If Mars were terraformed and had oceans.
Terraforming is the process of altering the environment of a planet to make it more clement and suitable for human habitation. The possible future terraforming of Mars and Venus has been widely discussed. A major consideration before starting such a project would be its effects on any indigenous life.
As a concept, terraforming goes back more than half a century. In the 1920s, Hermann Oberth defined as the ultimate goal of space exploration: "To make available for life every place where life is possible. To make inhabitable all worlds as yet uninhabitable, and all life purposeful." In 1948 the astrophysicist Fritz Zwicky, in expansive mood, suggested a reconstruction and reconfiguration of the entire Universe, starting out by changing the positions of the planets, satellites, and asteroids of the Solar System with respect to the Sun. A more modest scheme to make Venus habitable by injecting colonies of algae to reduce atmospheric carbon dioxide concentration was proposed in 1961 by Carl Sagan. An initial effort in this direction will most likely be directed at Mars.