What role does virus mutation and adaptation play in host jumps?

Viruses desperately need to hijack their hosts’ cellular machinery and resources to replicate, over and over again. Without its host, a virus is nothing. Because of that dependence, some viruses have stuck with their hosts throughout evolution, mutating to make minor adjustments every time the host branched into a new species — a process called co-divergence. Humans and chimpanzees, for instance, have slightly different versions of the hepatitis B virus, both of which likely mutated from a version that infected their shared ancestor more than four million years ago.

The other option — cross-species transmission — occurs when a virus jumps into a completely new type of host largely unrelated to its former one. That kind of viral evolution is notoriously linked to severe emerging diseases like bird flu, HIV, Ebola fever and SARS. Given the extreme virulence of those diseases, the apparent rarity of cross-species transmission seemed fortunate.

But recently, when researchers in Australia conducted the first study of the long-term evolution of thousands of diverse viruses, they reached a startling conclusion: cross-species transmission has been more important and more frequent than anyone realized. Jumps between species have driven most major evolutionary innovations in the viruses. Meanwhile, co-divergence has been less common than was assumed and has mostly caused incremental changes.