"Rainbow Mars" by Larry Niven

Five-time Hugo Award-winner Larry Niven weaves together time travel and fantasy to create an utterly unique novel on the origin of the Martian "canals."

Hanville Svetz was born into a future earth that matches the sorriest predictions of Greenpeace. With most of Earth's original species extinct, Svetz travels back and forth in time retrieving them. Svetz learns that Mars was inhabited, and how the sapient Martian species were wiped out. He forsees that Earth could soon fall victim to the same fate.