|Some speculations required!
Also in a more detailed view our main problem persists: we know only one example of life for sure, and therefore our ability to judge the realistic chances for diverse types of life is severely limited. Thus we completely depend from theoretical considerations, which pose real problems for such complex questions.
If you want to know more about the reasons for the following prevailing assumptions, you should better read a book (there are some good books about this topic, for example from Donald Goldsmith and look also at the SETI Institute link respective site for more literature about the topic). It's more easy to read, than numerous HTML pages, I guess!
largely accepted basics
The general consensus is, that water and hydrocarbons are generally most suitable for the evolution of life, with a few other life-sustaining liquids than water, especially ammonia and the slight, but not impossible chance of substituting hydrocarbons by silicon compounds. So far organic or similar life is affected, there are a few further requirements: a planet of appropriate size and distance from a star in the stable main sequence state. The last has to remain stable for a sufficient time, i.e. several billion years, to enable a evolution of sufficient time, what restricts the type of stars to sun-like stars (spectra F, G and K, to speak more precisely). The frequent low-mass, long existing dwarfs of type M are not adaquate, because they have a too low luminosity and would require therefore a too low distance of the planet from its star, which had to rotate bound and that is no good... Besides the low-mass stars of type M are mostly flare stars, which would wipe out any life from their planets.
Minor chances may lie in objects like Europa, one of the major (Galileian) satellites of Jupiter, where a liquid ocean of water below the ice crust is likely, or even in the atmospheres of Jovian planets with higher temperatures than Jupiters or even brown dwarfs!?
This is the more conventional framework. Other possibilities in the discussion are more exotic and mostly causing so much difficulties for us to recognize or even to communicate with these eventual life forms, that I will leave them apart.
about evolution in general...
So far we understand the evolution on Earth now, we know only one thing for sure: certain single events in the past have respective had could caused changes of large extent to todays lifeforms. There is so much involved by accident, that even at identical conditions the result would be total different. This applies to external influences as giant impacts, and also as to more interior ones (but no ways biological too) like giant volcanism periods and even to the biological mutation/selection events, which are at least partly working random.
For our own planet the possible alternative evolution path for only one changed event in the past are hard to "predict". Taken into account, that there were many such events and on another worlds even the initial conditions change, it may seem hopeless, to think, that other life forms and even intelligent beings could resemble to ours at least slightly. But this highly unlikely!
The reason is the so-called convergence. Plants and animals of very different origin can evolve into partly rather similar organisms, if the environment conditions are such tight, that they require special abilities. Regardless of starting point, these different origins approach often with time, despite for example their interiors and genes remain partly very different.
That is our main hope. If another beings would evolve into totally other directions, we would be again also totally unable, to recognize or even to communicate with them.
...and to intelligent beings?
Another topic, which is unavoidable being prone to speculation, are the ways, in which life forms can aquire what we call intelligence. Again resorting to our only example, we know today, that whales with teeth - especially the orca - are the most intelligent animals living in the water and that other primates are the next to us on the land (especially the bonobos). A more speculative thing are the troodontes, living in the final cretacious time as the most intelligent dinosaurs. They may have had the chance to gain intelligence too, if they were not be wiped out by the now already infamous Yucatan impact event 65 million years ago.
The chances to develop real intelligence are not identical with the advent of the intelligence however. It may require certain events, to trigger this "final" step. Remember, that it has also immediate drawbacks, to spent so much to brain evolution: energy, time, lack of additional physical capabilities and so on, to name a few (possible) of them. Only we humans have done it so far on our planet, despite there may be others (see above), who could also be able to do so.
The most speculative thing is, what will other intelligent species do? Will they do similar things like us? At this point convergence considerations are no more very helpful.
about probabilities for life back to SETI overview back to main
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