So we are stuck. Life may indeed pop up readily in Earthlike conditions, or it may be a fluke, unique in the observable universe. Because we are a product of this cosmic accident, we cannot conclude that Earth is typical. No statistical evidence can be drawn from a sample of one.

The easiest way to settle the matter is to find a second sample of life, one that arose from scratch independent of known life. The inventory of extrasolar planets being discovered is an extremely useful first step. In the future, our telescopes should be able to analyze the atmospheres of some of these planets for telltale signs of biological activity.

But evidence favoring life’s high probability could exist closer to home. No planet is more Earthlike than Earth itself. If life does pop up readily in Earthlike conditions, then it should have started many times, right here on our own planet. It could be that intermingled among the seething microbes all around us are some that are so biochemically different they could be descended only from a separate origin. You couldn’t tell by looking, only by delving into their molecular innards and finding something weird enough to rule out a common precursor. The discovery of just a single “alien” microbe under our very noses would be enough to conclude that the universe was indeed teeming with life.