Darling spoke to Salon from his home in Brainard, Minn.

Can you explain what exactly astrobiology is and when it emerged?

Astrobiology is an attempt to include all of "life," whatever it may consist of and wherever it may be. It's looking for more general principles of life and is a truly universal science in the same way that physics and astronomy are. Astrobiology has become the standard name for the science, though an older name, exobiology, goes back to 1960. That term was invented by Joshua Lederberg, a geneticist, and had quite a bit to do with the early discussions of extraterrestrial life. It traces back to the late 1950s when the space age started and we were thinking of putting probes on Mars. Then, the issue was contamination -- to avoid bringing bugs back to Earth or to avoid contaminating Mars. That focused scientific attention on what might be out there.

In your book, you write, "It's beginning to seem more and more as if there's nothing special about what took place here on earth." What do you mean?

That statement is based on a number of lines of evidence. There's evidence that life on Earth goes back at least 3.8 billion years, which suggests that it got started 4 billion years ago at the latest. At that time, earth was a very hostile and unfriendly place and was being heavily bombarded from space. The speed with which life arose suggests that life may be "easy" or that there's nothing peculiar about the environment needed for life to arise. Also, the basic building blocks of life as we understand them -- liquid water, some source of energy and organic material -- seem to be increasingly common in the universe as a whole. This circumstantial evidence is raising the optimism that perhaps there was nothing special about how life got started here and that it could start in other places.

This has brought the definition of "life" into question. Why is evolution seen as such a key factor in determining what is alive?

It's the one characteristic that seems to separate life as we know it, and life as we can imagine it, from nonlife. In school they give you a laundry list of things that are supposed to tell you what's alive and what isn't. The problem with those sorts of lists is that too many things fall through the cracks; for example, crystals grow and there are a lot of individuals that can't reproduce for one reason or another. You need something more basic and that basic thing appears to be evolution in the Darwinian sense: You have a population of competing individuals with different genetic makeups and the fittest survive. Of course, it's a difficult thing to detect; you can't land on a planet and detect evolution. But if you've got evolution going on, then you must have a population of self-reproducing individuals and to reproduce you must have a metabolism and so on. It leads to things you can measure.

Maya Angelou reads from "The Heart of a Woman"