Two recently published papers in the journal Science state that the coronavirus pandemic came from live animals sold and butchered in the Huanan Seafood Market in Wuhan, China, in late 2019. The researchers found that there were at least two, but possibly up to two dozen, separate viral spillover events.

There has been tremendous debate on where the pandemic originated. Despite undergoing five months of peer review and several revisions by the authors, the papers are unlikely to stop the dispute. According to Kristian Andersen, an immunologist at Scripps Research, “Have we disproven the lab leak theory? No, we have not. Will we ever be able to? No. But there are ‘possible’ scenarios and there are ‘plausible’ scenarios. … ‘Possible’ does not mean equally likely.”

A natural origin of the pandemic, or “zoonosis,” has been the favored theory because most pandemics start that way, including the SARS coronavirus outbreak of 2002-2003. The authors of the papers considered many forms of evidence, including the clustering of early Covid-19 cases around the market. Therefore, the market origin theory is not only the most likely scenario but the only one that fits the data.

Early in the pandemic, once the market was first identified as the location of a bunch of cases, Chinese investigators took environmental samples looking for traces of the virus. The new study shows that a disproportionate number of positive virus traces came from the part of the market where live animals had been sold. Michael Worobey, an evolutionary virologist at the University of Arizona, states, “The virus started spreading in people who worked at the market, but then started spreading in the surrounding local community as vendors went to local shops, and infected people who worked in those shops.”

The second paper points to genetic evidence and computer modeling that signals the virus spilled into the human population on multiple occasions in late 2019. Genomic examination of early cases shows two distinct lines, called A and B, that came from separate spillovers. Both were found in environmental samples taken from the market.

However, there are still many unknowns that will need to be investigated further. Some issues that the authors highlight are not knowing which animals were involved, where did they come from, and how they separate spillover events are connected.