When new species evolve, where do their viruses come from? As little more than free-ranging bundles of genetic material, 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.

“They showed rather convincingly that co-divergence is the exception rather than the rule,” said Pleuni Pennings, an evolutionary biologist and assistant professor at San Francisco State University who was not involved with the study.

The finding does not necessarily mean that emerging diseases from cross-species transmission are a more grave or imminent threat than medical science has assumed. However, it does reveal that the dynamics of virus evolution can be surprisingly complex. If scientists have been underestimating how often viruses can move into new hosts, then understanding which viruses are most primed to do so becomes a higher priority.

There’s no shortage of reasons why cross-species jumps would seem unlikely to influence viral evolution much. The odds against a virus leaping successfully to a new host species are formidable. If the virus can’t manipulate the host’s genetic material and replicate itself, then that’s the end of the line. A virus might need to make multiple attempts to infect a novel host over decades or longer, accumulating appropriate mutations all the while, before it could finally establish itself, replicate and spread.