Giant Amoeba

The Giant Amoeba is the final antagonist in Ivan Reitman's 2001 film Evolution.

Description
The Giant Amoeba is an alien creature with unknown origins. Like all the aliens in the film, it's a Nitrogen-based species. It's very similar to a normal amoeba and, at first, was a microscopic organism. However, it evolves much faster than Earth species and an exposition to high temperatures causes it to increase size tremendously. Even when it is giant, it still can duplicate itself like all amoebas. Its only known weakness is the Selenium, which is lethal for it.

History
The Giant Amoeba, along with with all the other aliens in the movie, arrives on Earth inside a meteorite. Near the end of the film, the military try to destroy the aliens by sending them large amounts of napalm. However, the explosion makes them grow up even more quickly, and the Amoeba becomes so huge that it crushes all its congeners. Now mountain-sized, the Giant Amoeba makes a lot of damage in the surrounding city and kills several militaries. Then, it begins to duplicate itself. However, the protagonists arrives armed with Selenium. They successfully poisons the Giant Amoeba, which makes it explode.