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US evolutionary biologist Michael Worobey has been firming up his opinion on the origins of the coronavirus since May, when he was among 17 scientists calling for an investigation that covered both natural source and lab-leak hypotheses.

Some two months after that appeal, published as a letter in the journal Science, the University of Arizona scientist said the newly emerging evidence has convinced him that natural origin is the more compelling explanation for the virus.

With this shift in thinking, Worobey joined 21 other scientists to publish a paper－The Origins of SARS-CoV-2: A Critical Review－in early July. "The most parsimonious explanation for the origin of SARS-CoV-2 is a zoonotic event," the scientists concluded, in referring to the name of the virus behind the pandemic. A zoonotic virus is one that crosses from animals to humans.

The authors of the review, led by Edward Holmes from the University of Sydney, are scientists from universities and research institutes in countries including the United States, the United Kingdom, New Zealand, Austria, Australia and China.

The May letter stated that in an investigation by the World Health Organization into the origins of the virus, there were no findings in clear support of either a natural spillover or a lab accident. They thought that the two theories weren't given balanced consideration and "we must take hypotheses about both natural and laboratory spillovers seriously until we have sufficient data".

The May letter was taken as evidence by some media that the scientists were advocating lab-leak theories. That's not what they meant, according to Worobey.

"Why am I on both papers? The answer is not that I've undergone a dramatic conversion. I've always thought a zoonotic emergence was more likely than a lab leak. In light of recent evidence/thinking, I simply view it as even more likely,"Worobey wrote on Twitter recently.

Pamela Bjorkman, a biology professor who also signed the May letter, recently wrote a letter to This Week in Virology and expressed her regret in signing the May letter.

Bjorkman said she signed the letter to prompt "more funding for searching for natural viruses in animal reservoirs".

"Perhaps naively, I did not anticipate that the letter would be used to promote the lab-origin hypothesis," she said. "I now think that I should have realized this would happen and should have been more proactive－either not signed the letter at all or else requested more wording changes to make my position clear."

Second thoughts

Worobey said a couple of things moved his thinking about the origin of COVID-19.

First, Shi Zhengli of the Wuhan Institute of Virology, or WIV, and other Chinese scientists recently published a paper concerning the genomes of several bat SARS-related coronaviruses from the same mine in Yunnan province where RaTG13－closest to SARS-CoV-2－came from. He found that the genomes turned out to be less close to SARS-CoV-2.

Another recently published study also helped to move Worobey's thinking. Xiao Xiao, Chris Newman and three other scientists traced animal sales from 17 shops in Wuhan's wet markets.

The study documented "47,381 individuals from 38 species, including 31 protected species sold between May 2017 and November 2019 in Wuhan's markets", according to the paper published in Nature.

Worobey said that traded animals such as raccoon dogs and civets are "plausible intermediate hosts".

Worobey told NPR that his colleague Robert Garry, a microbiologist at Tulane University, took data from the WHO and plotted it on a map showing where people with infections lived in Wuhan.

The data model shows that early COVID-19 cases in Wuhan started near the Huanan Seafood Wholesale Market. The map showed virtually no cases near the WIV, which is over 16 kilometers from the market.

"There are no cases around the WIV," Worobey said. "If the outbreak did start in the lab, the bottom line is, it would be odd for it not to be spreading from there rather than from elsewhere."