Plagues helped shape human genomes

Genetics has always been my favorite part of biology. I enjoy Punnett squares more than I really should  (as I explain in this post about my first batch of axolotl eggs), I got to compare my mitochondrial DNA to an international database that included Neanderthal DNA in my genetics class last spring, and overall I find inheritance and genes to be really interesting. As such, this post from IFLS about how the Black Death tweaked European and Rroma genomes was something I wanted to look into.

What these researchers found was that the presence of Yersinia pestis, the bacterium responsible for killing off more than 20 percent of the world's population at the time, selected for genes involved in the immune response, specifically a Toll-like receptor that helps in the pathway to tell the cell that it is under attack. As one might guess, people in the 1300s who didn't have these genes were killed by the plague, while those who did survived and passed on these genes to their children. It's natural selection for humans.

The nasty little bacterium responsible for decimating Europe. Public domain photo from the NIH

The cool part? This selection happened in two genetically distinct populations. While Europeans and Rroma both lived in Europe, the Rroma were originally from the northwest corner of India, so their initial geographic isolation from the native European group provided for a greater genetic diversity between the two groups. However, because they occupied the same space during the wrath of the Black Death, both were subject to the natural selection process above, thus picking out the immune people from both populations. Because these genes came about separately (geographic isolation and all), this shows convergent evolution; another example of this process would be how birds, bats, and bugs all have wings, but they all developed them separately rather than inheriting them from a common flying ancestor.

So no, we don't have bacterial DNA in our genome (… alright, technically we do because some of the genes are the same for all organisms, but you get my drift), but this goes to show that even a small organism like this can bring about large changes in human genetic history.