The theory goes that some type of animal, potentially rodents, was infected with the SARS-CoV-2 virus sometime in mid-2020. In this new species, the virus evolved, accumulating roughly 50 mutations on the spike protein before spilling back over into people.

Kristian Andersen, an immunologist at the Scripps Research Institute, is among those who has been raising the idea that Omicron may have emerged from a reverse zoonotic event.

(A zoonotic event is when an animal pathogen starts to infect and spread among people. A reverse zoonosis is when such a virus passes back into an animal species.)

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“I know that most people think that these [come from] immunocompromised individuals, and I do think that that’s plausible, but to be perfectly honest, I actually think this reverse zoonosis followed by new zoonosis seems more likely to me given just the available evidence of the really deep branch, and then the mutations themselves, because some of them are quite unusual,” Andersen told STAT.