Overall, it wiped out 95 percent of the gorilla population within a year, they reported in the journal Current Biology.

"Thousands of gorillas have probably disappeared," they wrote.

The study shows that the deadly virus spreads directly from gorilla to gorilla and does not necessarily depend on a still-unidentified third species of animal, perhaps a bat, that can transmit the virus without getting ill from it.

It also may shed light on how early humans evolved, they suggested. The findings may show that pre-humans were slow to live in large social groups because disease outbreaks could wipe out those who did.

Ebola hemorrhagic fever is one of the most virulent viruses ever seen, killing between 50 percent and 90 percent of victims. The World Health Organization says about 1,850 people have been infected and 1,200 have died since the Ebola virus was discovered in 1976.

WHO and other experts say people probably start outbreaks when they hunt and butcher chimpanzees. The virus is transmitted in blood, tissue and other fluids.

Caillaud's team said Ebola is a serious threat to the survival of endangered gorillas and chimpanzees, along with hunting and the destruction of the forests they live in.

The researchers documented one outbreak of Ebola and its effects on gorillas and people in Odzala-Kokoua National Park.

"On October 13, 2003, two villagers from Mbandza hunting at an undetermined site inside the park got contaminated and became index cases of an outbreak that killed 29 people in seven weeks," they wrote.

They were able to identify 400 individual gorillas.

"Overall, 109 distinct gorilla social units visiting Lokoue were reliably identified and monitored during a 1,360-day period."

By July 2004, nearly all the gorillas were dead. "Due to intra-group transmission, the death rate was highest among gorillas living in groups," the researchers wrote.