The most probable Fermi Paradox Solution
The Fermi Paradox is one of the most interesting questions posed of numerous branches of science. The basic question is: where are the aliens? If the universe had 13.7 billion years to evolve, there should be some evidence of life evolving elsewhere. People have argued that there are many great filters that may have held back life from progressing. I believe this view to be accurate, and I believe the most important great filter to be in our past. This is the filter of intelligent life evolving at all. I will present two convincing arguments supporting this view.
It's often said that humanity went from cave dwelling species to intelligent and advanced civilization within maybe 100,000 years, the "blink of an eye" in evolutionary terms. While that may be true, it took a long, long time to get there. It's incredibly apparent to me that life took an extremely long time to go from its genesis to evolve into any animal. Earth has harbored life for most of its life, and it's no spring chicken. Earth is a 4.2 billion year old rock. Yes, the universe is over 3 times older than earth, but only just 3 times older. Any variables that could potentially double or triple the evolutionary timespan would be enough to explain the entire paradox, as life itself has been present on earth for 3.7 billion years. The first explanation of the fermi paradox must come from the macro side of things: the universe is not a friendly place, and earth, as it cooled was barely habitable. Looking outside of earth, planets in particular were not always capable of harboring life, and took a long time to cool and settle. It's easy to imagine how maybe a planet could be relatively uninhabitable for 5 billion years, and then habitable for the most recent 1 billion years. Or maybe, most planets never became habitable in the first place. You may wonder then, surely even if most planets never become habitable, that the ones that are make it extremely likely that life could begin in an early enough phase to start into motion a timeline resulting in intelligent life before us, right? Yes - this is true, except for a couple things. Most importantly, following this macro-scale argument, it cannot be understated how long it took life to evolve here on earth. 3.7 billion years ago there was life, and nearly 3 billion years after that the first animal followed. It seems obvious that number must be somewhat arbitrary. I think it's extremely likely - far more likely than not - for planets that may harbor life to never harbor life more complex than single celled organisms, given earths own history. It's difficult to think of a planet more suited for life than our own blue planet. Surely earth is an anomaly in this regard too. Perhaps earth was just perfect enough for multi-celled organisms, but most planets trap single celled organisms with no opportunity for progression after that.
Let's fast forward in earths timespan and examine animal behavior even after multi-celled organisms evolved.
Large land animals may have begun with the dinosaurs on earth, but what we think of as intelligence, did not. Due to the extremely long reign of the dinosaurs and evidence that no intelligent reptilian life existed among them, it seems likely that if that asteroid did not hit the earth 65 million years ago, there would still be no intelligent life on earth. It appears that it's quite possible, even likely, given the lack of evidence of intelligent life from time periods in which dinosaurs roamed the earth, that intelligent life is extremely rare. Indeed there could be a stable ecosystem for millions if not billions of years where no single species rises above the rest. Without the asteroid hitting the earth 65 millions of years ago, there's no reason to believe dinosaurs were on a path to intelligence. Dinosaurs appeared content to continue their trajectory of finding new and different ways to live off the earth and eat each other. The asteroid, hitting with just enough force to reset the large land animals, but not enough force to make all life on earth extinct, seems like a miracle. An asteroid of that size striking a planet at all is far from guaranteed in the universe, especially in today's worlds where most of the mass is already concentrated in moons, planets, and stars. Earth harbored complex life for many hundreds of millions of years and yet only within the last one or two did intelligent life emerge. It's not an exaggeration to say that the extinction event 65 million years ago was quintessential to this revolution.
One thing seems clear: our ability to be curious, to reflect, to ponder the stars in peace, and to think about how we got here was truly a gift from a less violent world brought upon us by the destruction of the dinosaurs. The evolutionary game changed from conquering the competing species, to fighting against the elements and earth itself in order to survive, in a less violent and less populated world.
The legacy of humans could still have ended in many different ways. The ability to use and hold objects seems to be one of the extremely necessary adaptations in order for life to advance - however if animals rely solely on their natural physiology and abilities, it seems less likely for them to be intelligent, as was the case for the vast majority of life up until today. This is because ecosystems balance out due to predation primarily. For non-predators to ascend to intelligent life, there needs to be protection, and the most reliable form of protection is a residence or tools in which to protect from being eaten. For large animals to be able to build such things requires the ability to move objects around them. In order for predators to ascend to intelligent life, they would require an understanding of animal breeding great enough to be able to contain and breed their food, and this too requires the ability to move objects. I would have to also make the argument that it seems less likely for predators to gain this ability because if a species remains at the top of the food chain, there's less of reason for it to need to evolve, unless the food itself is evolving.
Conclusion: life is drastically more likely to occur and exist in stable ecosystems than to allow for the emergence of intelligent life. Life in this way would be invisible from out telescopes and not emit any unusual radio signals that would give away their existence. The most significant great filter is that of getting to an ecosystem that supports intelligent life, and therefore it follows that, although life in the universe may be common, intelligent life is less common than we may believe it to be.