fraction of ancient DNA sequences from chromosome 18 showing that it carried three copies of this chromosome, causing Edwards syndrome (also known as trisomy 18). He died in the 40th week of gestation.
This type of aneuploidy (change in the number of chromosomes) is a condition associated with more serious health problems than Down syndrome and currently has an incidence of less than one case in every 3,000 births. “We still don’t know why we find so many cases in similar sites,” says Roberto Risch, archaeologist at the Autonomous University of Barcelona.
“What we do know is that they belonged to the few children who received the privilege of being buried inside homes after death. This is already an indication that they were perceived as special babies,” adds the researcher, a specialist in intramural funeral rites.
As the number of DNA samples from ancient individuals continues to increase, the authors plan to further expand their research in the future. “What we would like to know is how ancient societies reacted to individuals who perhaps needed help or were simply a little different,” concludes Kay Prüfer, who coordinated the analyzes of the chromosome sequences.
