Researchers have devised a new tool for discerning between naturally occurring viral outbreaks and those resulting from lab accidents.

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The researchers found no peculiar changes in SARS-CoV-2 before it jumped into humans. It gained mutations as it spread from bat to bat, just like other bat coronaviruses did; only after the virus emerged in humans did it undergo a marked shift. Within a year, radically new variants were evolving, with mutations that made them exquisitely well-adapted to humans.