The earliest theory describing the “String” nature of our universe was the “Bosonic String Theory”. As the name suggests, the vibrational patterns of Strings in the Bosonic String Theory only described Bosonic particles. Bosons are particles with whole number spins (0,1,2). They include all the four exchange or messenger particles- photons, W and Z bosons, gluons and Gravitons (hypothetical)- and also particles like the Higgs Boson.
The Bosonic String theory requires the universe to be 26 dimensional to make sense.
Shortcomings of the Bosonic String Theory
The first shortcoming was that Bosons account for less than half the particles that exist. For String Theory to be a “Theory Of Everything” or T.O.E, it had to incorporate all known particles. By this we mean that the String description had to somehow incorporate the matter particles or the Fermionic particles– particles with half integer spin (1/2,3/2…)- as well.
Another problem with The theory was that it gave rise to a particle with negative mass. These hypothetical particles are called “Tachyons”. Few physicists tried to show that tachyons existed together with the particles of positive mass, but, the concept seemed to be completely illogical.
Towards supermymmetric nature
Few physicists took up the challenge of getting rid of Tachyons and incorporating Fermionic particles within the framework of String Theory. One of these physicists was Pierre Ramond. He found a “supersymmetric” relation in String Theory. Similar to how every particle might have a superpartner in the standard model, there is a Fermionic vibration for every Bosonic vibration. They seemed to come in pairs. This discovery gave birth to the “Supersymmetric String Theory” or the “Superstring Theory” in short. This theory required 9 dimensions of space and 1 of time to describe the physical universe.
Note: Understand that we are not talking about the supersymmetry in particle physics where you have Superpartner particles for every existing particle. The supersymmetry in Strings have pairs of particles (fermions and bosons) similar to supersymmetry in quantum mechanics (all particles and their superpartners).
Five Superstring Theories?
Physicists found later on that there are five ways in which this new “supersymmetry” can be incorporated into String Theory. Five equally convincing and elegant ways of describing nature. These five types are: Type I, Type II A, Type II B, Heterotic O and Heterotic E. These five theories arise due to the purtubative (approximation) approach to String Theory. Later on, in mid 1990s, the five String Theories were merged into one M-theory (M is short for Membrane). This M-theory added another dimension to space, bringing the total upto 11 dimensions.