Instead of asking when biological evolution decided that terrestrial vertebrates have four limbs, we might ask what the probability was that at some prescribed earlier time, say the Cambrian, they would develop six limbs. More precisely the question is this: ¿can one hope for a reasonable description of life at the end of the Cambrian, such that the probability of the number of limbs of future terrestrial vertebrates can be estimated in a stable manner? Animals with six limbs would be interesting because they could adapt two limbs for manipulative purposes (centaurs) or for flying (dragons). But since centaurs and dragons do not exists, it is tempting to say that they must have had negligible probability to come into being, and dismiss the question. One cannot so easily dismiss the problem of how life on Earth would have evolved if the great cataclysm and extinction at the end of the Cretaceous had not taken place. Are such problems outside of the scope of scientific investigation? At the very least, their solution would require a considerable increase in depth of our understanding of biological evolution.
We must admit that questions of hypothetical history, or historical probabilities, are in many cases unanswerable, at least in our present state of knowledge. Yet, these questions are not a priori meaningless. It is possible that one can make sense of some probabilities that arise in the history of life on Earth. And we may hope to evaluate historical probabilities in other areas of interest. After all, such probabilities can already be estimated in the simple situations of weather prediction and astronomy of the solar system.
(Lem y los dragones de la probabilidad - Instan - Paul Davies y otras formas de vida en la tierra)