Ancient rivers that once flowed across an Earthlike Mars may have harbored primitive forms of life that flourished briefly before the red planet turned bitterly cold and dry, scientists say.

Steve Squyers, a planetary scientist at Cornell University, said that most evidence now indicates that Mars and the Earth were very similar shortly after their formation some 4.5 billion years ago and that conditions that led to the formation of life on Earth may also have existed on Mars.Squyers was among the scientists discussing planetary research at a meeting Monday of the American Geophysical Union. In an interview, he said the most likely place for finding proof of Martian life is soil laid down billions of years ago when water is thought to have flowed freely across the planet's surface.

"We know that life developed on Earth very early, and we know that conditions on Mars very early were, if not Earthlike, were very much more Earthlike than they are at the present," Squyers said.

No evidence of life on Mars has been found so far in studies by satellites and by robot craft that landed there. But Squyers and others believe Mars holds out that possibility, and even if life never evolved there, that finding also would be important.

"If you don't find life, you've found out that despite the fact that you had really Earthlike conditions, they weren't Earthlike enough to produce life," he said. "That will provide more information on the type of conditions you must have to enable life to develop."

Susan Postawko of the University of Hawaii said most scientists studying the planet now accept the idea that water once flowed on Mars in massive rivers that cut channels across the surface. She said that idea once was considered outrageous because of conditions on Mars now.

"It's virtually impossible to have water flowing on Mars under the current conditions," said Torrence Johnson, a Jet Propulsion Laboratory scientist. The planet has only seven one-thousandths of the atmospheric pressure of Earth and is very cold. Water would either evaporate or freeze, said Johnson.