WASHINGTON -- Fire helped early humans evolve and become more intelligent not because it allowed them to barbecue meat but because it allowed them to cook vegetables, researchers said Tuesday.

Learning how to cook probably also allowed humans to develop their unique monogamous society, the researchers reported in the journal Current Anthropology, to be published in December."The process of human evolution had much to do with food and how it was prepared," Gregory Laden of the University of Minnesota, who helped lead the study, said in a statement.

Laden, Richard Wrangham of Harvard University and colleagues noted that very early pre-humans, including the australopithecines such as "Lucy," had huge teeth and powerful jaws.

"This indicates that they ate a lot of food over many hours of the day," Laden said in a telephone interview.

"It has got to be low-quality food," he added, including raw foods such as roots, nuts and seeds.

By 1.9 million years ago, when Homo erectus appeared, teeth became smaller and jawbones less robust. Females got bigger -- closer in size to males. Brains and bodies both grew.

Laden and Wrangham said the changes occurred because the pre-humans had discovered fire and learned how to make roots and other vegetables easier to eat and more nutritious.

While some anthropologists argue it was because meat entered the diet, Laden and a team of anthropologists, nutritionists and primatologists argued otherwise.

"However you put meat in doesn't fit in," he said.

Laden pointed to recent studies that indicated pre-humans ate some meat -- probably scavenged from carcasses -- more than 2.5 million years ago. But their bodies did not change until much later.

"At 2 million years we see no change in the distribution of (animal) bones (at pre-human sites), and we don't see meat as being high-energy food source. It has to be (a) high-energy food source to explain this doubling of body size," he said.

In addition, evidence is building that humans started to use fire just at the time their bodies changed. Meat is just as nutritious whether it is raw or cooked, but plant food is not.

"(Cooking) also makes a lot of things less toxic and more chewable. If you are an ape with fire, there is a much longer list of foods you can eat," Laden said.

"We strongly suspect hominids began using fire about 1.9 million years ago, when Homo erectus appeared," he added.

He said colleagues working in Kenya have recently contacted his team and said they have evidence that humans were controlling fire that long ago. The most recent accepted evidence puts fire use at just 200,000 to 500,000 years ago.