In part of this thread I'm talking about the odds of evolution or abiogenesis.
the odds really don't matter, though.
Pick any number you want.
A million to one.
A trillion to one.
One point seven zero one gabbazillion to one against life developing under certain conditions.
Then compare that number to the number of planets in the galaxy. And the number of galaxies in the universe. How's your odds stand up?
I mean if you give something one in a million odds, and there are a billion opportunities for the event, it becomes LIKELY to happen a thousand times, no?
Evolution is an unavoidable certainty, once you have a system that reproduces itself imperfectly, in an environment where some of those offspring are more likely to survive than others.
Abiogenesis is a whole different question, and it is deeply unhelpful to conflate the two; Religionists have an irritating habit of assuming that because their tales often have the creation of the universe, the solar system, the earth, life, and species diversity all in a single brief narrative, that all explanations of any of these things must somehow form a single narrative in order to be understood at all. But the reality is that these are each effectively unrelated events, each with their own unique explanations and often separated by thousands of millions, or even billions, of years.
Biology requires cyclic chemistry, which requires an energy source, plus a supply of elements and compounds, in an environment where these can react at energies that are sufficient to maintain the cycles, but not so high as to smash everything up.
One such environment is provided by liquid water as a substrate; Water is a particularly good solvent for a very wide range of compounds, so it can support a lot of different chemical reactions at moderate energies. Carbon is also a very useful component for cyclic chemistry, as it can make up to four bonds - so it can form branching chains with a variety of groups hanging off them, and these can become very large and complex. While other solvents than water, and 'framework' elements than carbon, have been mooted as possible environments in which life could arise, it seems that the most likely environment for abiogenesis is one with large volumes of permanently liquid water, with a variety of other chemical compounds, particularly ones containing carbon, dissolved in it, and with a constant source of energy.
The absence of highly reactive chemicals and physical phenomena that tend to smash everything up, such as strong oxidizers and high fluxes of ionizing radiation, is probably also useful.
Given those requirements, only Earth and Enceladus (a moon of Saturn) are good candidates for life to arise in our Solar System; We have not yet had a good enough look at Enceladus to know whether or not life exists there, and we certainly don't yet have the technology to detect life outside our Solar System, so right now the probability of abiogenesis given only a liquid water ocean as a prerequisite is unknown - but I suspect that the probability of life, given an environment that has liquid water available for at least a billion years, is very close to one. The necessary basic chemistry simply isn't that uncommon, unlikely, or difficult, and once life gets going, evolution tends to make it good at surviving very quickly.
A billion years is a LONG time. Even very unlikely events are likely given that much time in which to occur - if you buy a Lotto ticket here, your chances of winning this week are about 45 million to one. But if you buy a ticket every week for a billion years, the chances of NEVER winning are so close to zero as to be negligible. Life on Earth arose within somewhat less than a billion years after the formation of liquid water on the planet's surface - we know it was present about 800 million years after the first water, but it might have been around for quite some time before that.
A billion is a very large number, and many people struggle to grasp just how much longer a billion years is than a million. To get a feel for the difference, consider that a million seconds is about 12 days; but a billion seconds is more than 37 years.