What is life? Even though an answer has been passed on to generations of biology students, they weren’t getting the full story. When scientists invented the modern field of astrobiology, they had to wrestle with a fundamental problem: There is no scientific consensus about precisely what makes something alive. Given that unsettling absence, did it really make sense for astrobiologists to apply to the rest of the universe the never-quite-exact definitions we had come up with on Earth? To make matters all the more confounding, what would be a sure signature of biology on our planet could be totally nonbiological on Mars, and vice versa. So how do you find life in the beyond if you can’t agree on what life is on Earth? Because it’s in the business of trying to find “life” beyond Earth, NASA has probably done more to try to define it than any other organization. Here is an unofficial working definition: Life is “a self-sustaining chemical system with the capacity to evolve in a Darwinian manner.”
