WHAT IS LIFE?
There is grandeur in this view of life, with its sev-
eral powers, having been originally breathed by the
Creator into a few forms or into one; and that,
whilst this planet has gone cycling on according to
the fixed law of gravity, from so simple a beginning
endless forms most beautiful and most wonderful
have been, and are being, evolved.
—Charles Darwin, The Origin of Species, 1859
If intelligent aliens someday land in Times Square or Red Square or Tiananmen Square and announce themselves to the world, the implications to life in the universe will be immediately clear. Their advanced technology will prove they are from a distant world, and even if they themselves are robots or androids or genetically engineered organisms bearing little resemblance to their forebears, they’ll still represent indisputable proof that life has arisen elsewhere. A SETI success in receiving a message from another civilization would be only slightly less dramatic, and equally profound in demonstrating that we are not alone in our universe. But unless or until something this spectacular occurs, our only realistic hope of finding life beyond Earth lies with using spacecraft or telescopes to search for it. And in that case, it’s important to know exactly what it is that we are searching for.
You might think that it would be easy to define life, but it’s not. Consider a cat and a car, which turn out to have a lot in common. Both require energy to function—the cat gets energy from food, and the car gets energy from gasoline. Both can move at varying speeds and can turn corners. Both expel waste products. But a cat clearly is alive, while a car clearly is not. What’s the difference?
In the case of a cat and a car, we can find many important differences without looking too far. For example, cats reproduce themselves, while cars