(CNN) The coronavirus pandemic almost certainly originated from an animal, probably at a wildlife market in China, and not from a laboratory leak, a group of virus experts said Wednesday.
Theories about a lab leak are almost all based on coincidence, not hard evidence, the group of 20 top experts from the US, UK, Australia, New Zealand and elsewhere said.
They had been following discussions -- going all the way to the White House -- about the possibility of a laboratory origin of the virus, and worked together to analyze the evidence.
"I think you can make a pretty strong argument that it didn't leak from a lab," Robert Garry, professor of microbiology and immunology at Tulane Medical School and one of the scientists who signed the paper, told CNN.
The experts lay out the published evidence in a pre-print review posted online.
It is signed by some of the leading experts in coronaviruses and the genetics of viruses, including Kristian Andersen of The Scripps Research Institute in La Jolla, California; evolutionary biologist Michael Worobey of the University of Arizona; Andrew Rambaut of the Institute of Evolutionary Biology at the University of Edinburgh; Stephen Goldstein of the Department of Human Genetics at the University of Utah; Angela Rasmussen of the University of Saskatchewan; Joel Wertheim of the University of California, San Diego; and Jeremy Farrar of Britain's Wellcome Trust.
Many have done their own investigations into the possible origin of the SARS-CoV-2 coronavirus.
"We contend that there is substantial body of scientific evidence supporting a zoonotic (animal) origin for SARS-CoV-2," they wrote.
"There is currently no evidence that SARS-CoV-2 has a laboratory origin. There is no evidence that any early cases had any connection to the Wuhan Institute of Virology (WIV), in contrast to the clear epidemiological links to animal markets in Wuhan, nor evidence that the Wuhan Institute of Virology possessed or worked on a progenitor of SARS-CoV-2 prior to the pandemic," they added.
"The suspicion that SARS-CoV-2 might have a laboratory origin stems from the coincidence that it was first detected in a city that houses a major virological laboratory that studies coronaviruses."
But it's not a surprise that a new virus might emerge in Wuhan, they noted.
"Wuhan is the largest city in central China with multiple animal markets and is a major hub for travel and commerce, well connected to other areas both within China and internationally," they wrote.
"The link to Wuhan therefore more likely reflects the fact that pathogens often require heavily populated areas to become established."
Garry and colleagues also say it is "extremely unlikely" that the lab was working on a virus of natural origin that accidentally leaked out. "You have to have an unlikely set of circumstances in place for something like that to happen. If it was a person at the lab, then how did it get to all the animal markets?" Garry asked. "There are other, much more likely ways that this virus got into the human population. Just like SARS 1, it happened the same way with SARS 2."
What did get lost was some of the valuable evidence that disappeared when the animal markets in the area were cleared and sanitized, Garry said. "They shut down farms. They cleaned those animals out of that market," he said. Samples from the Huanan Seafood Market and others did turn up evidence of the virus, the experts said, but the animals that might have been infected and that could have provided the needed smoking gun were removed.
"We'd like to know more. We wish the Chinese government was a little bit more open about the wildlife trade," Garry said.
The World Health Organization has been leading efforts to find the origins of the coronavirus and issued a report in March saying it was most likely the virus originated in an animal and passed to people, as other coronaviruses have. Least likely, it said, is the possibility that a virus was engineered in a lab and leaked out.
Much of the investigation has focused on early cases at Wuhan's Huanan Seafood market.
It's not the first joint letter from scientists fed up with speculation. On Monday, in another letter to the Lancet, a group of scientists called for reason and science in the investigation.
"Careful and transparent collection of scientific information is essential to understand how the virus has spread and to develop strategies to mitigate the ongoing impact of COVID-19, whether it occurred wholly within nature or might somehow have reached the community via an alternative route, and prevent future pandemics," they wrote.
"Allegations and conjecture are of no help, as they do not facilitate access to information and objective assessment of the pathway from a bat virus to a human pathogen that might help to prevent a future pandemic."