Researchers in the UK looked at mutations in the virus and found evidence of rapid spread, but no evidence that the virus is more easily transmitted or more likely to cause serious illness.
"The virus is changing, but this in itself does not mean it is getting worse," genetics researcher Francois Balloux at the University College London Genetics Institute told CNN.
Balloux and colleagues extracted virus sequences from a gigantic global database used by scientists around the world to share data. They looked at samples taken at different times and from different places, and said they indicate that the virus only started to infect humans at the end of last year.
"This precludes any scenario that assumes that SARSCoV-2 may have been in circulation long before it was identified, and has already infected large populations," Balloux's team wrote in its report, published in the journal Infection, Genetics and Evolution.
That's one piece of bad news. Some doctors had hoped the virus had circulated for many months, and may have been infected by infecting many more people than has been reported. It would give hope that there may already be some immunity in some populations.
"Everyone was hoping for it. So was I," Balloux said.
Their findings pour cold water on such an idea. At most, 10% of the global population has been exposed to the virus, Balloux estimates.
Between humans and bats
Many different studies have shown that the new coronavirus, often called SARS-CoV-2 by scientists, originated in a bat, but had to have infected another animal before it jumped into humans. The first human cases were reported in Wuhan, China, in December last year.
Viruses make mistakes every time they replicate, and these mutations can be used as what is called a molecular clock to track a virus through time and geography.
"Our results are in line with previous estimates and point to all sequences sharing a common ancestor towards the end of 2019, supporting this as the period when SARS-CoV-2 jumped into its human host," the team wrote.
"It's very recent," Balloux said. "We are very confident that the host jump happened late last year."
That's because virus samples taken from all corners of the globe show several mutations, and they are similar mutations. "Everything is everywhere," the team wrote.
"It has been introduced and introduced and introduced in almost every country," Balloux added.
They also found genetic evidence to support the suspicion that the virus infected humans in Europe, the United States and elsewhere weeks or even months before the first official cases were reported in January and February. It would be impossible to find the "first" patient in any country, Balloux said.
"All these ideas about trying to find a patient zero are meaningless because there are so many patient zeros," he said.
The Balloux team's findings were reviewed by other experts, a process called peer review, before being published in the journal. He said some reports from other teams, published online in what are called preprinted websites, may have drawn incorrect conclusions.
"All viruses mutate naturally. Mutations in themselves are not a bad thing, and there is nothing to suggest that SARS-CoV-2 mutates faster or slower than expected. So far we cannot say whether SARS-CoV-2 will be more or less deadly and contagious, "Balloux said.
Lane Warmbrod, an analyst at the Johns Hopkins Center for Health Security who has been following the reports on the genetics of the new coronavirus. She said more animal studies are needed to demonstrate how changes in the genetics of the virus can make it more or less contagious or pathogenic.
"Just because these studies tell us that these mutations are rapidly spreading or becoming dominant doesn't mean anything other than that we know it happened. It doesn't tell us anything about what happens biologically," Warmbrod told CNN.
Reports of mutations can be important for teams working on drugs and vaccines to fight the coronavirus. Vaccines are especially necessary to target parts of the virus that have been preserved – which do not change over time.