It’s one of the perplexing mysteries of the Covid pandemic: Where did Omicron emerge from, almost one year ago? The fast-moving, extremely contagious variant arrived just after Thanksgiving 2021, bristling with weird mutations. When scientists untangled the array, they found that Omicron wasn’t related to Delta or Alpha, the two waves that preceded it. Instead, its divergence from its closest common ancestor dated back more than a year, to the first few months of the pandemic—practically a geologic era in viral-replication time.
That was a conundrum. How could something be so communicable that it ripped through more than 120 countries in two
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