After bringing in fish, we could begin introducing land-dwellers to Mars, starting with insects and, as more and more oxygen became available, progressing to warm-blooded creatures. Using this process, Zubrin says, humans could walk around without supplemental oxygen within 1,000 years. But he also predicts that we'll come up with other ways to speed up our settlement of Mars, such as genetically engineered plants that photosynthesize faster. "We can grapple with it and come up with a way to do it using 20th-century technology," he says. "Yet these are solutions that people in the future will consider prophetic but quaint."