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A New Family Tree for Ants

6 April 2006 (All day)
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S. Bruce Archibald

Great ant.
A 34-million-year-old fossil helped confirm a new ant family tree.

Ants come in all sizes, from smaller than a grain of sand to larger than a thumb. Now, by rebuilding the ant family tree, researchers have discovered that the ancestral ants that led to all this diversity date back 30 million years longer than previously thought. The study also indicates that it wasn't until the rise of flowering plants that ants truly became diverse.

Systematists have struggled for years to determine the relationships between various ant species. Family trees based on morphological or DNA data are often at odds with each other, thwarting efforts to reconstruct the history of this pervasive insect. Harvard graduate student Corrie Moreau's solution was to use a lot more DNA from 139 ant genera--about half the known genera--in her analysis. She compared sequences from six genes to estimate which ants were ancestors, cousins, or distant kin. She then combined this information with the estimated ages of 43 fossils and ants preserved in amber to come up with a timeline of ant evolution.

Unexpectedly, the little-known subterranean ants are the most ancient and gave rise to the two other major groups: predatory hunters and a catchall category containing carpenter, army, leafcutter, and other familiar ants. Although most ants can sting, some use the stingers in other ways, and the data indicated that loss of the stinging ability, thought to have happened only once, evolved twice, in rather distantly related groups.

Fossils indicate that the modern ant is about 100 million years old, and the prototype ant dates back 20 million years earlier. But the new analysis shows that the modern ant had its start 140 million to 168 million years ago. It gave rise to the hundreds of genera known today much later, 60 million to 100 million years ago--about the same time that flowering plants had spread across ancient continents. These hardwood forests provided new habitats--leaf litter and canopies, for example--that ever-more-diverse ants and their prey could call home. "As angiosperms rose to dominance, the ants came along and rode the back of the wave," says Moreau, whose findings appear 7 April in Science.

"It's the largest phylogenetic analysis of ants published to date and also the one that attempts to incorporate most of the information available from our growing fossil ant record," says Roberto Keller, an ant systematist from the American Museum of Natural History in New York City. As such, adds Phil Ward, an ant systematist at the University of California, Davis, "this is a landmark paper that offers a new framework for ant evolution."

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