The Cold War looks like a pillow fight compared to the ancient arms race between humans and the vicious microbes that mate and feed inside them. Consider the pervasive, mysterious brain parasite Toxoplasma gondii. Once a little-known cat parasite, T. gondii is now implicated in behaviors from pregnant womens' innate fear of cat feces (where the parasite matures after reproducing in cat intestines) to schizophrenia. Or how about the most prolific annual murderer of humans in the world, Plasmodium falciparum, the parasitic source of malaria. It seems that as biology moves forward, more and more parasites are discovered, and more and more human pathologies are tied to them.Differences between human populations evolve for a combination of reasons, partly by random genetic drift, but largely influenced by localized, environmental factors. It isn't surprising that evolutionary theorists have moved away from the traditional factors that affect selection, like climate and local diet, towards parasites and other microscopic pathogens. In a new article in Plos Genetics, Rasmus Nielsen and his team argue that “pathogen-driven selection” is and was one of the foremost local selection pressures in our species' evolution. In other words, the differences between a population in, say, the tundra, and a population in the rainforest, may be more related to local pests than local climates. Wait, really?Using genetic assays of 1500 human subjects from over 50 populations, and statistical models of different environmental variables (mainly local climate, diet, and pathogens), the authors found that differences in genes were most strongly correlated with the pathogen factor. By a lot.A total of 103 genes are significantly correlated in frequency with pathogen predictors while none correlates with climate or subsistence strategies. This predominant role of pathogen-driven selection in the human genome is confirmed when testing each variable within each environmental category separately (229, 10 and 9 genes significantly correlated in frequency with at least one pathogen, subsistence and climate variable, respectively).Interestingly, the authors argue that it is larger microorganisms, like parasitic worms and cysts, that affected evolution more than bacteria or viruses, because they evolve slower and are easier for us to catch up to. They contend that:The pressure imposed by parasitic worms (helminthes) on human genes has been stronger than the one due to viral, protozoa or bacterial agents. Perhaps this is due to the fact that helminthes evolve slower than unicellular/viral agents and that they often have complex life cycles which results in a relatively stable geographic distribution. Evolutionary changes in the helminthes, therefore, occur at a similar time-scale to that of humans, allowing for a true co-evolutionary interaction between humans and the pathogen.The result of this fact is that humans end up "over-adapting" their immune system to fight the slow wormy guys, but cause other problems in the process. The Sickle cell adaptation is a perfect example: it strongly increases your chance of surviving malaria (a parasitic disease) by altering the shape of the blood cells the parasites live in, but can lead to a number of serious health complications.The Red Queen theory, which has been nicely supported by research as of late, goes as far as to say that arms races with parasites are the reason sex evolved.In other words, we can sum it all up with a hypothetical: Sex increases genetic diversity, which in turn gives us more diverse proteins in our offspring, such as the MHC surface proteins on immune cells, which help us fight the parasites that have plagued us for eons. These same proteins have been implicated in pheromone secretion and mate choice, which helps further diversify the immune system as we seek those with different MHC complexes. Basically, tiny deadly worms are the indirect though ultimate cause of your romantic endeavors and desires. Isn’t biology sexy?
Advertisement
Advertisement