The ant trail

One factor in the spectacular success of ants is their social life.

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 earths dry land,colonising virtually every imaginable ecosystem. By one rough estimate,there are 10,000 trillion ants on earth at any moment.

One factor in the spectacular success of ants is their social life. They live in large colonies in which they divide the labour 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.

Its thus no surprise that many biologists 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.

The authors conclude that the ancestors of ants were wasps. Not just any wasps,though: the closest relatives turn out to include mud dauber wasps,which make pipe-shaped nests on building walls.

The oldest ant fossils are 100 million years old. Scientists were able to narrow the field by comparing ants with other living insects. They came to agree that ants were closely related to stinging wasps and bees.

In the 1990s,Ward and other researchers started sequencing genes from insects to gather more evidence. Recently,Ward and his colleagues took advantage of the explosion in DNA sequencing power to take a fresh look at the old problem of ants.

Ward travelled around California and Nevada to catch ants,wasps and bees. He and his colleagues then sequenced more than 300 genes from one species of ant and 10 species of bees and wasps,each representing a different major lineage. Their research clearly pointed to mud dauber wasps and bees as the closest cousins to ants.

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Female mud daubers first create a mud cylinder in which to house the egg. The ancestors of ants may have started out as similar insects,building simple nests and delivering food to their offspring.

Carl Zimmer