Faced with extinction, Ebola mutated. Just as bacteria mutate under a constant barrage of antibiotics to emerge resistant and more pathogenic — think MRSA or extensively drug-resistant tuberculosis — so too did Ebola change under stress to become fitter, more resistant to animal immune systems and, most importantly, capable of infecting primates. Genetic evidence tells us that in approximately 1970, a particularly strong variant emerged for each of the three major Ebola species, including both main African species from Congo and Sudan as well as the one in the Philippines. These variants, because of their greater fitness, quickly spread through bat populations, occasionally spilling over into primates, including tragically in 2013 in Guinea. All but two known Ebola outbreaks, which are from rarer Ebola species and may be the result of the same process, are descended from these three dominant variants.