Despite bosonic string theory’s apparent failures, some brave physicists stayed committed to their work. Why? Well, physicists can be a passionate bunch (nearly obsessive, some might say). Another reason was that by the time these problems were fully realized, many string theorists had already moved on from bosonic string theory anyway.
With the development of supersymmetry in 1971, which allows for bosons and fermions to coexist, string theorists were able to develop supersymmetric string theory, or, for short, superstring theory, which took care of the major problems that destroyed bosonic string theory. This work opened up whole new possibilities for string theory.
Almost every time you hear or read the phrase “string theory,” it probably really means “superstring theory.” Since the discovery of supersymmetry, it has been applied to virtually all forms of string theory. The only string theory that really has nothing to do with supersymmetry is bosonic string theory, which was created before supersymmetry.
The origins of supersymmetry are a bit confusing, because it was discovered around the same time by four separate groups.
In 1971, Russians Evgeny Likhtman and Yuri Golfand created a consistent theory containing supersymmetry. A year later, they were followed by two more Russians, Vladimir Akulov and Dmitri Volkov. These theories were in only two dimensions, however.
Due to the Cold War, communication between Russia and the non-communist world wasn’t very good, so many physicists didn't hear about the Russian work. European physicists Julius Wess and Bruno Zumino were able to create a 4-dimensional supersymmetric quantum theory in 1973, probably aware of the Russian work. Theirs was noticed by the Western physics community at large.
Then, of course, we have Pierre Ramond, John Schwarz, and Andre Neveu, who developed supersymmetry in 1970 and 1971, in the context of their superstring theories. It was only on later analysis that physicists realized their work and the later work hypothesized the same relationships.