An Alien Experiment Gone Wrong?
Sometimes I like to imagine that the Thing is some kind of Aliens experiment into genetic manipulation and cell reproduction. Like some alien species was attempting to create donor cells that can "become any cell" they need it to be...but its goes horribly wrong! The cells absorbed an alien and gained its first sentience when replicating the brain cells. Then all heck broke loose! It eventually becomes this apocalyptic being that travels to other planets and takes all lifeforms. Becoming the most powerful organic being to roam the cosmos!
Theres no evidence in the movie to support my theory, since no answer is given on where this Thing came from, whether a lab or a natural evolved species from another world. But Its fun to consider.
https://youtu.be/cHf3aitzSCE