A group of prominent international scientists say that after reviewing the evidence they believe the coronavirus most likely came from animals rather than a laboratory leak.

In a paper posted online without review, they said there was substantial evidence supporting a zoonotic origin for the Sars-CoV-2 coronavirus, while no current evidence had shown that it escaped from a lab.

Failing to investigate the zoonotic origin “would leave the world vulnerable to future pandemics arising from the same human activities that have repeatedly put us on a collision course with novel viruses”, they said, referring to the wildlife trade.

The 21 scientists from Australia, Austria, Britain, Canada, China, New Zealand and the United States posted the paper on the Zenodo research repository on Wednesday.

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The World Health Organization has repeatedly said that finding the origins of the virus that causes Covid-19 – which has now killed more than 4 million people globally – is essential in efforts to reduce the risk of future pandemics.

The UN agency sent a team of international scientists to Wuhan, the Chinese city where the first known outbreak occurred, in January to investigate the origins. After their report was released in March, WHO chief Tedros Adhanom Ghebreyesus said “all hypotheses remain on the table”.

“Although the team has concluded that a laboratory leak is the least likely hypothesis, this requires further investigation, potentially with additional missions involving specialist experts, which I am ready to deploy,” he said.

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Theories that the virus might not have a natural origin – particularly that it could have leaked from a Chinese laboratory – have gained traction in recent months and a growing chorus of scientists, as well as the US government, are now calling for a deeper examination of the possibility.

Critics say the lab leak theory was dismissed too quickly, including by scientists in a letter published in The Lancet in February 2020, less than two months after the world became aware of the first Covid-19 cases in China. Some of those scientists published another Lancet letter on Monday standing by their original view that the virus emerged in nature.

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The lead author of the latest paper, Edward Holmes, an evolutionary biologist and virologist from the University of Sydney, said “our careful and critical analysis of the currently available data provided no evidence for the idea that Sars-CoV-2 originated in a laboratory”. Holmes has widely published on the genome and origins of the virus.

The scientists also said that early cases of Covid-19 had clear epidemiological links to animal markets in Wuhan. “[The] most parsimonious explanation for the origin of Sars-CoV-2 is a zoonotic event. The documented epidemiological history of the virus is comparable to previous animal market-associated outbreaks of coronaviruses with a simple route for human exposure,” they said, referring to the Sars epidemic in 2003.

They said no evidence had shown that any early cases had a connection to the Wuhan Institute of Virology, a leading research institute into bat coronaviruses that has been linked to the lab leak theory.

“Under any laboratory escape scenario Sars-CoV-2 would have to have been present in a laboratory prior to the pandemic, yet no evidence exists to support such a notion and no sequence has been identified that could have served as a precursor,” according to the scientists.

They said “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” and serves as a major travel hub.

“The link to Wuhan therefore more likely reflects the fact that pathogens often require heavily populated areas to become established,” they said.

The University of Sydney said the paper would be submitted to a leading journal for peer review and publication.

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