Did life originate on Earth, or did it come from somewhere else?
In a recent issue of "Astronomy Now" there was an article by an associate of Hoyle, Chandra Wickramasinghe, explaining the theory of panspermia, that earth was seeded by life having developed in the bowels of a comet which struck the earth a glancing blow billions of years ago.
My question is: Is this theory mainstream? And if not, what do most astronomers believe was the origin of live on earth?
A quick correction: panspermia is simply the theory that life came to Earth from somewhere else--be it from inside a comet, in a meteorite from another planet in our Solar Sytem, in the form of bacterial spores carried by light pressure from another solar system, by deliberate or accidental inoculation by an alien intelligence, or by some other mechanism.
The origin of life on Earth is still very much an open question, and I don't think that there are too many astronomers who feel that panspermia could be conclusively ruled out with the current evidence we have in hand--mostly because we have so little evidence.
The evidence we do have raises many interesting questions. DNA studies indicate that the oldest life on Earth is a class of single-celled organisms called archae, or extremophile bacteria. They are called extremophiles because they can survive--even thrive--in environments previously thought too harsh for life: extreme temperatures, high salinity, extreme pH, etc. Some species of archae can even survive high doses of radiation.
Were archae the first form of life to evolve on Earth? Or did many other primitive lifeforms evolve, only to be wiped out by one catastrophe after another, leaving only the hardiest organisms? Or did extremophiles come to Earth from somewhere else, using their unique abilities to survive the hazards of space?
I would say that most astronomers find panspermia to be improbable, but not necessarily impossible. The default assumption is usually that life arose on Earth from organic compounds, probably borne by comets.
Very few astronomers think that life could arise in comets, however. Comets spend most of their time in a deep freeze, after all! Dr. Wickramasinghe also claims to have collected extraterrestrial bacteria in the upper atmosphere. The bacteria, he says, are originally from interplanetary dust and from comets that have burned up in the atmosphere. As would be predicted by the panspermia hypothesis, these bacteria are indistinguisable from terrestrial bacteria. ;) Most aspects of Dr. Wickramasinghe's work are considered fringe science.
Most astronomers are also quite skeptical of the notion that life could be efficiently transfered from one solar system to another. If life came to Earth this way, it seems like a one-in-a-billion shot, and we don't like theories that require such long odds. They might be correct, but a mechanism that doesn't require the Earth to be a cosmic oddball, a lucky chance, would be more satisfying. Panspermia is a somewhat unsatisfying answer to the question of the genesis of life on Earth, because it simply pushes the question back a step. If life came to Earth from elsewhere, how did it arise there? Why did it not arise independently on Earth? Would life have arisen on Earth if it had not been colonized by alien life?
These objections do not rule panspermia out, of course. I must emphasize the primitive state of our understanding of this question. At this stage of the game, we are guided more by intuition and aesthetic arguments than by the hard evidence we crave. In the coming decades, we will be discovering the first earth-sized planets around other stars, and, with any luck, we will be able to determine the compositions of the atmospheres of some of these planets. If we find the telltale signature of oxygen, we will have a much better idea of how common life as we know it is in the Galaxy.
Even more exciting, we might find life on other bodies in the Solar System. Life on Mars or fossil evidence of past martian life would give us great insight into the origins and evolution of life. Some astronomers think that there is a liquid ocean beneath the icy surface of Europa, a moon of Jupiter, which may harbor life. Finding life in the Solar System would be a great boon, because we would be able to retreive samples and study it up close.
Any information about life elsewhere would give us a lot more to work on. If life is common and seems to be very similar, then the panspermia hypothesis will have to be examined more closely. However, it could just be that there's really only one type of life that could be successful, and it evolved in many places independently.
Without much evidence to guide us, we can speculate endlessly. I'm hoping that this is a quesition that we will be able to address with a great deal more scientific rigour in the near future!
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