Early next year I have been invited to give a talk on biogenesis. How life gets started is of interest, because now we know there are a number of planets around other stars, if we know how it can get started, we can know whether life could or should form on a given planet. Of course, nobody actually knows, which has the obvious benefit is that if I get something a little wrong, nobody (including me) will know, and the second one is, one can use logic to cut away a lot of some rather silly stuff being said elsewhere. The most obvious silly suggestion (in my opinion) to me is panspermia, which is when life came from somewhere else, travelled through space on a meteor, and landed on Earth, whereupon the life form flourished in a new environment. Supporters of this theory point out that very primitive life forms can survive in a vacuum, and that DNA has been shown to be able to survive through extended time in vacuum and radiation, if buried inside a rock.
So, what is wrong with this theory? First, let us think about “extended periods of time”. A number of meteorites have been found that originated on Mars, and these have taken millions of years to get here. (Not there has been any sign of viable life on them.) We have no evidence whatsoever that life forms could last that long, nor have we any reason to believe that Mars was a better place for life to get started than here. Had the life come from another solar system, it had to survive for hundreds, or thousands of millions of years, because of the huge distances in space. This also shows another problem: if they take that long to arrive, and there is not that many of them, the concentration of them is very low. Why does it take that long? Basically, because ejecta that escapes Mars goes into orbit around the sun, and stays in that orbit indefinitely until it hits something else. Compared with the size of the solar system, Earth is a tiny microscopic dot. If it comes from another star system, it will arrive with a velocity greater than the escape velocity of our star, so it will come in at extreme velocity and either hit the Earth or never pass it again.
The fact that DNA can survive (although it has hardly been shown to be viable for that length of time) is insufficient to bring life here. Life will contain a set of enzymes, and they get denatured on warming. Yes, there are special enzymes in hot pools, but these have evolved there. Most enzymes denature and then refuse to work if heated half-way to boiling, and a meteorite coming into the Earth’s atmosphere will get a lot hotter than that. But let us suppose it survives and either hits land or water. Now what? A life form has to support itself by consuming what it needs from its environment. It needs a boundary to contain it (skin, in our case), it needs chemicals to replace it, it needs some means of obtaining energy, and it needs chemicals to enable it to use energy. If any of these are missing, then the life form will die.
Energy for life on Earth comes from the sun, via photosynthesis. We know that on Earth life may well have been around in some form or other for something like a billion years before photosynthesis evolved. That means that if the original life forms came from space, they had no means of using solar energy that we know of, hence they would have to rely on chemical processing. There would then be the problem of finding nitrogen-rich organic compounds from which to make things like protein and more nucleic acids. Either those things were around or they were not. If they were not, the life form would die out, as it would be impossible to reproduce. If they were around, then why did they not evolve to form life? Thus any conditions present that would permit an alien life form to grow would also be sufficient for the life form to evolve from what is there. I cannot prove it did, but from Occam’s razor, life coming from outer space is an unnecessary assumption unless there is proof, and since it is sufficient for life to form here if the chemicals required for alien life to survive are here, it is simpler to assume that is what happened. I am convinced we are ourselves, and not some remnants of some space catastrophe.