As Covid-19 was spreading fear and spurring lockdowns across the United States in early 2020, the scientific journal Nature Medicine published a paper on March 17 titled “The Proximal Origin of SARS-CoV-2.” Written by five renowned academic scientists, it played an important early role in shaping the debate about a fiercely controversial topic: the origin of the virus that has killed millions since it emerged in Wuhan, China, in late 2019. Did it spill from animals to humans in nature, on a farm, in a market? Or did it leak from a lab like the Wuhan Institute of Virology, a leading center of coronavirus research in China? Drawing on “comparative analysis of genomic data,” the paper’s authors wrote that “our analyses clearly show that SARS-CoV-2 is not a laboratory construct or a purposefully manipulated construct.” Toward the end of the paper, they added, “we do not believe that any type of laboratory-based scenario is plausible” in explaining the origin of the virus. Instead, the scientists strongly favored a natural origin, arguing that the virus likely spilled from bats into humans, possibly by way of an intermediate animal host.