4. The Origin of Life

Have you ever had one of those dreams where you try to run from a monster and your legs go 'round and 'round but you don't get anywhere? The quest to understand the origin of life isn't much different.

In fairness, it must be pointed out that there is little data to work with. Earth does not retain a record of what went on billions of years ago, when life got going.

Meanwhile, there is no shortage of wild ideas. Scientists now generally agree that life could survive a trip to Earth from Mars, in the belly of a rock kicked up by an asteroid impact. A study in November revealed why a Mars rock lands on Earth once a month, on average. A wilder idea, that bugs simply rain down from space inside comet dust, gained support from a second scientist in December, who claimed to have found some of these space bugs in Earth's atmosphere.

Most mainstream scientists, however, figure there's a good chance that life on Earth was cooked up in a soup of pre-biotic chemicals right here on the planet. The ingredients -- water and organic chemicals -- may well have come from space, but Earth likely acted as the incubator.

The answer (and a lot of well-funded researchers are asking the question and debating the possibilities) bears on how likely it is that life might have begun elsewhere, on Mars or around another star.

Next Page: Keys to Life's Origin … on the Moon?