You remember your blood type scrupulously (don’t you?). This is important in the event of hospitalization or blood donation. We readily talk about the nature of blood groups, their characteristics, their rarity. But we less often ask ourselves why such diversity. Why didn’t we all end up joining one universal blood group? As is often the case in human biology, the answer is neither simple nor unique.
These groups are the product of tens of millions of years of evolution, which immunology, genetics and evolutionary biology attempt to decipher. Asking what blood types “are for” risks lending an intention to evolution that does not exist. However, the scientific consensus does not say that these molecular differences are arbitrary. We now know that blood types began to differentiate through subtle molecular variations on the surface of our cells. Millennia later, as biologist Scott Travers explains in Forbes magazine, they are proof that natural selection almost always prefers diversity to perfection.
A 2015 state of the art review examined the biological roles of the ABO blood group system. Conclusion: its influence extends well beyond transfusion compatibility. The antigens—those small carbohydrate structures found on the surface of red blood cells—that define blood types are found throughout the body, such as in epithelial tissues and body fluids. Which means that blood groups are misnamed: it’s not just about blood. They are part of a broader biological interface between the body and its environment.
Group A carries the A antigens. Group B the B antigens. Group AB, the rarest, carries both antigens; and group O, the most common, has neither… These antigens can influence the way cells interact with each other and, importantly, with pathogens. As the 2015 study points out, these groups have been undergoing natural selection for millions of years thanks to this role, however diffuse and context-dependent it may be.
Biological weaponry or genetic drift?
So what are the evolutionary forces that caused such a complex ABO system to develop so early? Two theories clash in the scientific community. A Royal Society study explored them in 2004. Both envisage the persistence of the ABO system as arising from interactions between the human body and its environment.
First hypothesis: blood groups are reactions to infectious diseases. Certain microbes can use specific antigens as a gateway to the body. Thus, individuals of a certain blood group are more vulnerable, when other blood groups are completely immune. These interactions justify keeping several different blood groups in the same population, depending on the pathogens most present in their environment. Each has its advantages and disadvantages: it can be seen as a genetic arms race between hosts and microbes.
The second theory is more conservative. It postulates that the diversity of the ABO system is only due to neutral processes, such as genetic drift. This polymorphism would be neither particularly advantageous nor disadvantageous, it would simply persist because it does not hinder survival or reproduction. Nevertheless, the authors of the 2004 study suggest that neutral processes are not sufficient to explain the remarkable stability and overall distribution of the ABO system.
As has been said, natural selection often favors diversity. These two hypotheses show it. If we were all of the same blood type with the same biological profile, theoretically it would take just one tailor-made microbe to wipe out humanity. This is called balanced selection: a process that encourages several genetic variations to coexist within a population so that the species is prepared for any eventuality. Even nature doesn’t put all its eggs in one basket!
Blood groups are therefore a strategic portfolio. Thus, evolution does not necessarily seek to bring about the emergence of best characteristic. She sometimes favors the multitude of possibilities. When the future is uncertain, flexibility is key.
