Nathaniel Essex was a biologist in Victorian England who developed advanced theories on human evolution. A geneticist of unparalleled genius, he foresaw the coming mutations that would transform mankind. However, his unorthodox experiments resulted in his ouster from the Royal Society and caused his wife to shun him. These rejections led him to ally himself with Apocalypse, an immortal mutant who has witnessed virtually the whole of human history. Apocalypse transformed Essex into the ageless being known as Mr. Sinister.