Abiogenesis

Abiogenesis is the process through which life emerges from non-living materials. Critics of evolution have sometimes focused on the implausibility of abiogenesis, and suggested that it is so unlikely to occur that it must not be the way that life came about – comparable in probability to a tornado ripping through a junkyard and assembling a 747. 

Whether or not the critics are right about the probability of abiogenesis, the conclusion that life must have arisen otherwise doesn’t follow. Whether or not abiogenesis occurs is a matter not just of its probability, but of the number of chances it’s given to happen. If the universe is a big enough place, then it is likely that it will happen somewhere, no matter how unlikely it is to occur at any given place. The observable universe isn’t too big, but the unobservable reaches of the universe might be huge.

This also means that even if we think evolution is almost certainly true, we cannot conclude that abiogenesis is not too unlikely.  Though admittedly I do not have much relevant background, I don’t find tornado analogy crazy. It seems very plausible that abiogenesis required a remarkable coincidence in which amino acids happen to come together in just the right way to be both stable and self-replicating.

But, short of discovering the mechanisms through which biogenesis occurs, what evidence do we have to go on? Here are three possible things to consider.

1.      Evidence from Abiogenesis on Earth

It seems safe to assume that abiogenesis did happen on Earth. This fact might be taken to count as evidence that abiogenesis is fairly likely, at least in Earth-like conditions.

There are two ways to think about our evidence. On the one hand, we might treat our evidence as evidence that a randomly selected planet contained life. If we think about things this way, then the fact that Earth has life is strong evidence that life is likely to arise. After all, if we found life on Mars, we would probably conclude that abiogenesis was near inevitable.

On the other hand, we might treat our evidence as evidence that a planet randomly selected from the planets which have life has life. Thinking about things this way erases almost all of the evidential value that life on Earth has. If there are any planets with life, a planet randomly selected from those with life will have life.

Which way is the right way to think about it? I’m inclined to think there is no fact of the matter about how to think about it. Probability theory isn’t an all-powerful technique that will resolve all epistemological questions. If this is right, no clear lesson can be drawn about the probability of abiogenesis.

2. Backward Induction

Evolution is pretty unintuitive. The fact that all of the information needed to build a human body can be written in dna is pretty amazing. The fact that there exists a sequence of strings of dna, each with which includes only minute changes from the preceding, and all of which produce evolutionarily viable creatures ranging from single celled organisms to humans, is remarkable. It seems like evolution should be unworkable. The problem isn’t that blind mutation isn’t sufficient to hit upon the right sequence.  The problem is that there is such sequence. But evolution surely has occurred and must have been driven by gradual changes.

The argument that I have in mind goes as follows. Evolution seems pretty implausible in all its different manifestations. We have good evidence that it has happened many times, and so it isn’t so unlikely as it may first appear. Therefore, we should think that abiogenesis is likely too.

There is something good about this argument and something bad about this argument.  I don’t think that we can infer much from the ease with which evolution occurs somewhere (say, the evolution of the eye) about how easy it occurs elsewhere (the evolution of language). So even if it was inevitable, once the first cells evolved, that photosynthesis, multicellularity, intelligence, etc. would evolve, it doesn’t follow that abiogenesis is any more probable.

However, there is an argument to be made that we intuitively underestimate the probabilities when it comes to evolution. The fact that we have underestimated some of the probabilities casts doubt on the others. We can’t trust our intuitions here. So we should be wary about relying on our intuitions to make the case that abiogenesis is unlikely.

While this inductive argument provides great evidence that our intuitive ability to estimate biological odds is pretty bad, it doesn’t actually provide much evidence for thinking that abiogenesis is likely. It leaves open the very reasonable possibility that the first step was a leap, and that only by the greatest coincidence the Milky Way has ever seen was life able to emerge.

3.      Timing

The fact that life emerged on Earth may not be much evidence, but its timing tells us something. Life emerged very early in Earth’s history. Not long after the Earth cooled enough for life to be possible, it came to be. If abiogenesis were extremely easy, this is exactly what we should expect. If abiogenesis were extremely unlikely, then it as likely to have occurred early on as later.  If you buy a computer and it dies within a year, it is some evidence that the parts were cheaply made even though a quality computer may also fail quickly for any number of reasons.

This evidence is somewhat weak, but it is far from insignificant. Apart from actually working out the process through which biogenesis occurs, it is perhaps the best we have to go on.