The finding suggests that Severe Acute Respiratory Syndrome arose just as many new strains of influenza do - when a bird virus recombines with a virus in a mammal to take on a form that can infect humans.
SARS swept across China and was exported as far as Canada between November 2002 and May 2003, killing 774 people and infecting as many as 8000.
Experts eventually identified a new kind of coronavirus as the cause of the disease, essentially an unusually severe form of pneumonia. Such viruses cause a range of veterinary diseases but usually nothing worse than the common cold in people.
David Guttman, a professor of evolutionary genomics at the University of Toronto, compared the SARS virus genome to related coronaviruses.
About half the DNA looked like coronavirus sequences taken from mammals, while half looked like coronaviruses normally found in birds, Guttman reported in the January issue of the Journal of Virology.
A key gene called the spike gene looked to be a mix. This gene seems to be important in allowing the virus to infect cells, experts have said.
The merging of mammalian and avian viruses probably allowed the spike gene to sneak past immune system sentinels.
"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"
Professor of evolutionary genomics at the University of Toronto
"These recombination events have the potential to create an entirely new structure essentially instantaneously," Guttman said in a statement.
"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.
"It's possible that a civet picked up the virus from a bird," Guttman said.
"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.
"This will allow us to design more effective treatments and respond more effectively to future outbreaks," Guttman said.