400 billion stars in one galaxy out of 100 billion galaxies we have found so far.. and ... we're it? Humans are the only intelligent lifeforms? To think we, humanity, are the most intelligent species in the universe is arrogance of the highest order, IMHO. So for me, this is not a question that needs answering.
Humanity is what, 200,000 years old? In terms of things happening in out in space that is an extremely short period of time.
"Are we the only ones" is a fascinating question, we can all agree with that, but what I can't get my head around is timing.
A good example of what I mean is humanity and dinosaurs... We never 'crossed over' during our existences. So lets say the dinosaurs never existed on Earth, they existed on... Venus or Mars and as humans we found evidence of this... Humanity would have found evidence of a "primitive alien race" that was already extinct... called "dinosaurs", who unfortunately didn't live long enough to meet humanity... so dinosaurs may as well be aliens to us humans, as all we can do is research what they were, they're alien enough in that they were really just big animals that started to prosper, until the planet they lived on was hit by an asteroid (using the generally accepted reason why the dinosaurs died out there)
Now imagine humanity as dinosaurs (raptor claws out, everyone, grrr) - What I'm saying is that humanity will die out, before it meets any other form of intelligent life because lets say humanity prospers in total for roughly 3 million years (before we all die from some sort of epidemic, kill each other, get hit by an asteroid like our dinosaur friends, run out of resources and never manage to establish off-world colonies etc etc) even 3 million years is a short period of time in terms of the universe and space etc.
Another advanced civilisation may overlap our existence, but we never meet because we never find them or vice versa and humanity ends up dying out, or they die out. Maybe an advanced civilisation starts to prosper 1 million years after humanity dies out? Maybe an advanced civilisation has already died out?
So, yes I agree it may be arrogant to think we are the only forms of -life- in the universe... as there will be microbes and bacteria and all manner of 'living' things out in space and on other planets, but -intelligent life-... mmmph.
Realistic caps on, humanity will likely live and die before we see life anywhere else... I believe we will literally 'miss' anything else that exists because as humans, we won't live long enough.
Dark, sober thoughts for a Thursday morning. Sorry
