When we go, we will bring some of Earth’s species to Mars, such as the microbes on our skin, and we might even find some life already there. For all of history, species in all of our planet’s ecosystems could be placed neatly into one of these three buckets. It is now estimated that one million species may be lost due as a result of human activity, and one of every four birds in North America is now gone due to hunting and habitat destruction. We track and restrict invasive species across borders, nurture endangered species back to health, and set up nature preserves around the world. If we find no life on Mars, or anywhere else, then life is that much rarer and more precious.