In the year 2020, it was found out that a harmful virus had infected the human race, killing them slowly and bringing them towards a dark future. But with the technology they had in hand, in an attempt to save the civilized race, the people chose to go through with "Beast Transformation." Knowing that other than humans, all animals were safe from the virus, the humans made the decision to inject the genes of animals into themselves to become part-human, part-animal. In the end, the idea was "Either you do beast-transformation or die." With the race known as Homo Sapiens gone, by the year 2035, being a beast-human was normal.

(Source: MangaHelpers)