The origins of the Covid-19 pandemic are still unknown, with many scientists now openly discussing whether the virus was engineered, or leaked from a Wuhan lab.
But a new theory gives an entirely different explanation.
Dr Jonathan Latham, the executive director of the US Bioscience Resource Project, believes that Covid-19 may have evolved in the body of an infected Chinese mineworker nearly a decade before the pandemic began.
Crucially, samples of the puzzling miner's disease were sent to viral researchers in Wuhan for study, from where they may have escaped into the population, he argues.
Dr Latham says the emergence of the Alpha variant in Kent last Autumn proves that the virus can make "strange evolutionary leaps" and quickly develop large numbers of mutations when inside an individual for a long time.
Earlier this year, Cambridge University concluded that the super-infectious Alpha variant probably evolved in a single immunocompromised patient who had the disease for many months.
Hundreds of mutations
Speaking at a BMJ webinar on the origins of the pandemic, Dr Latham said: "The theory requires many hundreds of mutations in one miner to turn into Sars-Cov-2. Decades were crammed into about six months.
"But we have heard of the surprising phenomenon of isolated cases of greatly accelerated evolution in viruses in Britain. As much evolution occurred in that one individual in England as had occurred in the millions of other infections.
"Our theory proposes that a similar evolution was happening inside the lungs of miners following the mystery disease in 2012 and argues that the virus leaked from a medical sample obtained from the miners infected by the outbreak."
In 2012, six miners who were shovelling bat guano in the Tongguan mineshaft in Mojiang, Yunnan, became seriously ill with a pneumonia-like illness that bore a striking similarity to Covid-19. Three of them died and the others were hospitalised for up to six months.
A Chinese researcher who investigated the deaths for his master's thesis concluded they were probably infected by a Sars-like coronavirus originating in horseshoe bats.
Virus discovered in caves
Just a year after the deaths, Wuhan scientists discovered a virus named RaTG13 in the same caves, which was later found to be a 96 per cent match for Covid-19, but must have diverged 40 years earlier.
However, the new theory suggests that the evolution from a virus such as RaTG13 could have occurred far more quickly inside the body of a miner.
"We know that coronaviruses were diverse and abundant near the mine, and we know that some of the miners underwent lengthy hospitalisations," added Dr Latham.
"Their treatment lasted six months and allowed the evolution of novel adapted human coronaviruses.
"We know many medical samples were sent to the Wuhan institute of Virology, so the question is, what was in the samples, and what was done with any viruses that were found?"
Scientists investigating the origin of the pandemic have repeatedly asked for details of virus sequences housed and studied at the Wuhan Institute of Virology. But a database containing details of the samples was taken offline shortly before the pandemic began.
Alison Young, the Hurley Chair in Public Affairs Reporting at the Missouri School of Journalism, also told the webinar that lab leaks of dangerous viruses were common throughout the world, and had previously led to outbreaks.
"One of the arguments is that lab experiments are extremely rare events," she said. "Lab accidents are not rare. In the US in 2020 there were 134 reported lab exposure incidences of viruses, bacteria and toxins that the US government regulates.
"Lab accidents and exposures happen frequently."
She pointed to the 2007 outbreak of foot and mouth disease in England which occurred after sewage leaked from an Institute for Animal Health (IAH) lab at Pirbright in Surrey as well as leaks of the original Sars virus in China. But she said many lab leaks went unreported.
"Laboratories do not like making their leaks public," she added.
French genome analyst Dr Jacques van Helden, who recently co-authored a letter in the Lancet calling for an objective debate on the origins of Covid-19, said new regulation was needed to stop labs from doing work that could spark a pandemic.
"I want to understand what really happened and so far I don't think we have the answer," he said.
"We need to revise our internal regulations. Irrespective of the origin of this virus we know there are several labs doing experiments of concern because they are generating viruses that are potential pandemics."
His views were also echoed this week by Kevin Esvelt of MIT, who helped invent gene drive technology.
Writing on Twitter, Dr Esvelt said: "It doesn't matter whether it actually came from a lab - mere plausibility is enough. No nuclear warhead could kill as many people as Sars-CoV-2.
"Pandemic virus discovery equals disseminating blueprints for nuke-equivalents accessible to individuals. Let's not."