Like people, ants have often fought over food and territory. But ants began fighting long before humans: at least 99 million years ago, according to Phillip Barden, a fossil insect expert who works in the Insect and Evolution Lab of Jessica L. Ware, an assistant professor in the Department of Biological Sciences at Rutgers University-Newark.

"That's a trait of ants," Barden said. "Many ant species do that all the time. They're always warring with either other individuals of the same species from different colonies or with different species."

The ant wars began in the Cretaceous period, when enormous dinosaurs thrived on Earth, according to a study published online in the journal Current Biology on Thursday. Barden, the lead author, is affiliated with the American Museum of Natural History in New York City. Co-author David A. Grimaldi is a curator at the museum and is also affiliated with Cornell University and the City University of New York.

The fighting ants and others trapped in ancient Burmese amber from Myanmar are among the earliest known ants.

"These early ants belong to lineages distinct from modern ants," he said. "That is, they aren't necessarily the direct ancestors of modern ants. They're kind of their own branch doing their own thing."

The study also provides strong evidence that ancient ants - like modern ants - were social, according to Barden, who began a two-year, National Science Foundation Postdoctoral Fellowship in biology at Rutgers-Newark in September.

"We have one piece of amber with as many as 21 worker ants trapped, and that's significant because at this time period, ants are very rare to find in fossils. They make up less than 1 percent of all insects in amber," he said. "So to find 20 in one piece is highly suggestive of social behavior."

Today, scientists have described 13,000 species of living ants and some researchers believe at least twice as many exist, Barden said. Scientists think that some of today's ants are related to ones that lived 99 million years ago.

People think that ants' social behavior is one reason why they've done so well. Instead of competing as one individual, they compete, in some cases, as tens of thousands, if not millions, and most of them are not reproducing, he said. Instead, most work for a colony - and that's a beneficial trait.