“There is zero probability [SARS-CoV-2] was circulating in fall 2019,” tweeted Trevor Bedford, a computational biologist at Fred Hutchinson Cancer Research Center who has been tracking SARS-CoV-2’s genetic code as it has spread. Allison Black, a genomic epidemiologist working in Bedford’s lab, says this is apparent from researchers’ data. As the virus spreads, it also mutates, much like the way words change in a game of Telephone. By sequencing the virus’s genome from different individual samples, researchers can track strains of the coronavirus back to its origins. They have been continually updating their findings on Nextstrain. (In case you’re wondering, the strains have nothing to do with severity of illness. They’re simply a way to track the virus’s mutations over time.)

Richard Neher, an evolutionary biologist at the University of Basel in Switzerland, told the Scientist that Nextstrain researchers’ work has tracked the virus back to a single source “somewhere between mid-November and early December,” which then spread in China. The earliest cases in the U.S. appeared in January 2020, according to Nextstrain’s sequencing work. Washington state, where the first known COVID-19 case in the U.S. was identified, has at least six strains. A similar analysis of California’s coronavirus cases—which has yet to be peer-reviewed—identified at least eight strains in the state, suggesting transmission from Washington state, New York, Europe, and China.