The new coronavirus that has killed 33 people since September may have emerged almost a year earlier than the first known case and circulated unnoticed for that time, a study has shown.

The genetic sequence of a virus taken from a 73-year-old patient who died in Munich in March, together with sequences from four other patients, suggests a common ancestor halfway through 2011, researchers at the University of Bonn Medical Center in Germany wrote in The Lancet Infectious Diseases journal Monday. The first known case was in Jordan in April 2012.

The finding provides another clue as researchers try to figure out where the virus originated and how it is spreading. While most cases have been identified in Saudi Arabia, they have also been detected in Jordan, Qatar, the United Arab Emirates, France, Germany, Italy, Tunisia and the United Kingdom.

The virus appears to be most closely related to one taken in from a bat in the Netherlands in 2008. It may have crossed from bats to a domesticated or agricultural animal, and from there spread into humans.