Researchers in China say a deadly virus outbreak which has killed 18 people so far originated in snakes, but others aren't so sure.
The virus, known as 2019-nCoV, is closely related to SARS and the common cold, but had never been seen before it first appeared in the city of Wuhan in December.
Scientists have traced its origins to the sprawling city's food markets, where various species are traded - including dogs, pigs, chickens, bats, snakes and even koala, often illegally. The snake species they're pointing the finger at are the many-banded krait and Chinese cobra - both of which are venomous and common in southern China.
Identifying the virus' source is essential to developing a vaccine and effective treatments.
"Results derived from our evolutionary analysis suggest for the first time that snake is the most probable wildlife animal reservoir for the 2019-nCoV," Peking University's Wei Ji and his team wrote in their study, published in the Journal of Medical Virology.
They looked at the virus' genome and compared it to known viruses, taking into account their host's location and species.
"Results obtained from our analyses suggest that 2019‐nCoV appears to be a recombinant virus between the bat coronavirus and an origin‐unknown coronavirus," they wrote.
A recombinant virus is essentially built using DNA from two or more different sources - a kind of hybrid.
"The recombination occurred within the viral spike glycoprotein, which recognises cell surface receptor."
That's the part of the virus which lets it attach to cells and infect new hosts. The mutation, the scientists say, is what it gave it the ability to leap across the species divide into humans.
But Paulo Eduardo Brandão, a virologist at the University of São Paulo, says there's no evidence this coronavirus can even infect a snake.
"They have no evidence snakes can be infected by this new coronavirus and serve as a host for it," he told journal Nature. "There's no consistent evidence of coronaviruses in hosts other than mammals and [birds]."
And Peter Daszak, president of the EcoHealth Alliance in New York, said there isn't "evidence ample enough to suggest a snake reservoir" for the virus.
"This work is really interesting, but when we compare the genetic sequence of this new virus with all other known coronaviruses, all of its closest relatives have origins in mammals, specifically bats."
But Prof Nigel McMillan, director of infectious disease and immunology at Griffith University's Menzies Health Institute in Australia, said it was "entirely possible but how it occurred is a mystery".
"It has happened before and will happen again. HIV came about the same way. Eating contaminated exotic meats is a clear way for this to occur."
SARS, which originated in China in 2002 and killed 774 people, was in 2017 traced back to a remote cave in Yunnan province. It had a fatality rate of about 10 percent, compared to 2019-nCoV's 3 percent to date.
2009's swine flu epidemic, which killed up to half a million people, was caused by a virus made up of bird, swine and human origins.