How should we judge the success of an animal? Philip S. Ward, a biologist at the University of California, Davis, offers what could be called the Picnic Test. “Have a picnic anywhere in the world,” he suggests. “Who would pick up the crumbs?”

Unless you happen to lay down your picnic blanket in Greenland, Antarctica, or a few remote islands in the Pacific, the answer will be ants. Ants have spread to just about every corner of earth’s dry land, colonizing virtually every imaginable ecosystem. By one rough estimate, there are 10,000 trillion ants on earth at any moment. In one study in a Brazilian rain forest, scientists discovered that the total mass of the ants that lived there was about four times greater than that of all the mammals, reptiles and amphibians combined.

One factor in the spectacular success of ants is their social life. They live in large colonies in which they divide the labor of finding food, rearing their young and defending their nests. Their societies are so complex that some scientists have studied ants as a way to understand the factors behind our own evolution into a social species.

It’s thus no surprise that many biologists — Dr. Ward among them — have long wondered how ants evolved. In the journal Current Biology, Dr. Ward and colleagues at the University of California, Davis, and the American Museum of Natural History, have now published an evolutionary tree of ants and their closest relatives that may provide the answer.