Down syndrome linked to ancient societies through DNA
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How far back do records of people with Down syndrome go? How were they treated in past societies?
Key points:
- A trisomy is a chromosomal condition characterised by an additional chromosome
- A person with a trisomy has 47 chromosomes instead of 46
- Down syndrome, Edward syndrome and Patau syndrome are the most common forms of trisomy
A new study has highlighted the respect that ancient civilisations had for people with Down syndrome, looking at the remains of past societies and case studies from across the globe.
The team identified six cases of Down syndrome and one case of Edwards syndrome in human populations that were living in Spain, Bulgaria, Finland and Greece from as long ago as 4,500 years before today.
The research indicated that these individuals were buried with care, and often with special grave goods, showing that they were appreciated as members of their ancient societies.
The global collaborative study, led by first author Dr Adam ‘Ben’ Rohrlach of the University of Adelaide and senior author Dr Kay Prüfer of the Max Planck Institute for Evolutionary Anthropology, involved screening DNA from approximately 10,000 ancient and pre-modern humans for evidence of autosomal trisomies, a condition where people carry an extra copy of one of the first 22 chromosomes.
“Using a new statistical model, we screened the DNA extracted from human remains from the Mesolithic, Neolithic, Bronze and Iron Ages all the way up to the mid-1800s. We identified six cases of Down syndrome,” Dr Rohrlach, a statistician from the University of Adelaide’s School of Mathematical Sciences, said.
Recently, researchers at the Francis Crick Institute, working with the University of Oxford, the University of York and Oxford Archaeology, developed a new technique to measure the number of chromosomes in ancient genomes more precisely, using it to identify the first prehistoric person with mosaic Turner syndrome — characterised by one X chromosome instead of two, who lived about 2500 years ago.
However, the University of Adelaide study involved researchers identifying skeletal remains and assessing the structures for signs that indicated trisomy.
“While we expected that people with Down syndrome certainly existed in the past, this is the first time we’ve been able to reliably detect cases in ancient remains, as they can’t be confidently diagnosed by looking at the skeletal remains alone.”
Down syndrome occurs when an individual carries an extra copy of chromosome 21. The researchers were able to find these six cases using a novel Bayesian approach to accurately and efficiently screen tens of thousands of ancient DNA samples.
Bayesian inference is the process of using statistics to detect the likelihood of a result using distribution and conditional probability.
“The statistical model identifies when an individual has approximately 50 percent too much DNA that comes from one specific chromosome,” Dr Patxuka de-Miguel-Ibáñez, of the University of Alicante and lead osteologist for the Spanish sites, added.
“We then compared the remains of the individuals with Down syndrome for common skeletal abnormalities such as irregular bone growth or porosity of the skull bones, which may help to identify future cases of Down syndrome when ancient DNA can’t be recovered.”
The study also uncovered one case of Edwards syndrome, a rare condition caused by three copies of chromosome 18, that comes with far more severe symptoms than Down syndrome. The remains indicated severe abnormalities in bone growth and an age of death of approximately 40 weeks gestation.
All of the cases were detected in perinatal or infant burials but from different cultures and periods.
“These individuals were buried according to either the standard practices of their time or were in some way treated specially. This indicates that they were acknowledged as members of their community and were not treated differently in death,” Dr Rohrlach explained.
“Interestingly, we discovered the only case of Edwards syndrome and a noticeable increase in cases of Down syndrome in individuals from the Early Iron Age in Spain.
The remains could not confirm that these babies survived to birth, but they were among the infants buried within homes at the settlement or within other important buildings,” Professor Roberto Risch, co-author and archaeologist from The Autonomous University of Barcelona, said.
“We don’t know why this happened, as most people were cremated during this time, but it appears as if they were purposefully choosing these infants for special burials.”
Previous research discovered that the first probable case of Down syndrome was that of a five to seven-year-old child who lived in medieval France some 1,500 years ago, seemingly buried respectfully and commonly.
The condition was first coined Down syndrome by English physician John Langdon Down in 1862, who brought it to the public eye in a paper on the condition in 1866, describing Down syndrome as ‘unique’ in nature.
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