The Sars virus could be the result of a merger between viruses carried by birds and mammals, say researchers.
Coronavirus causes Sars
The finding is based on a genetic analysis of the coronavirus, which causes the disease, and others that are closely related.
A similar phenomenon is responsible for the emergence of new types of flu virus.
The research, by a team from the University of Toronto, is published in the Journal of Virology.
Sars killed 774 people and infected as many as 8,000 in a world-wide outbreak earlier this year. China and Hong Kong bore the brunt of its effect, but Taiwan, Singapore and Canada also recorded many deaths.
The virus was eventually identified after a concerted international effort as a new kind of coronavirus.
Such viruses usually cause a range of veterinary diseases - but usually nothing worse than the common cold in people.
Lead researcher Professor David Guttman, an expert in evolutionary genetics, found that about half the DNA in the Sars virus looked like coronavirus sequences taken from mammals.
But the other half looked liked coronoviruses normally found in birds.
And a key gene in the virus, known as the spike gene, seemed to be a mix of the two.
The spike gene is thought to control the virus' ability to infect cells.
Immune system fooled
Professor Guttman believes the merging of mammalian and avian viruses probably allowed the spike gene to sneak past the immune system defences.
He said: "These recombination events have the potential to create an
entirely new structure essentially instantaneously.
"Since our immune systems have never seen this new viral form, it is more difficult for them to respond to it in a timely and effective manner."
Sars coronavirus was found in raccoon-like animals called civets in wildlife markets.
Professor Guttman said: "It's possible that a civet picked up the virus from a
"This could have created the opportunity for a very rare recombination event that produced a virus with a new host range.
"Basically, the recombinant virus is infectious to humans, while the two parent viruses are not.
"This new virus likely then spread to humans due to poor hygiene and close
quarters in the food markets of southern China."
Sars has not been seen since it was brought under control in June but health officials are watching warily for it to re-emerge.
Professor Guttman hopes his work will lead to more effective treatments.