Leaf-cutter ants - world's first farmers

Long, long ago - millions of years before the Sumerians and Ur people of the Fertile Crescent discovered agriculture - a tribe of clever ants practiced a highly sophisticated form of farming, and their evolutionary descendants are still at it.

Those ants were the world's first farmers, harvesting leaves from trees and bushes, chopping the leaves into nutritious pulp, feeding the pulp to nourish their cultivated crops of fungus, and harvesting the fungus to feed themselves.

It's the oldest example of agriculture in the world by far, and entomologists at the Smithsonian Institution have now constructed an evolutionary tree to reveal that the very first ancestral ant farmers emerged in the world some 50 million years ago - incidentally, during a long period of natural global warming - and over the past 25 million years gave rise to at least four different farmer tribes.

Most recent among the farmers, the Smithsonian scientists find, are two tribes of common leaf-cutter ants in the tropical Americas that probably evolved less than 8 million years ago, and their descendants are now the most widespread and dominant plant eaters of that part of the world. Ant farming technology achieved its evolutionary apex in these tiny leaf-cutters, say the scientists.

And while the modern leaf-cutting farmers live primarily in the tropics, at least one species has been found as far north as California around Sacramento, as well as in New York and Illinois, the researchers say.

This remarkable story of ant evolution is being published online this week in the Proceedings of the National Academy of Sciences by Ted Schultz and Sean Brady of the Smithsonian's Museum of Natural History in Washington. The two scientists have spent 15 years using the most advanced techniques to derive molecular clocks for tracing the ancestry of the ants through the mutation rates of their DNA sequences.

Their work evokes more than enthusiasm from Brian Fisher, curator of entomology and a leading ant expert at the California Academy of Sciences, who provided the Smithsonian team with samples of ants he has collected for years throughout Central and South America.

"This is a great study," Fisher said, "because it provides us with major new keys to understanding the origins and evolution of biodiversity in some of the world's most important species."