By now, you’re hopefully all aware of how evolution works. An animal gets some random mutation that helps it survive and produce more offspring. After a whole bunch of generations, that mutation is passed on to others until the whole species is reaping the benefits.
This line of events, however, isn’t the only way in which evolution helps a species to thrive. Sometimes, it’s important to keep a healthy mix of genetic traits within a population. Take human blood types, for example. We all have some form of A, B, or O combinations of positives and negatives, describing the presence or absence of different antigenic substances on the surface of red blood cells. You’d think that evolution would weed out the weaker, more susceptible versions of blood type, but that’s not the case. Here, it is more important to keep a healthy genetic variation, as different combinations provide different types of protections from diseases.
In fact, it is so important, that the genetic variation related to our blood-group system is even present in chimpanzees, meaning that it has been inherited over millions of years from a common ancestor. If strong variation is conserved for that long, it must be really important.
That got some researchers from the University of Chicago thinking; are there any other examples of genetic variation that have survived the millions of years of evolution separating humans and chimpanzees?
To find out, they took the genomes of 10 chimpanzees from Western Africa and 59 humans from sub-Saharan Africa and compared their genetic blueprints. They grouped similar snippets together and then looked to see if the similarities were from a single species or if a single cluster grouped both humans and chimps together. Most of them contained only humans or only chimps. There were a few, however, where the clusters contained a healthy mix of both species. In those regions, some humans were more closely related to some chimpanzees than to other humans.
These six clusters represent areas of the genetic code that have a very strong chance of having been inherited from our common ancestor. So what do they do? That question remains up for debate, but if prior knowledge is any indicator, they probably have something to do with our immune systems.
Keeping genetic diversity in the immune system is extremely important. If every person on the planet were extremely susceptible to the same disease, we’d be in real trouble when an outbreak occurred. Most of the time, genetics work to ward this off, as in the case of malaria. For this, there are two version of the hemoglobin gene: a normal version and hemoglobin S., a mutation that distorts the shape and function of red blood cells. Those who inherit two normal hemoglobin genes are at high risk for malaria, which infects more than 200 million people each year. Those who inherit one normal gene and one hemoglobin S. gene are partially protected from malaria—a potentially life-saving benefit. Those with two copies of the gene suffer from sickle-cell anemia, a serious and lifelong circulatory disease. If hemoglobin S. genes did not bestow a genetic advantage—keeping malaria at bay—it would have disappeared from the population long ago.
We’re in constant war with the pathogens around us. As we develop new immune techniques to keep them out, they develop new ways to get in; it’s a constant arms race. Maintaining a strong genetic variety gives the species a Swiss army knife of options for finding ways to adapt to new attacks. So while it’s a Darwinian world with survival of the fittest, when it comes to building an army, it pays off to have nerds along with the meatheads.