Zoonotic viruses do not require extensive adaptation before spreading among humans, according to a new study analyzing the evolutionary history of major pathogens.
Researchers used phylogenetic methods to examine selection patterns in Ebola, Marburg, mpox, influenza A, and SARS-CoV-2. They found no evidence of accelerated viral evolution just before human outbreaks-indicating pre-human adaptation is not necessary for human-to-human transmission.
SARS-CoV was an exception, showing signs of adaptation in an intermediate host. But this appears situational, not universal.
The study also revealed that lab-passaged viruses-like the 1977 H1N1 flu strain-carry distinct evolutionary signatures, allowing scientists to differentiate natural spillovers from potential lab incidents.
These insights call for stronger surveillance of animal reservoirs and early containment strategies that assume immediate transmissibility, not delayed risk.