Mewtwo was created from the DNA of Mew in the original Pokémon story.
In Pokémon Red and Blue, research documents on Cinnabar Island describe experiments that produced Mewtwo. The creature was genetically engineered from Mew's genetic material and altered through laboratory research, giving it a distinct identity rather than making it a normal evolution of Mew.
Mewtwo and Mew are separate Pokémon species in the Pokédex. Mew is National Pokédex No. 151, while Mewtwo is No. 150, so their numbering does not reflect a simple evolutionary sequence. Mewtwo is a Psychic-type Pokémon in its original form.
The concept also drives Pokémon: The First Movie, whose story focuses on Mewtwo's origin, identity, and resentment toward the humans who created it. Ditto, Deoxys, and Genesect have other unusual biological or scientific stories, but they are not the answer to this specific origin question.