Simple life arose early on earth after it cooled, some 3.8 billion to 4 billion years ago. However, it took some three billion years for prokaryotes to evolve to eukaryotes, the basis for all complex life, including us, of course. This strongly suggests (though of course does not prove) that it will be staggeringly unlikely for life on any planet to be much more than very simple. Eukaryote evolution may have been an astounding fluke accident.
To put it into further perspective, even after eukaryotes evolved, it took further hundreds of millions of years for proto-humans to evolve. Had a meteor not wiped out the dinosaurs, there would be no humans. It is unknown whether some species of dinosaur could have evolved to human-style intelligence, but I think it highly unlikely. After all, we’re the only species that ever lived that evolved the capacity to send radio signals into outer space.
More perspective: if you compressed the entire history of the earth into a single calendar year, with earth forming on Jan. 1, then modern humans made their first appearance at about one-tenth of one second to midnight on the last day of the year, Dec. 31.
Just considering human history by itself, the vast majority of our time was spent as hunter-gatherers, without even a concept of settled communities or agriculture. This notwithstanding the fact that humans of 200,000 years ago were just as intelligent as we are.
Radio only first appeared less than 150 years ago!
This strikes me as good evidence for the extreme unlikelihood, in the Milky Way, of other radio-signal-sending, spacefaring aliens apart from us. The odds against such entities seem far too great, judging by the evidence we have here on earth. There is also the issue of time. Maybe such beings existed 200 million years ago and then perished; maybe we will perish and some other species somewhere will achieve what we did 200 million years from now. Any civilizations, if they exist at all, must be separated not just by vast gulfs of space but of time as well.
There is also the problem of mutual intelligibility. Even if somewhere in the Milky Way there is a species with comparable cognitive abilities to our own, they may well be so different from us, and we from them, that we would not even recognize each other. It is often said that math would be a universal language, a Rosetta’s stone that would facilitate mutual communication even among radically different species, but there are good reasons to think this is false.