Dr Stephan Gunther, of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, Germany, said: "The source of the virus is still not known."
He led an international team of researchers who studied the genetics of the virus and reported results online in the New England Journal of Medicine.
The outbreak has caused panic and killed more than 120 people in West Africa, mostly in Guinea, according to the World Health Organisation.
Ebola causes internal bleeding and organ failure and is fatal in 30% to 90% of cases, depending on the strain. It spreads through direct contact with infected people, and some earlier cases have been linked to certain fruit bats in West Africa.
There is no cure or vaccine, so containing the outbreak has focused on supportive care for those infected with the virus and isolating them to limit its spread.