Fossil find extends ants’ ancient lineage

When David A. Grimaldi, an entomologist at the American Museum of Natural History in New York, gazed into a 92-million-year-old piece of amber that had been donated to the museum about 6 months before, he saw something rather unexpected. “But I knew exactly what it was,” he adds.

It was the fossil of a primitive ant. The surprise is that the species of ant must be 40 million years older than any others of its subfamily, Grimaldi and his museum colleague Donat Agosti report in the December 15 Proceedings of the National Academy of Sciences. The finding forces researchers to reevaluate their ideas about the early evolution of these tiny insects.

Grimaldi says the fossil has many features that suggest the female ant was a working member of a social colony. First, the insect has a large gaster, or abdomen and lower thorax, a defining feature of such ants. Second, it has a metapleural gland, which secretes a chemical that identifies social ants to others in their colony. Finally, it sported no wings, so the ant wasn’t a queen.

A gland at the end of its abdomen identifies the amber-entombed insect as a formicine ant, so called because it can defend itself by squirting formic acid. Other formicine ants include wood ants, carpenter ants, and the almost ubiquitous species that Grimaldi terms sidewalk ants.

Formicine ants are “amazingly adaptable,” he says. “That’s what makes them so successful.” Today, there are more than 9,500 known living species of formicine ants, and there are probably between 3,000 and 9,000 others yet to be discovered, says Ted R. Schultz, a research entomologist for the Smithsonian’s National Museum of Natural History in Washington, D.C.

The fossil record shows that ants were rare—and anatomically quite primitive—until their explosion of evolutionary success about 45 million years ago. But today, they dominate, Grimaldi notes. Ants are leading predators of invertebrates in many ecosystems, and in the tropics of the Western Hemisphere, they are leading herbivores as well. Up to 15 percent of the fresh vegetation there is carried off by leaf-cutter ants, Schultz says.

Worldwide, 15 to 20 percent of the total weight of all land animals comes from ants, Schultz notes. In the tropics, it’s 25 percent or more. Grimaldi agrees: “In the tropical lowlands, ants rule.”

The new fossil joins previously discovered early specimens of two other large, modern subfamilies of ants. Therefore, the lineages for most types of modern ants were already in place at least 90 million years ago. This has profound implications for ant evolution, says Grimaldi.

Formicine ants actually are stingless wasps, he says, and their earliest ancestors couldn’t have arisen before about 110 million years ago, when wasps first appeared. The discovery in the amber indicates that the diversification of ants took place over a much shorter time—only 20 million years—compared with the 70 million years that entomologists had previously presumed.

The finding also intensifies the mystery of why it took so long for ants to evolve their dominant role in so many ecosystems, including picnics. All the ingredients for success were apparently in place. The ancient species had a social structure, the climate was quite warm, and the ants’ ecological niche wasn’t occupied by other species, Grimaldi says.

“All of their ecological opportunities were already available [92 million years ago],” he notes. “We simply don’t know what’s responsible for their present-day success.”