Only 15 percent of people of European descent carry Rh-negative blood, compared to roughly six percent of the global population, according to biologist Scott Travers writing in Forbes.
The presence or absence of a protein named RhD on the surface of red blood cells determines whether blood is Rh-positive or Rh-negative. Red blood cells, also called erythrocytes, carry oxygen throughout the body, and the RhD protein rests on their outer membranes.
Travers noted that Rh-negativity historically created medical challenges during pregnancy for Rh-negative women. If an Rh-negative mother carries an Rh-positive baby, her immune system can produce antibodies after contact with the baby's blood, most often during delivery.
During a subsequent pregnancy with an Rh-positive child, those maternal antibodies can cross the placenta and damage fetal red blood cells. The placenta is the organ that develops during pregnancy to supply oxygen and nutrients from mother to child.
From an evolutionary perspective, this biological conflict should have exerted natural selection pressure to reduce the frequency of Rh-negative blood over time. However, the genetic variant remains relatively common across certain human populations.
Evolutionary persistence and population genetics
Travers explained that one reason for this persistence may be the relatively weak effect of natural selection in ancestral history. Because rhesus conflict primarily affected subsequent pregnancies, it had less impact in past eras when many women did not have third or fourth children.
Random chance within small ancient human populations may have also played an important role in preserving the trait. Additionally, a separate hypothesis suggests that Rh-negativity might offer a compensatory health advantage, though Travers emphasized that the supporting study was correlational and remains a theoretical assumption.
The prevalence of Rh-negative blood varies substantially depending on the population group. While the blood type is rare across East Asia, the highest rates are recorded among the Basque people of the Pyrenees, where between one-fifth and one-third of the population has it depending on the sample.
The Pyrenees mountain range creates a natural geographic boundary between modern France and Spain. The Basque people are an indigenous ethnic group in the region known for distinct genetic patterns and a language unrelated to neighboring European tongues.
Medical progress and daily health impact
In everyday life, individuals who only lack the RhD protein experience no noticeable negative health effects. Travers contrasted this common variation with the extremely rare condition known as Rh-null, where the entire Rh protein complex is absent, causing red blood cells to become deformed and fragile.
The absence of RhD does not indicate a separate human lineage or ancient hybridization. Instead, it is a genetic feature that spread much more strongly in some human populations than in others.
Medical advancements have dramatically changed the management of rhesus conflict during pregnancy. Since the late 1960s, preventative injections of anti-D immunoglobulin have helped prevent sensitization in Rh-negative mothers.
Because of anti-D immunoglobulin therapy, severe manifestations of rhesus disease have become rare wherever routine prenatal care is available.
