Coronaviruses capable of infecting humans may have circulated undetected in bats for decades.
Research suggests that one of the closest known ancestors of the virus that causes Covid-19 emerged in bats 40 to 70 years ago.
It has been prepared for human crossing for some time, scientists said.
And this casts further doubt on conspiracy theories that the virus causing Covid-19 was bioengineered or escaped from a laboratory, they added.
Professor David Robertson of the University of Glasgow worked on the study, published in the journal Nature Microbiology.
He said that while Sars-CoV-2 (the pandemic coronavirus) is genetically very close to the closest known bat viruses, they are separated in time by several decades.
“That suggests that these viruses with the potential to emerge in humans have been around for some time,” he told BBC News.
“We really need to understand where or how the virus has been crossed in the human population. If we now believe that this generalized virus is circulating in bats, we need to improve on monitoring that.”
The work points to the need for increased surveillance for emerging human diseases and for more sampling within wild bat populations if we are to avoid future pandemics, he said.
“If these viruses have been around for decades, that means they have had many opportunities to find new host species, including humans,” said Professor Robertson.
The researchers compared the genetic makeup of Sars-CoV-2 with that of a close relative in bats, a virus known as RaTG13 and other bat-related viruses.
They came out with the date the two shared a common ancestor, and found that they followed their own evolutionary path several decades ago.
Professor Mark Pagel of the University of Reading, who was not part of the study, said the work suggests that coronaviruses capable of infecting humans have been present in bats for about 40 to 70 years, but have not been detected.
“This is significant in pointing out the scale and nature of the problems that zoonotic transmission to humans presents: There may be numerous as yet undetected viruses capable of infecting humans residing in animal hosts.”
Viruses may have infected other wildlife, particularly those that come into close contact through the illegal wildlife trade. But to date, all the evidence points to bats being the important reservoir.
Previous research has suggested that pangolins may have played a role in the evolution of Sars-CoV-2, but the latest study suggests that this is not the case.
Instead, pangolins may have contracted the virus more recently through contact with other wildlife through wildlife trafficking in China.
Follow Helen on Twitter.
