Deinococcus radiodurans, episode 2: How'd it get so tough?
Well, it was probably just luck. We now know that D. radiodurans shares a recent common ancestor with T. aquaticus (in fact, the two impressive organisms are grouped into one phylum, Deinococcus-Thermus). The ancestor likely had some genetic predisposition to tolerating stressful environments that it passed on to these and other species. From there, D. radiodurans got yet more stress response genes from other bacteria through a process called horizontal gene transfer. So that explains its resistance to some common conditions that many extremophiles are able to survive, like cold and acidity. But what about all that radiation, which doesn’t occur anywhere in nature? That seems to be a happy accident from the evolution of resistance to conditions that we do find in nature: extreme dryness, or desiccation. Desiccation causes DNA to break into small pieces in the same way radiation does, so the ability to repair that damage would help an organism survive both kinds of extreme conditions. That organism might then earn an awesome nickname, like “Conan the Bacterium”: