This article investigates the evolutionary history of zoonotic viruses to determine if they must adapt within animal hosts before infecting humans. By analyzing the natural selection patterns of major pathogens like SARS-CoV-2, Ebola, and influenza, researchers found that most viruses do not require significant genetic changes to become capable of human-to-human transmission. Instead, many viruses circulating in the wild are already pre-adapted to jump the species barrier. The study also highlights a unique evolutionary signature for the 1977 H1N1 influenza outbreak, suggesting it was likely the result of laboratory passage rather than natural spillover. These findings provide a phylogenetic framework for distinguishing between natural viral emergence and artificial selection. Ultimately, the research suggests that the selective sieve for most pandemics occurs at the moment of contact rather than through long-term adaptation in a reservoir.

References:

• Havens J L, Pond S L K, Zehr J D, et al. Dynamics of natural selection preceding human viral epidemics and pandemics[J]. bioRxiv, 2025.