Top international scientists have again poured cold water on widespread claims the Covid-19 pandemic began in a Chinese laboratory.
At first dismissed, the "lab leak theory" has been given more credence after intense media coverage in the US over the past few months.
US President Joe Biden directed scientists to take a closer look, an open letter published in The Lancet called for more transparency, reports said a top Chinese official defected with sensitive information and a leaked US intelligence memo talked of several researchers at the Wuhan Institute of Virology (WIV) becoming ill shortly before the outbreak.
One theory was that Sars-CoV-2, the virus that causes Covid-19, had been deliberately engineered; another was that that scientists could simply have been growing a culture of the virus, and it escaped from there.
But, as a team of international scientists have set out in a review just published online, ahead of peer review, there's still scant evidence to support either possibility.
"All of the evidence points toward a natural, zoonotic origin," Otago University virologist and review co-author Dr Jemma Geoghegan told the Herald.
She and the other authors noted how the virus' documented epidemiological history – which began at the Wuhan Huanan Seafood Wholesale Market – was comparable to other coronavirus outbreaks linked to animal markets.
In particular, there were "striking" similarities to the early spread of the 2003 Sars epidemic to markets in Guangdong, near where the earliest-infected people lived and worked.
Already, the World Health Organisation's report on the Covid-19 pandemic's beginnings had identified that live animals like ferret-badgers and rabbits were being traded in the markets.
The animals could provide an intermediate host for the virus to jump to humans – and the market was just the type of setting for a zoonosis event to happen.
By contrast, all lab escapes documented to date had almost exclusively involved viruses that were being studied specifically because of their known human infectivity.
The researchers said there was still no evidence that any of the early cases had any connection to the WIV, nor was there any evidence the institute had or worked on a progenitor of Sars-CoV-2 virus before the pandemic.
"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," they said.
"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 within China and internationally.
"The link to Wuhan therefore more likely reflects the fact that pathogens often require heavily populated areas to become established."
Going deeper, the researchers addressed widespread speculation about the supposedly unusual molecular make-up of Sars-CoV-2.
In one of the earliest major studies into the virus, scientists analysed its genetic template for spike proteins, which it used to grab and penetrate the outer walls of human and animal cells.
More specifically, they focused on its receptor-binding domain (RBD) - a kind of grappling hook that grips on to host cells - and what's called the furin cleavage site, a molecular can opener that allows the virus to crack open and enter host cells.
This earliest research suggested the RBD portion of the spike proteins had evolved to effectively target a molecular feature on the outside of human cells called ACE2 - a receptor involved in regulating blood pressure.
The Sars-CoV-2 spike protein was so effective at binding the human cells, in fact, that the scientists concluded it was the result of natural selection, and not the product of genetic engineering.
Yet many theorists have surmised that the furin cleavage site was so unusual it must have been artificially inserted by scientists.
The review authors did note the cleavage site was absent from the closest known relatives of Sars-CoV-2, but they added that wasn't surprising, given the lineage leading to the virus had been poorly sampled.
Further, they added, furin cleavage sites were common in other coronavirus spike proteins, including the last coronavirus to spark a global incident - MERS-CoV – and also feline alphacoronaviruses, most strains of mouse hepatitis virus, and endemic human betacoronaviruses.
A near identical nucleotide sequence to Sars-CoV-2's had also been found in the spike gene of the bat coronavirus HKU9-172 – and there were indications the evolution of both viruses had involved "recombination", or the natural exchange of genetic material.
"Hence, simple evolutionary mechanisms can readily explain the evolution of an out-of-frame insertion of a furin cleavage site in Sars-CoV-2," the researchers said.
They added there was "no logical reason" why an engineered virus would use such a poor furin cleavage site as Sars-CoV-2's.
That would have entailed an "unusual and needlessly complex" feat of genetic engineering, for which there were few precedents.
"Further, there is no evidence of prior research at the WIV involving the artificial insertion of complete furin cleavage sites into coronaviruses."
Although they said the possibility of a lab accident still couldn't be entirely dismissed, they concluded the explanation remained "highly unlikely, relative to the numerous and repeated human-animal contacts that occur routinely in the wildlife trade".
"Failure to comprehensively investigate the zoonotic origin through collaborative and carefully coordinated studies 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."