The discovery of a second genesis of life besides the one on Earth, this time on Mars, would have profound scientific and philosophical implications. Scientifically, it would provide a second example of biochemistry and of evolutionary history. Many important biological questions may be answerable through the comparison of biochemistry between the life forms on the two planets. Philosophically, the discovery of a second genesis of life in our solar system would suggest that the phenomenon of life is distributed throughout the (...) universe. We could finally be confident that we are not alone. To protect a second genesis as we search for it, the robotic and human exploration of Mars should be done in a way that is biologically reversible, i.e., we must be able to undo our contamination of Mars if we discover a second genesis of life there. It is important to note that human exploration can be done in a way that is biologically reversible. Further, the discovery of a second genesis of life on Mars poses new questions in ethics. One question is: what ethical consideration is due to an alien life form when that life is distinctly different from Earth life, and the members of that life are no more advanced than microorganisms? Will we choose to terraform Mars to enhance the richness and diversity of the indigenous life we find there? In considering our answers to these questions, we should note that for most of Earth’s history our ancestors were microscopic. (shrink)