QUESTION:
How could there be life on Mars if there is only an atmosphere of 
carbon dioxide?

ANSWER from Dr. Bob Anderson on December 19, 1996: 
Bacteria is one kind of life that could exist in an atmosphere of only 
carbon dioxide.  There are certain bacteria on Earth
that can only survive in an atmosphere that does not contain oxygen. It 
"might" be possible for bacteria similar to these to be on Mars. But 
they could not survive on the surface of Mars today due to the high 
influx of ultraviolet radiation.  The high UV radiation would not 
let life, as we know it, to exist on the surface.

Dr. Bob Anderson
Mars Pathfinder Mission