here is what sucks about talking about whether or not there are other sentient being out there, the argument is that with the vastness of space and the shear volume of the # of galaxies with billions of stars each and each one of those possibly having planets that orbit each of those stars and all of them have the possibility that there is or once was life on that planet and the fact that we know there can be life because we are here, how could there not be right?
but if we are going big bang/evolution theory here the series of events that it took to get us here are overwhelmingly against us. first a big ball of dust has to form, heat, then cool to form the planet. in that dust has to be key elements to support life, and at the same time that planet has be at the exact perfect distance from the exact perfect size, and perfect temperature star(or what will eventually be a temperature for supporting life). then while all this is going on we are waiting for a few more key elements for supporting life which we are hoping to get from asteroid and meteor strikes but not just any asteroid and meteor, we need ones carrying the exact right elements we need to support life, and they need to get here in the exact right window of opportunity when the sun is hot/cool(who knows which) enough to support the growth of life. BUT WAIT. our planet is too big. now we need to hope a huge asteroid slams into earth, and breaks off a chunk of it the size of the moon(arr,arr,arr) but the asteroid cant be too big cause it would break the whole planet, but it cant be too small cause it wouldn't break off enough of the planet. so now we have everything we need now we wait for evolution to kick in and hope nothing happens or changes so that the planet no longer can support life and that we evolve fast enough to adapt to any changes. oh and BTW somewhere in there the ozone layer was was formed(who knows how it got there), it shields us from the suns harmful rays, something that without we could not support life.
so. how long did all this take? well the universe is believed to be just under 14 billion years old, the earth and sun are between 3.5 and 4.5 billion. oldest bacteria found is 3.5 billion years old. oldest dino fossil, about a half billion. so three billion years from micro organism to dinosaur and another half billion from dino to us.
we got here late people. our sun is only expectd to last about another billion years, that means that at some point it is going to start cooling off and wont give off enough heat to support life.
there are billions and billions of possibilities for other life out there, but billions and billions of things had to happen just right for us to be here(big bang/evolution theory) and we happened pretty late by astronomical standards, if we had happened earlier in the creation of the universe i would say there is a good chance that life is out there but truth is we are most likely the only intelligent life in the universe right now; but there is a chance there is life that is evolving right now and there is also a chance that life has already come and gone on another planet somewhere, and eventually, after our sun has long been burned out that the cosmos will align again and support life somewhere else similar to ours.
