The New York Times has a cogent article on the origins of Life As We Know It, and some clues as to finding Life As We Don't Know It.
Geologists believe life established itself on Earth about four billion years ago. Australian rocks dated at 3.5 billion years contain fossilized traces suggesting that microbes were already well ensconced by then. But the ancient Earth was no Garden of Eden. Huge asteroids and comets mercilessly pounded the planet, creating conditions more reminiscent of hell. The biggest impacts would have swathed our globe in incandescent rock vapor, boiling the oceans dry and sterilizing the surface worldwide.The article postulates that Life may have been created (and destroyed) on Earth multiple times. And that some Lifeforms here might be derived from ancestors different from our own. It's possible: we certainly don't have a good handle on the Lithophillic bacteria found in the deep strata. Twenty years ago, we had no idea that the peculiar organisms around "black smokers" on the sea bottoms existed, ones which do not get their energy ultimately from the Sun, but the Earth's heat. Chemosynthesis, not Photosynthesis.
How did life emerge amid this mayhem? Quite probably it was a stop-and-go affair, with life first forming during a lull in the bombardment, only to be annihilated by the next big impact. Then the process was repeated, over and over. As the bombardment began to abate and the impacts diminished in severity, so isolated colonies of primitive microbes sheltering deep underground managed to cling on. One of these colonies was destined to become life as we know it.
The author, Dr Davies, really should have mentioned the work of Hoyle and Wickramasingh though, as I have before. We know that space is full of pre-life chemicals. The real question is, is it also full of Life?
Nobody knows how life began. Somehow a mixture of lifeless chemicals assembled itself into a primitive organism, presumably through a long and complex sequence of chemical reactions. Our ignorance of this process is so great that scientists can't even agree on whether it was a gigantic, one-time fluke, or the expected and frequent outcome of intrinsically bio-friendly natural laws, as the astrobiologists hope. Jacques Monod, a Nobel Prize-winning biologist, was adamant that life is a bizarre accident confined to Earth. On the other hand Christian de Duve, another Nobel laureate, declares life to be "a cosmic imperative," bound to occur wherever Earth-like conditions prevail."Cosmic Imperative"? Perhaps not a felicitous turn of phrase. More like an inescapable conclusion - there's so much life-friendly volume out there, in the Oort clouds and closer in, that for Life *not* to have developed would require an unbelievable chain of coincidences. Now whether it developed indpendently here on Earth once, or many times, or whether there was infall of existing biota from Space, or some combination, that we're not sure of. If I was a betting man, I'd say the infall hypotheses were somewhat more likely, the numbers indicate that. But perhaps a template - such as the regular patterns in clay - increases the probability of Life forming so much that it usually happens on planets, rather than in ices and gas clouds out in space.
I am prepared to believe though that multicellular life - the type that leads to creatures with more intellect than an amoeba - might be quite rare, and requires very special conditions for it to become probable. It might need a double planet system (for tides and continental drift), a second sun, failed star, or really huge gas giant such as Jupiter to sweep up most of the asteroidal and cometary dinosaur-killers. A place where water exists as solid, liquid, and gas.
A place in fact, rather like Earth.
That's not to say that these things are required (for example, not all life may be carbon-based), but it might be like buying a trillion lottery tickets as opposed to one. It increases the chances, and a lot.