A pair of studies in the latest issue of the journal Science appear to back up the theory that an animal market in the Chinese city of Wuhan really was the source of the coronavirus pandemic.
The authors say finding out whether the disease spilled over naturally from animals to humans or was the result of a lab accident could be vital to preventing future deadly pandemics.
What did the studies show?
The first paper examined the spatial pattern of COVID-19 cases in the outbreak's first month, December 2019.
The team mapped the first 174 cases identified by the World Health Organization, finding that 155 of them were in Wuhan.
The early cases were geographically clustered in a tight pattern around Wuhan's Huanan Seafood Wholesale Market. Some early patients with no recent history of visiting the market lived very close to it.
Mammals that can carry the coronavirus — including red foxes, hog badgers and raccoon dogs — were all sold live in the market, the team showed.
The authors also connected positive samples from patients in early 2020 to part of the market where traders sold freshly butchered animals in late 2019.
The second study looked at an apparent discrepancy in the virus's early evolution. Scientists found there were two lineages, A and B, of the virus early in the pandemic.
Researchers used a technique called "molecular clock analysis," which counts genetic mutations to build a timeline of evolution.
They found that both lineages jumped from animals at the market to humans on separate occasions, and that one did not come from the other. From the analysis, they determined that there was unlikely to have been any human circulation prior to November 2019.
What do the findings mean?
Evolutionary biologist Michael Worobey of the University of Arizona, who co-authored both papers, had previously urged the scientific community to be more open to the possibility that the virus had spread as a result of a laboratory leak.
Worobey said the recent findings had moved him "to the point where now I also think it's just not plausible that this virus was introduced any other way than through the wildlife trade at the Wuhan market."
He said the idea that the market was not the origin, given that the two studies indicated as much, was as improbable as lightning striking twice in the same place.
Co-author Kristian Andersen of Scripps Research, based near San Diego in the United States, said the findings did not disprove the lab-leak theory but gave a plausible explanation for the origin of the SARS-CoV-2 virus in humans.
"All this evidence tells us the same thing: It points right to this particular market in the middle of Wuhan,'' said Andersen, a professor in the Department of Immunology and Microbiology.
"I was quite convinced of the lab leak myself until we dove into this very carefully and looked at it much closer," Andersen said.
The World Health Organization (WHO) stated in its strongest terms so far in June that a deeper probe was needed into whether a lab accident may be to blame.
Critics had previously accused the WHO of being too quick to dismiss or downplay the lab-leak theory, which had put Beijing on the defensive.
Former US President Donald Trump speculated repeatedly— offering no evidence — that COVID-19 was started in a Chinese lab. His successor, Joe Biden, has previously called for a more thorough investigation into the origin of the pandemic.
China dismissed the lab leak explanation as "a lie concocted by anti-China forces for political purposes."
