Neanderthals, our closest ancient relatives, offer a window into the evolutionary forces that shaped our species. Analyzing the remnants of their ancient DNA allows scientists to decipher the specific genetic codes that determined their blood groups. This genetic information, preserved in fossilized remains, provides insights into their origins, population structure, and vulnerabilities. It also serves as a molecular roadmap detailing the connections between Neanderthals and modern humans, especially concerning health and migration.
Identifying Neanderthal Blood Groups
Initial assumptions suggested Neanderthals might have been uniformly blood type O, a pattern seen in some other primate species. However, recent analysis of high-quality Neanderthal genomes reveals they possessed genetic diversity in their ABO system similar to modern humans. Genetic markers show Neanderthals carried alleles for Type O, Type A, and Type B blood. This finding suggests that the ABO polymorphism, allowing for A, B, AB, and O blood types, is an ancient trait predating the split between Neanderthals and Homo sapiens. Scientists also analyzed gene variants for seven other major blood group systems, including Rhesus (Rh), Kell, Duffy, and Kidd. The Rh system analysis showed Neanderthals possessed a unique RHD allele, referred to as “Rhesus plus incomplete,” which is now extremely rare in modern human populations. They also carried specific ABO-related alleles associated with protection against certain viral gut infections, suggesting their blood traits were influenced by local pathogens.
The Process of Ancient DNA Analysis
Determining the blood groups of an extinct hominin requires ancient DNA (aDNA) analysis. The process begins by extracting minute, highly degraded fragments of genetic material from fossilized bone or tooth remains. This DNA is fragmented and chemically damaged after tens of thousands of years, making sequencing technically challenging. Samples are often contaminated with DNA from soil microbes and modern humans who handled the fossils.
Researchers employ advanced sequencing technologies and computational methods to filter out contamination and reconstruct the original Neanderthal sequence. To determine the blood group, scientists search for single-nucleotide polymorphisms (SNPs) in genes like ABO, RHD, and RHCE that encode red blood cell antigens. Identifying the specific alleles at these locations allows researchers to genotype the individual and predict their blood type for multiple systems.
What Blood Types Reveal About Neanderthal Populations
The genetic profile of Neanderthal blood groups offers clues about the demography and health of their populations. A common blood group pattern across geographically distant individuals indicates low genetic diversity within the species, consistent with small, isolated populations, limited gene flow, and likely inbreeding. Low genetic diversity would have made the population less adaptable to environmental changes and new pathogens.
The Rh system analysis revealed a reproductive vulnerability: Rh incompatibility. If a Neanderthal mother carried a fetus with a different Rh factor inherited from a non-Neanderthal father, her immune system could attack the fetal red blood cells. This condition, known as hemolytic disease of the fetus and newborn (HDFN), often results in miscarriage, stillbirth, or severe infant mortality without modern medical intervention. This potential for high infant death rates could have lowered the reproductive success of Neanderthal groups, especially where they interbred with other hominin species.
The discovery of specific blood group alleles also supports the hypothesis that Neanderthals and Denisovans share deep ancestral roots with modern human populations in Africa. Many of these archaic hominin blood group alleles are recurrent in modern Sub-Saharan African populations, suggesting the genetic variants predate the migration of Homo sapiens out of Africa.
Neanderthal Genetic Contributions to Modern Blood Traits
Interbreeding between Neanderthals and early Homo sapiens left a genetic legacy scattered throughout the genome of non-African modern humans, including some blood-related traits. While the direct influence on common ABO blood groups is minimal, an archaic genetic segment related to the Rh system has been identified in modern populations. The unique Neanderthal RHD allele, now rare, has been found in a small number of present-day Aboriginal Australian and Papuan individuals. This suggests a specific episode of introgression, where this archaic gene segment was passed down.
This transferred Neanderthal DNA segment in the Rh gene provides evidence of interbreeding that occurred before the ancestors of these modern groups migrated into Southeast Asia and Oceania. Beyond specific blood groups, the overall archaic gene flow influences various modern human traits, including immune responses and disease resistance. Neanderthal alleles have been implicated in the modern human immune system, often reintroducing older variants that may have provided a selective advantage in Eurasian environments.