Scientists in China have claimed COVID-19 did not jump from animals to humans at a wet market in Wuhan, as has been reported widely since the beginning of the outbreak.
While the first cases were reported amongst people linked to the market in Huanan, where live animals are traded, it might merely have been the site of a "superspreader" event where one infected person passed it onto many others, a US expert has suggested, backing the Chinese research into the disease's origins.
COVID-19 is caused by a coronavirus named SARS-CoV-2, which was first detected late last year in China's Wuhan province. It went international in January, and has now killed at least 360,000 people worldwide.
It's related to the SARS virus which emerged nearly 20 year ago, and is believed to have originated in bats before leaping to humans, probably via an intermediate animal such as a pangolin. Bats are able to carry many kinds of viruses without getting sick, and live in large groups, allowing them to spread and mutate rapidly.
But Chinese scientists this week said testing on animal samples collected at the market in January showed no trace of the virus.
"At first, we assumed the seafood market might have the virus, but now the market is more like a victim," said Gao Fu, head of the Chinese Center for Disease Control and Prevention, in comments reported by the state-run Global Times.
The virus was detected in sewage and other environmental samples, but not the animals - suggesting it arrived in the bustling market via other means, likely a person.
Infectious disease expert Colin Carlson of Georgetown University told LiveScience it took 15 years to definitively trace where SARS came from, and the same might be the case for COVID-19.
"I haven't seen anything that makes me feel, as a researcher who studies zoonotic disease, that this market is a likely option... None of the animals tested positive. So since January, this has not actually been particularly conclusive. But this has developed into a narrative."
Another theory, promoted by US President Donald Trump, is that the virus leaked from the nearby Wuhan Institute of Virology, and could have been artificial.
"This is pure fabrication," Wang Yanyi, director of the Wuhan Institute of Virology, told CCTV this week.
"Our institute first received the clinical sample of the unknown pneumonia on December 30 last year. After we checked the pathogen within the sample, we found it contained a new coronavirus, which is now called SARS-CoV-2. We didn't have any knowledge before that, nor had we ever encountered, researched or kept the virus. In fact, like everyone else, we didn't even know the virus existed. How could it have leaked from our lab when we never had it?"
The institute was studying a related virus, RaTG-13, which shares more than 96 percent of its genome with SARS-CoV-2. Chinese researchers sequenced the SARS-CoV-2 genome early in the pandemic, making it available for researchers worldwide.
Both pathogens are very large by virus standards, so even a 4 percent difference amounts to more than 1100 changes, Dr Wang said.
" In the natural world, it takes a long period of time for a virus to naturally evolve and mutate to become SARS-CoV-2."
SARS-CoV-2 and RaTG-13's most common ancestor would have existed between 20 and 50 years ago, according to University of Sydney expert Prof Edward Holmes - that's how different they are.
"There is no evidence that SARS-CoV-2, the virus that causes COVID-19 in humans, originated in a laboratory in Wuhan, China," he wrote in April.
But Dr Wang said while the Wuhan Institute of Virology was studying RaTG-13, it didn't actually have any copies of it - just the RNA sequence.
"Thus, there is no possibility of us leaking RaTG-13."
