Researchers led by Rebecca Garten of the Centers for Disease Control and Prevention studied samples of the flu isolated in Mexico and the United States.
The findings suggest that in the future pig populations will need to be closely monitored for emerging influenza viruses, according to the report, released Friday by the journal Science.
First detected last month, the H1N1 flu has sickened more than 11,000 people in 41 countries and killed 85, according to the World Health Organization, whose figures often trail those of individual countries. Mexico has reported 75 swine flu deaths, the United States 10, and there has been one death each in Canada and Costa Rica.
Garten's team said the exact combination of the virus' eight gene segments has not previously been reported among swine or human influenza viruses.
They said all of the segments originated in bird hosts and then began circulating in pigs at various times in history, from 1918 through 1998. Infected pigs might not have shown signs of illness, but gave the viruses an opportunity to mix with other viruses and create more dangerous strains.
Six of the eight segments include genetic material from human, avian and swine viruses as the result of these viruses' tendency to swap pieces of their genomes with each other, the researchers said. These have been circulating since at least 1998.
The other two segments originated from Eurasian swine viruses, they reported.
The sequences for the gene segments did not reveal the signatures of high transmissibility or virulence that have been found in other influenza A viruses, suggesting that other, yet-unknown sequences are responsible for the new virus' ability to replicate and spread in humans.
The H in H1N1 stands for the protein hemagglutinin, which is responsible for the virus' ability to bind to and infect its host cell. The researchers focused on that protein in experiments and say they will need to continue to look for changes in the hemagglutinin protein in the new virus, which may affect the selection of vaccine candidates, the authors say.
While the journal Science normally publishes on Thursday, the new study was released immediately because of the widespread interest in the topic.