How does M-theory connect different versions of superstring theory?

In 1995, physicist Edward Witten proposed M-theory as a framework to unify the various, seemingly distinct versions of superstring theory that had been developed over previous decades.12

Before this development, the existence of multiple self-consistent superstring theories—each incorporating supersymmetry and gravity—threatened the goal of establishing a single, unique "theory of everything". Witten demonstrated that these disparate theories are actually different limiting cases of a more general, underlying 11-dimensional theory.2

Key features of this unification include:
* Higher Dimensions: M-theory reveals that the theory requires seven extra spatial dimensions, rather than the six previously assumed in 10-dimensional superstring models.13
* Brane Objects: The theory incorporates not only strings but also membranelike objects of various dimensions, collectively known as branes.12
* Refined Equations: By utilizing new mathematical techniques, theorists were able to refine the approximate equations of earlier models, showing that the different versions of string theory were essentially the same.1

While M-theory provides a more coherent picture, it remains a theoretical construct that has yet to be supported by experimental observation.1