I read the book "Alone in the Universe" by John Gribbin recently. He argues that the conditions that produced intelligent (sort of) life on earth are incredibly rare, so this explains the "Fermi paradox": if there are aliens out there, why don't we see them? (assuming as seems reasonable, that we don't).
He gives a lot of plausible reasons for why the rareness of life. Like, only a certain part of the Milky Way is a good home for life. Our planet has a magnetic field that shields it from the solar wind - charged particles that stream out of the sun. Continental drift may not be common, and it's crucial for regulating the concentration of carbon dioxide in the atmosphere, which is a kind of natural thermostat for Earth. He thinks there were serendipitous mass extinctions of life forms that were unlikely to become intelligent, like the Ediacarans. They were a weird kind of multicellular life that were around before the Cambrian. Strangely shaped things. And so on.
Some of it is very questionable to me. Like, he thinks that land life is much more likely to become technological, and that the reasons are obvious. I'm not sure what the obvious reasons are and I looked online and didn't find them. I wondered if part of the reason was that aquatic creatures would be less likely to develop hands, but then I found that octopi can manipulate objects with their tentacles. A lot of technology wouldn't work underwater, but being underwater would make some things easier, too. Aquatic intelligent creatures might be very good at spatial thinking since they live in 3 dimensions. He mentions the idea of an advanced civilization putting self-replicating machines out into the universe that would colonize everything in time, and report back what they found to the home planet. So why aren't there self-replicating alien machines on Earth, if there are alien civilizations? But I'm not convinced that an advanced civilization would necessarily want to do this, or that it would be at all practical or feasible.
He talks about a dinosaur called Troodon, which lived right before a 6-mile wide rock (probably) smashed into the earth, causing the end-Cretaceous extinction. He says based on its brain size, Troodon was about as smart as a small baboon. So the dinosaurs were on their way to evolving intelligence when they were wiped out.
It would be interesting to speculate what an intelligent creature that Troodon could have evolved into, would have been like. They might have been more warlike than us (NOT a happy thought) since they were carnivorous and we evolved as omnivores. Carnivores generally seem to be rather aggressive towards others of the same species.
There are obviously a lot of pitfalls with arguing that life is rare, based on lucky accidents that produced us. Evolution is very creative, and examples of convergent evolution show how the same solution to a problem that life is faced with, are found over and over. Why shouldn't intelligence be a solution that convergent evolution finds over and over, on different planets, even? And, different solutions to different problems posed by different circumstances, could be found by alien life, and the limitations of our imagination are no argument against this.
To me, the slow speed of light compared to the size of the cosmos seems a good reason why we'd be left alone by Them. The Milky Way galaxy is about 100,000 light years in diameter. One could perhaps make a good argument that other intelligent life would exist on a roughly similar time scale to us, so that 100,000 years would look like a long time to Them, too.
If it's true that conscious life is very rare, then it returns the specialness to us that science took away so much, by discovering the hugeness of the universe, that the sun and stars don't revolve around us, etc. The wonderful nature of our world is partly explained by the fact that we are here to see its wonders - which is made possible by the enormous size of the universe.
For me, thinking about this kind of spacey thing is a mind-refreshing relief from the humdrum forced on me by health problems. And you can enjoy exploring the cosmos too in your imagination.
With all due respect, I am down on memetics big time. So I will recuse myself from the discussion.
ps It would be very useful for a creature to have 4 legs to move around with, and also 2 arms with hands to manipulate things. It seems like an accident of evolution that we had 4 legs to move around with, and 2 of those legs were converted to arms with hands. It seems like there are some features, like a basic 4-ness of limbs, that are very unlikely to change in evolution, although this feature could become hidden, as in snakes.
If I remember correctly, Carl Sagan also did not expect we would contact any aliens because of the vastness of the universe. I stopped worrying about UFO's when I heard that.
The Fermi Paradox is not something that can be rejected. It is a statement of fact: there are no extraterrestrials here. The "paradox" part comes from recognizing that while the Milky Way is vast, time is more vast. At very modest fractions of the speed of light, a sufficiently technologically advanced civilization could completely colonize the galaxy in a tiny fraction of the age of the galaxy. That has not happened. Fermi was saying, "why not?". There are many possible explanations, but the vastness of space is not one.
Of course you can reject a paradox, if you don't think that there are sufficient problems with the proposition for it to be a paradox.
What I don't buy is his assumptions that make it a paradox. Thus, I reject the paradox.
Time is not sufficiently vast, on a logarithmic scale, relative to the size of the galaxy, after you account for a few things, such as the synthesis of elements higher than hydrogen and the formation of later generation stars and planets that are capable of generating life.