We, all of us, are what happens when a primordial mixture of hydrogen and helium evolves for so long that it begins to ask where it came from.

Earlier generations of stars in the galaxy could well have had planets. But really, there was only hydrogen and helium to work with, so they'd all be gas giants and not small, rocky planets.

We are made out of stardust. The iron in the hemoglobin molecules in the blood in your right hand came from a star that blew up 8 billion years ago. The iron in your left hand came from another star.

Ultimately, we actually all belong to only one tribe, to Earthlings.

As I was leaving graduate school in 1974, I was recruited to join a fledgling SETI project at the Hat Creek Observatory in California, mainly because I knew how to program an ancient PDP8/S computer that had been donated to the project.

Life has evolved to thrive in environments that are extreme only by our limited human standards: in the boiling battery acid of Yellowstone hot springs, in the cracks of permanent ice sheets, in the cooling waters of nuclear reactors, miles beneath the Earth's crust, in pure salt crystals, and inside the rocks of the dry valleys of Antarctica.