The study -- led by Richard Pancost and colleagues at the University of Bristol -- showed carbon isotope values of hopanoids (compounds made by bacteria) suddenly decreased in a manner that the scientists said can only be explained by switching to a diet of methane. That, they said, suggests methane emissions must have increased at that time.

The team that included scientists from the University of London and other institutions analyzed the geochemical composition of sediments taken from the Cobham Lignite wetland in southeast England.

"Fifty-five million years ago, a massive release of carbon into the atmosphere caused significant global warming," said Pancost. "It is likely that this warming and associated climate change caused a change in environmental conditions that brought about increased methane emissions. This in turn, may reflect an increase in methane production and subsequent release from the terrestrial biosphere."

The research is detailed in the current issue of the journal Nature.