Even in real life, humans are slowly destroying the planet.  However, in Parasyte, Earth may have a chance due to alien invaders.  These aliens are parasitic, sometimes symbiotic, creatures that attempted to take over human bodies when they first invaded.  Most Parasites succeeded and took control over human brains. Others failed to take over humans but succeed in controlling other animals, although this is considered a failure by the Parasite population. These Parasites develop a need to eat others of the same species as their host.  Thus, Parasites in human hosts eat other humans.

Another failure is when a Parasite fails in taking over a human’s brain and instead controls a different part of their body. This is the case for Shinichi Izumi, who has a Parasite in his right hand.