Photo: Nipah virus, computer model / Getty Images
And that’s the concerning part.
Scientists say viruses that jump from animals to humans don’t need special mutations to make that transition — which suggests humanity may be at greater risk than previously believed, according to ScienceAlert.
The study found that most viruses responsible for major outbreaks in recent years showed no unusual features before infecting humans. Researchers identified no prior adaptive changes that would have signaled their ability to make the jump.
This challenges the long-standing assumption that viruses must evolve new traits to infect humans. Instead, the findings indicate that the risk of transmission from animals to humans may be higher than previously thought. If spillover depends less on new mutations and more on contact with already compatible pathogens, humans may be more vulnerable than expected.
The research also supports the idea that outbreaks are primarily driven by zoonotic viruses — those transmitted from animals — rather than pathogens engineered in laboratories.
“This work is directly relevant to ongoing debates about the origin of COVID-19,” said Joel Wertheim, professor of medicine at the University of California, San Diego. “From an evolutionary perspective, we found no evidence that SARS-CoV-2 was shaped by laboratory selection or prolonged evolution in an intermediate host before emerging. This absence of evidence is exactly what we would expect from a natural zoonotic event.”
During the study, scientists analyzed viral genomes from outbreaks including Influenza A, Ebola, Marburg virus disease, Smallpox, SARS, and SARS-CoV-2. They focused on the period just before each virus spilled over into humans, searching for any evolutionary signals that might have prepared them for transmission.
Using a phylogenetic model, researchers measured selective pressure across three stages: in natural animal hosts, immediately before transmission to humans, and during the early spread among people.
Across all cases, the intensity of natural selection remained typical before the jump to humans, with no detectable evolutionary warning signs. Changes in selective pressure only appeared after the viruses had already begun spreading within human populations.
