What a terrifying theory and great answer, By Åke Rosenius form Quora

What about the Fermi Paradox is absolutely terrifying?

Here’s a schematic of all the current types of life on Earth:

Note the little ‘Animals’ twig in the upper right corner. We’re part of that one, as are lizards, octopi, bugs and pretty much any other Earthly lifeform that people base their idea of “aliens” on. The whole discussion about alien creatures and civilizations out there is oddly rooted in the assumption that life on other planets will, as a rule, end up having that very same twig. Or something very similar to it.

Life on Earth tells a different story. Life seems to have appeared as soon as it was possible given the conditions. That’s a good argument for life being (at least relatively) plentiful in the universe. Life then thrived and developed and populated every corner, nook and cranny of the Earth, inventing photosynthesis and all kinds of nifty stuff along the way… but it took about two billion years for it to invent cellular nuclei (the ‘eukaryota’ branch pictured above), and some more time for those eukaryotes to develop this weird symbiosis with the things now known as mitochondria (originally probably some kind of bacteria) living inside their cells. And those are basic requirements for multicellular creatures as we know them. Life on Earth has always been absolutely dominated by microscopic single-celled creatures, and remains so to this day.

If Earth is anything to go by, the answer to the Fermi Paradox is really easy: the Great Filter exists, and it’s multicellularity.

That gives us a universe that is abundant with life, just be careful that you don’t slip on it if you go visit it. No grays in sight, or guys with crabs in their foreheads, or horny tentacles, or bug-eyed monsters. Just a universe populated by smelly goos and slimes and unmoving blobs.

However (and I’m honestly not sure whether this makes it less or more terrifying), one has to ask the question — if multicellular life was such an odd, unlikely development on Earth, what other truly odd, unlikely developments could life have successfully come up with on other planets in those billions of years? If the answer to that question is anything other than “nothing”, then perhaps we have not found anything because we can’t possibly imagine anything as weird as the life that really is out there…