That's the Spirit!

The planet Mars, I scarcely need remind the reader, revolves about the sun at a mean distance of 140,000,000 miles, and the light and heat it receives from the sun is barely half of that received by this world. It must be, if the nebular hypothesis has any truth, older than our world; and long before this earth ceased to be molten, life upon its surface must have begun its course. The fact that it is scarcely one seventh of the volume of the earth must have accelerated its cooling to the temperature at which life could begin. It has air and water and all that is necessary for the support of animated existence.
-H. G. Wells
The War of the Worlds

Exceeding all expectations, NASA's Mars rover Spirit has crossed the one year mark. Close behind is Opportunity, which is running almost as well as it did when it landed. Not bad when you consider the planned life expectency was 90 days! Success like this is what leads people to take space exploration for granted. Almost daily there are new images and data from these rovers. It is so frequent we tend to lose our sense of wonder at it.

Researchers have now concluded that Mars once supported a watery environment. There is more and more discussion about what forms of life may have existed and may still yet exist on the Red Planet. There is even discussion about life being exchanged between the planets. Microbes could be flung from one planet to the other as a result of asteroid or comet impacts.

When Apollo 12 astronauts Charles Conrad Jr. and Alan Bean went to the moon, the found the unmanned Surveyor 3 probe. They retrieved several items from the craft to study the effects of solar radiation on the metal. Scientists examining these parts found they still contained microbes from when the it was assembled on earth. The microbes, when placed in a petri dish, grew.

So it's not so much of a stretch to conceive of microbes being shared between the planets. It is possible that life developed earth then found it's way to Mars. Likewise, it's conceivable that life on earth finds it's origin on Mars, or elsewhere.

Maybe Battlestar Galactica was right. Maybe life here did begin out there.