Scientists have discovered the origins of lager – deep in South America's forests.

The mystery yeast, which 500 years ago combined with the local type used by Bavarian monks for brewing ale, has been discovered on beech trees in Patagonia.

Somehow, possibly via a fruit fly, it ­travelled 7,000 miles to Germany and by chance helped create one of our most popular drinks.

Until now, scientists were puzzled as to why they could not find the origins of lager yeast among the 1,000 known species in the world. The unique yeast allows lager to brew at cold temperatures.

Chris Todd Hittinger, of the University of Wisconsin-Madison, is co-author of the study published in US journal ­Proceedings of the National Academy of Sciences.

He said: "People have been hunting for this thing for decades – and now we've found it. It's clearly the missing species."