New study retraces Covid's origins to bats in southwest China or northern Laos
In a paper published in Cell journal, scientists compared the genomes of 250 coronaviruses to reconstruct how the pathogens evolved over time, potentially offering insights into how Covid-19 spilled into people – an unresolved question that's been thrust back into the spotlight since Donald Trump assumed office.
The researchers found that both Sars viruses were circulating and changing inside bats in southern China and neighbouring countries for hundreds of thousands of years before emerging in humans.
Bats have unusual immune systems which allow them to harbour coronaviruses, allowing them to mix and mutate into something new.
By unpicking this 'recombination' process, the scientists were able to estimate when and where each of the two coronaviruses had emerged in bats.
They found Sars was circulating in western China just one to two years before it jumped into humans in Guangdong, central China.
Sars-Cov-2 followed an 'extremely similar' route, they say. It made its final recombination between 2012 and 2014 in bats in southwest China or northern Laos, five to seven years before sparking a human pandemic Wuhan.
The researchers say it is striking that, in both instances, the virus was circulating in bats hundreds of miles from where humans were first infected.
In the case of Sars, there is strong evidence that wildlife sold in wet markets bridged this geographical gap and carried the virus to humans.
Researchers have previously established that the virus was present in palm civets and other wild mammals for sale in markets at the time of the first Sars outbreak in 2002.
They concluded that it was the wildlife trade which transported the pathogens hundreds of miles, from bat caves to people.
Prof Michael Worobey, head of the department of ecology and evolutionary biology at the University of Arizona and a co-author of the paper, said that 'we're seeing exactly the same pattern with Sars-Cov-2'.
Like Sars, Sars-Cov-2 evolved in bat caves hundreds of miles away from the spot humans were first infected. While the paper does not prove it was transported by the wildlife trade to the wet market around which the first human cases emerged in Wuhan, the authors said parallels between the two pandemics were too striking to be ignored.
'At the outset of the Covid-19 pandemic, there was a concern that the distance between Wuhan and the bat virus reservoir was too extreme for a zoonotic origin,' said Prof Joel Wertheim, a professor of medicine at UC San Diego School of Medicine's division of Infectious Diseases and Global Public Health.
'This paper shows that it isn't unusual and is, in fact, extremely similar to the emergence of Sars-Cov-1 in 2002,' he said.
The question of how Covid-19 emerged has long been contentious, but tensions have ratcheted up since President Trump assumed his second term in office.
Last month the White House created a website called 'Lab Leak: The True Origin of Covid 19', which suggests the pandemic was caused by human error in a Wuhan lab facility.
Beijing then resurfaced its own conspiracy theory – believed by many in China – that the US's own high security labs were to blame.
Prof Jonathan Ball, a professor of molecular virology at the Liverpool School of Tropical Medicine who was not involved in the paper, praised the new research for showing how the two Sars viruses had evolved. But he added that it could not settle the lab v natural origins debate.
'I sit in favour of the Huannan market [analysis], mainly because of all the other evidence. This paper adds another bit of evidence to that pile,' he said. 'But it's not going to quash the lab leak [hypothesis], and it won't persuade Donald [Trump].'
Prof Stuart Neil, head of the department of Infectious Diseases at King's College London, also said the paper 'can't fill the gap between the bats and the market'.
But he added that the evolution and geography of Sars and Sars-Cov-2 described in the paper clearly showed that both were able to fully emerge of their own accord in nature and that there was no reason it could not happen again.
'What [the paper] reinforces is that you need to control and monitor the most likely flash points for zoonotic emergence,' he said.
Yet some remain sceptical of the idea that the wildlife trade carried the Sars-CoV-2 to Wuhan as happened with Sars in the 2002 outbreak.
'It's a very sophisticated analysis of the evolutionary origins of Sars viruses in their natural reservoir, however the analysis leaves two big gaps,' said Prof Mark Woolhouse, a professor of infectious disease epidemiology at the University of Edinburgh.
'The story [for Covid] stops some years short of 2020, and it also leaves a very big gap in terms of space.
'I don't agree with the inference in the paper that the only plausible way that the virus could have gone from its natural habitat to Wuhan is through the wildlife trade.
'For me, there's no convincing evidence that the animals implicated in the early spread of Sars-Cov-2 [such as racoondogs] were farmed in the region where Sars-Cov-2 is thought to evolve. So for me, the most likely route that it got from one place to the other is via people,' Prof Woolhouse said.
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