Harvey will focus on later stages.
Origins of string theory were to explain strong interactions, i.e. hadronic resonances and Regge behavior of scattering at high energies and fixed angles. This approach floundered because mathematical consistency required 26 or 10 dimensions, and because no mechanism was found to remove the massless spin 1 and spin2 states that didn’t match the hadronic resonances.
In 1974, Scherk, Schwarz and Yoneya proposed a reinterpretation. Massless spin 2 particles was graviton, massless spin one particles are gauge fields, and string theory was a method to unify gravity with gauge theory in a relativistic framework.