Extra dimension were part of string theory right from the start. The idea that everything comes from vibrating strings cannot work if you only consider the four

Extra dimension were part of string theory right from the start. The idea that everything comes from vibrating strings cannot work if you only consider the four-dimensional relativistic spacetime. (The maths behind it is awful complex btw, I only have some second-hand knowledge thanks to my SO being a differential geometry geek and reading a bit of Yau's book The Shape of Inner Space)