In the recent debate on the origins of the coronavirus, a group of scientists this week presented an overview of scientific findings that they believe show that natural spread from animals to humans is a far more likely cause of the pandemic than a laboratory incident.

The scientists refer, among other things, to a recent report showing that markets in Wuhan, China, had sold live animals susceptible to the virus, including civet cats and raccoon dogs, in the two years before the pandemic began. They observed the striking similarity of the appearance of Covid-19 to other viral diseases caused by natural spillovers and pointed to a variety of newly discovered viruses in animals that are closely related to the virus that caused the new pandemic.

The back and forth among scientists takes place as intelligence agencies work with a deadline for the end of summer to give President Biden an assessment of the origin of the pandemic. There is now disagreement among intelligence officials as to which scenario is more likely for a viral origin.

The new paper, which went online on Wednesday but has yet to be published in a scientific journal, was written by a team of 21 virologists. Four of them also worked on a 2020 paper in Nature Medicine that largely ruled out the possibility that laboratory manipulation could turn the virus into a human pathogen.

In the new paper, the scientists provided further evidence that the virus was spilled from an animal host outside of a laboratory. Joel Wertheim, a virologist at the University of California, San Diego and co-author, said a key point in support of natural origin is the “uncanny similarity” between the Covid and SARS pandemics. Both viruses appeared in China in late autumn, he said, with the first known cases emerging near animal markets in cities – Wuhan in the case of Covid and Shenzen in the case of SARS.

In the SARS epidemic, the new paper suggests that scientists will eventually trace its origin back to viruses that infected bats far from Shenzen.

Due to the spread of viruses similar to the new coronavirus in Asia, Dr. Wertheim and his colleagues predict that the origin of SARS-CoV-2 will also be a long way from Wuhan.

Since first surfacing in the final months of 2019, the viral culprit of this pandemic has not yet been found in any animal.

In May, another team of 18 scientists published a letter arguing that the possibility of a laboratory leak must be taken seriously due to insufficient evidence of a natural origin for the coronavirus or a leak from a laboratory. Wuhan, where the pandemic was first documented, is home to the Wuhan Institute of Virology, WIV for short, where researchers have been studying coronaviruses from bats for years.

One of the signatories of the May 2021 letter, Michael Worobey of the University of Arizona, co-authored the new paper, which advocates natural spillover.

He said his views evolved as more information emerged. Among other reasons for Dr. Worobey’s shift was the growing evidence of the Huanan animal market in Wuhan. When the pandemic first appeared in Wuhan, Chinese officials tested hundreds of samples from animals sold in the market and did not find the coronavirus in any of them.

But last month, a team of researchers presented an inventory of 47,381 animals from 38 species that were sold in Wuhan’s markets between May 2017 and November 2019. This included species such as civets and raccoon dogs, which can act as intermediate hosts for coronaviruses.

Dr. Worobey called this study “a groundbreaking paper”.

He also pointed out the timing of the earliest cases of Covid in Wuhan. “The Huanan market is right in the epicenter of the outbreak, with later cases radiating into space from there,” said Dr. Worobey in an email.

“No early cases cluster near the WIV, which has been the focus of most speculation about a possible lab escape,” he said.

However, other scholars say that such arguments are speculative and that the new review is mostly a repetition of what is already known.

“Basically, it really boils down to an argument that because almost all previous pandemics have been natural in origin, it must be,” said David Relman, a Stanford University microbiologist who organized the May Letter to Science.

He noted that he does not reject the natural origin hypothesis as a plausible explanation for the pandemic jump. But dr. Relman believes the new paper is “a selective sample of outcomes to be used to argue one side”.

Dr. In their new paper, Worobey and his colleagues also presented evidence against the notion that so-called gain-of-function research, which intentionally changes the function of a virus, may have played a role in the pandemic. The researchers argue that the coronavirus genome does not have mandatory signatures of manipulation. And the diversity that coronavirus scientists have discovered in Asian bats could serve as an evolutionary source for Covid-19.

But Richard Ebright, a molecular biologist at Rutgers University and a staunch critic of attempts to reduce the likelihood of a laboratory leak, said this was a straw man argument.

Dr. Ebright said it was possible that a WIV laboratory worker caught the coronavirus on a field expedition to examine bats or while processing a virus in the laboratory. The new paper, he argued, did not address such possibilities.

“The review does not advance the discussion,” said Dr. Ebright.