In January 2020 two hypotheses emerged about the origin of the novel coronavirus: that it began in a bat, then infected another animal before spreading to humans in a Wuhan “wet market,” where wild animals are sold for meat; or that it emerged from the Wuhan laboratory. The wet-market story was pushed by the Chinese CDC and the World Health Organization. Public-health leaders argued that Covid-19 was like SARS and MERS, earlier coronaviruses that emerged from bats and spread through an intermediate animal.

But neither of those viruses has ever evolved to the point where it can transmit efficiently from one human to the next. There have been fewer than 10,000 cases of each virus world-wide since SARS was discovered in 2003 and MERS in 2012. What virus comes out of a bat cave and infects humans by the millions? It’s not biologically plausible. If instead it evolved slowly over many years in nature, how come no one knew of it?