Due to misoperation, the carrier virus accidentally infected ape test subjects. 

The virus had a key mutation on the ape, causing significant gene recombination for them. 

Infected apes and their future cubs grew 8 times in size, and became extremely aggressive. 

Few months later, giant ape with horns broke free from the lab, leaking the virus into the wild.

A few other species were infected and turned into Mutants, 

consuming large amounts of food resources and destroying the environment for 

human survival. Ozzy was also abandoned due to electricity outage. 

Humankind eventually nuked the tropical with cobalt bomb 

in order to stop the unlimited reproduction of Mutants, 

causing extreme climate change and the end of human civilization as we know it.