Based on the new paper reconstructing the most recent common ancestor of life on Earth - and its environment - there's a 1-in-3 chance of a similar organism living under Europa's ice right now.
We can now be much more certain that life on Earth originated in deep sea volcanoes, which makes the prospect of life in Europa's oceans much more exciting. Life had already appeared by at latest 500 million years after the formation of the planet. If we assume mediocrity (ie that by 500 million years into it, there was a 50% chance of LUCA having developed) then that gives us a 0.14% chance of life evolving per million years. Assuming that the chance of life evolving is directly proportional to the surface area of the ocean floor (rather than the volume, because it was around volcanoes), and that Europa has volcanoes, that means a 0.00854% chance of life evolving on (or in) Europa per million years. After 4.5 billion years, this gives us 1 in 3 odds of a LUCA-like organism living under that ice right now.
If we hold all else equal, but instead assume that the likelihood of life evolving is proportional to volume, then the chance of life on Europa today is essentially 100%.
Of course this holding "all else" equal is a bit of an assumption. I didn't try to account for the different chemical composition of Europa's oceans (which we don't know yet), the volcanic activity (which we also don't know yet), and reaction kinetics based on water temperature (which we also don't know, and is a real wild card since tidal heating is a big deal when you're orbiting Jupiter.)
Besides the obvious excitement about the possibility of ocean-floor life on Europa, this also means that life evolved WITHOUT sunlight, DURING the Late Heavy Bombardment. You need water with stuff in it, but not sunlight, and if space rocks keep crashing into it, that's fine. If you're covered with ice and kilometers of water, even better.
When are we getting probes there?