When Bethesda took the reigns of the Bethesda took over the Fallout series, changing it to a primarily first person perspective, it also moved across the country to Fallout 3's Capital Wasteland.
Bethesda's subsequent entries to the post-apocalyptic series would also take place near the east coast - Fallout 4 in the Boston Commonwealth, and Fallout 76 in the Appalachian region of West Virginia. Moving the series such a great distance offered Bethesda a chance to write new narratives, free from the constraints of most canonized events established in Fallout 1 and 2, but separate origins for some factions and enemies, such as the super mutants, would have to be created. The super mutants of the original Fallouts - and those in Obsidian's New Vegas - share a similar origin: the results of an experiment at the Mariposa Military Base.
The recognizable, usually green mutants of the Fallout universe are man-made, unlike radiation-created creatures like Ghouls. When Bethesda created factions of super mutants for the east coast locales, they were each given a different origin, though they are all still consistent with the established Fallout lore.
Bethesda's Super Mutant Origins in Fallout 3, 4, and 76
Super mutants in Fallout 3's Capital Wasteland hail from the Evolutionary Experiment Program conducted in Vault 87. There, some vault dwellers were subjected to the Forced Evolutionary Virus, the same that was used on the west coast to create super mutants. This strain of the virus, however, resulted in super mutants much less intelligent than their west coast counterparts. Thus, super mutants in Fallout 3 are almost entirely dim-witted and violent, kidnapping humans to expose them to the virus in order to create more mutants.
The Boston area super mutants, including Fallout 4 companion Strong, are products of the Institute, who conducted research on the Forced Evolutionary Virus while trying to create their synthetic servants. The super mutants wandering the Commonwealth are generally more intelligent than those in the Capital Wasteland, and are infected with a virus strain that can be cured, as shown by the Sole Survivor helping Virgil turn himself back into a human.
Fallout Super mutants came to 76's Appalachia by way of the town of Huntersville, where West Tek - the company that created the Forced Evolutionary Virus - tainted the town's water supply with its creation. The super mutants of West Virginia have varying dispositions, with some being hostile and others being willing to trade with humans. What started out as the main antagonistic faction under control of the Master in the first game has turned into a varied character type across multiple titles, especially in the Bethesda Fallout games set on the other side of post-nuclear America.
