Aliens are artificial species designed to clean whole ecosystem and then all die

1. Mural in Prometheus with picture of Facehugger & Xenomorph proves that they existed before Prometheus has landed on LV 426 (so they weren’t created by accident)

2. Aliens as a species can’t survive evolution in long term because of their reproductive system (egg, facehugger, xenomorph).

   2.1. Deployed on Earth they would reproduce using Earth organisms as hosts 1:1
   
   2.2. Due to Alien fighting skills supremacy:
	- they would reproduce exponentially and 
	- human number would decrease exponentially.
   
   2.3. At the end, all hosts would be dead leaving on Earth only:
	- organisms that can’t be hosts, 
	- eggs, facehuggers and xenomorphs.
   
   2.4. As no one is immortal all existing aliens would die from old age
   
   2.5. Which leaves Earth purified from humans.

3. Survived cat from Alien 1 and survived mice from Alien 2 suggest that Xenomorphs would maybe kill only humanoids or bigger animals.