Eurasia for thousands of years, adapting to extreme conditions before their extinction approximately 40,000 years ago. Recent genetic studies have revealed a previously unexplored factor in their demise: their blood type.
Research analyzing genomes of individuals from 120,000 to 20,000 years ago indicates that Neanderthals possessed a rare blood group that could have been lethal for their newborns. Findings published in Nature suggest this factor may have been a critical barrier to their survival and reproduction.
Human blood is classified based on the presence of proteins and sugars on red blood cells. The most recognized system is ABO, but other less-studied classification systems can influence health and blood compatibility. One such system is the Rh factor, determining whether a person is Rh positive or negative. Scientists have identified a rare RhD variant in Neanderthals.
“Neanderthals have an Rh blood group that is very rare in modern humans,” explained Stéphane Mazières, a geneticist at Aix-Marseille University and lead author of the study. This blood characteristic was incompatible with that of Denisovans and early Homo sapiens, suggesting that any interbreeding could lead to health issues in offspring.
A major risk associated with this incompatibility is hemolytic disease of the newborn (HDN), which occurs when an Rh-negative mother carries an Rh-positive baby, prompting her immune system to attack the fetus's red blood cells. Today, this condition is treated with Rh immunoglobulin, but no solution existed 100,000 years ago.
Analysis of blood diversity in 14 Neanderthals and 22 ancient Homo sapiens revealed that the blood group variability in Neanderthals remained unchanged for over 100,000 years. In contrast, modern humans developed a diversity of blood alleles that facilitated their expansion out of Africa and adaptation to new environments.
“In any case of inbreeding between a Neanderthal woman and a Homo sapiens or Denisovan man, there is a high risk of hemolytic disease of the newborn,” Mazières noted. This disease could lead to severe jaundice, serious anemia, brain damage, or even death, drastically reducing infant survival rates.
This issue, coupled with other factors such as competition with Homo sapiens, climate changes, and resource depletion, may have posed insurmountable obstacles for Neanderthals. Their populations' inability to renew themselves quickly enough may have doomed them to extinction.
While Neanderthals remained trapped in a cycle of low genetic variability and reproductive issues, Homo sapiens developed new adaptations in their blood cells. Studies show that blood allele diversification occurred shortly after the exit from Africa, possibly on the Persian Plateau, between 70,000 and 45,000 years ago.
Mazières suggested that this process may have been driven by demographic expansion and adaptation to new environments. “My first idea was that it was due to demographic expansion,” he commented. “Then, probably the new environments of Eurasia helped maintain these changes over generations.”
This discovery regarding Neanderthal blood groups provides a new perspective on their extinction. It was not solely the pressure from Homo sapiens or climate that determined their fate, but also an internal factor inscribed in their own biology.