Where to look for life in the sky? Many believe it should be a planet like our own. One that is in the right location, has the right size, and the right materials (rockiness, water, plenty of free energy). Given some time, life will eventually get started through a combination of automatic chemical processes that eventually evolve into microscopic organisms. There are, I think, two problems with this story of life. Both are connected by the fact Earth is not alone.
Problem one, the story sounds like Darwin's pretty pond. It's much too pleasant. It's like making a cup of tea and sipping it in the garden on a sunny afternoon. It is a gentleman's story. The other problem is it imagines a single planet doing all of the work on its own. One star, one planet. But this image tells us more about our culture than the cosmos. The former was shaped by an individualism that's promoted and reinforced by the top members of American society. The latter was set into motion by values that, though accidental in their proportions and force, made a very specific concatenation of cosmic events possible.
But there is another and more probable origin story that, on one side, involves what appear to humans to be catastrophes; and, on the other side, a solar system that's rich with planets and interplanetary bodies. True, being in the right place, or a habitable zone, is important, but it is only part of a much bigger picture that cosmologists are now beginning to appreciate. From our perspective, the origin of life looks more like hell than heaven.
From a recent article, "Planet crash that made moon left key elements for life on Earth, scientists say," in Guardian:
The cosmic collision that made the moon left a host of elements behind on Earth that were crucial for life to emerge, US scientists have claimed.In less than a billion years after this "cosmic collision," life began on earth. More importantly, this hypothesis makes it clear that being in the right place may not cut it. One earth-like planet orbiting one sun is likely to be lifeless. And its not the violence that is crucial. This cosmic collision hypothesis has nothing to do with the creative destruction of economics, or Harry Lime's awful story in The Third Man about how "warfare, terror, murder and bloodshed" in Italy produced "Michelangelo, Leonardo da Vinci and the Renaissance," while "brotherly love... and peace" Switzerland produced "the cuckoo clock." The reason why this collision happened is not because of planetary aggression but planetary togetherness. Life is a solar system that's rich with objects.
The impact 4.4bn years ago is thought to have occurred when an itinerant planet the size of Mars slammed into the fledgling Earth, scattering a shower of rocks into space. The debris later coalesced into the moon.
Beyond an act that shaped the sky, the smash-up transferred essential elements to the Earth’s surface, meaning that most of the carbon and nitrogen that makes up our bodies probably came from the passing planet, the researchers believe.
But this idea is not entirely new. There have been variations to it, one of which can be found in the 2015 book A New History of Life: The radical new discoveries about the origins and evolution of life on Earth by Peter Ward and Joe Kirschvink. These authors point out that the conditions of the early Earth were actually not ideal for the development, by automatic processes, of rudimentary RNA—a part of which, many believe, evolved into DNA. But weirdly Mars is. And so, they believe a collision on Mars ejected materials into space that eventually arrived on Earth. Some of these molecules had the stuff to get RNA going on the planet that happened to be in the right place and time.
There is also a hypothesis (which at this point is almost a theory) that water was delivered to Earth by comets. From Space.com:
Astronomers have been arguing for some years about comets brought Earth its water. Then in 2011, an international team of astronomers using the Herschel Space Observatory to study Comet Hartley 2 (103P/Hartley) published their results on the first comet confirmed to contain ocean-like water.
What all of this and more points to is this: Life requires a community that can contribute, piece by piece, its basic materials with encounters that, from our human scale, have the appearance of hellish violence. Indeed, to a mite, a human handshake is a catastrophe of the first order.