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Mars seems an unlikely place for life.

In fact, the red planet today is downright lethal for creatures that need oxygen to breathe and warm, moist places to thrive.Mars is a barren ball of red dirt and rock, unshielded from deadly cosmic and ultraviolet rays that zap its surface constantly. It is soaked in temperatures cold enough to freeze carbon dioxide and scoured by sandstorms that can rage for weeks across the whole planet.

Ice caps cover the poles, towering volcanoes punctuate its vast empty plains and sinuous riverlike channels snake across thousands of miles. And everywhere, like scars from a cosmic pox, are impact craters left from space boulders that pounded, pounded, pounded for eons.

And yet, Mars was not always this way. Once, experts believe, life was possible - and some believe quite likely - on this planet that is most similar in many ways to Earth.

David McKay of the Johnson Space Center in Houston is leader of a team of NASA researchers that claims to have found evidence of ancient Martian microbes in a potato-size rock that fell to Earth.

More than 3 billion years ago, he said at a news conference last week, Mars was warm, wet and nurturing. But after a short period of promise, said McKay, Mars "fell on bad times."

The possibility of life on Mars has intrigued humans for more than 300 years. Early astronomers spotted characteristics that reminded them of home.

In the 1600s, observers determined that Mars had a day of about 24 hours and that it had polar ice caps - just like Earth. In the 1700s, astronomers found that Mars was tilted on its axis and, thus, had seasons - just like Earth. And there seemed to be clouds and the dark areas on the planet were taken to be oceans and seas - just like on Earth.

And in 1878, Italian astronomer Giovanni Schiaparelli spotted a system of what he called "canali" on Mars. Canals meant there had to be intelligent life there, perhaps actual human beings - just like Earth.

"Their idea was not nuts," said Allan Treiman, a Mars expert at the Lunar and Planetary Institute in Houston. "Mars is the most Earthlike of the terrestrial planets. It is just a little smaller than Earth. The temperatures are cold but not that much colder than some places on Earth. And it had features that looked familiar."

There may, some hope, be moisture there yet.

And if there is water . . . .

"It could be a good place to find life," said Treiman.