The Great Filter is the idea that, in the development of life from the earliest stages of abiogenesis to reaching the highest levels of development on the Kardashev scale, there is a barrier to development that makes detectable extraterrestrial life exceedingly rare. The Great Filter is one possible resolution of the Fermi paradox.
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, "If life is so easy, someone from somewhere must have come calling by now."
Personally I think it's photosynthesis. Life itself developed and spread but photosynthesis started an inevitable chain of ever-greater and more-efficient life. I think a random chain of mutations that turns carbon-based proto-life into something that can harvest light energy is wildly unlikely, even after the wildly unlikely event of life beginning in the first place.
Boy, Lemmy sucks donkey dick. For every one legitimate answer there are two or three edgelord answers like "capitalism" and "the internet".
Here's an answer that hasn't come up yet: cooperation among mono cellular organisms. I don't mean the development of polyp analogues or colonies of single celled organisms; I mean getting down to mitochondria. Brace for wild oversimplification.
Before mitochondria, life had a hard time creating enough energy to do much more than barely stay alive. The current line of thinking is that one organism ate another and didn't digest it. The two organisms worked symbiotically, one handled energy production and the other handled getting food and staying alive.
Just about every living thing utilizes mitochondria and if the current idea that mitochondria were actually symbiotic organisms is true, that means that what was likely a chance "sparing" of prey is the underpinning of all complex life.
The odds of that happening are ridiculously low. There could be simple life in tons of places even within our own star system, but if the mitochondria-like symbiotic capture never happens for those extraterrestrial organisms, then complex life is probably unlikely to develop.
I'd hardly describe it that way. It took untold trillions of predator/prey interactions over the hundreds of millions of years that single celled life existed for it to happen. That's more or less brute forcing the problem and it took geologic timescales to happen.
If you ask me to point at a hurdle stopping civilizations from developing that looks awfully reasonable.
Ultimately we don't know much about that era of time, but I suspect it was less like fumbling around for millions of years looking for a light switch, and more like the gradual warming of the planet with warmer and cooler seasons/years.
Iirc at least one of the other things related to development of eukaryotes was that atmospheric oxygen had to first be generated by early cyanobacteria.
So maybe that proverbial light switch was being flipped millions of times through random encounters but only became more viable after the voltage (atmospheric oxygen levels) became high enough. Maybe that's the reason it took hundreds of millions of years, because transforming by bacteria just takes that long.
We just don't know unfortunately. However, we DO know about species getting wiped out by asteroids or human cultures getting wiped out by disease or conflict with superior cultures. Any of these filters seems more of a hurdle to me than the development of eukaryotes.