Here is a very brief outline of the development of string theory, the
details of which will eventually fill many large volumes written by
many people directly and indirectly involved in this rich and fascinating
story.

1921 


Electromagnetism can be derived from
gravity in a unified theory if there are four space dimensions
instead of three, and the fourth is curled into a tiny circle. Kaluza
and Klein made this discovery independently of each other. 
1970 


Three particle theorists independently
realize that the dual theories developed in 1968 to describe the particle
spectrum also describe the quantum mechanics of oscillating strings. This
marks the official birth of string theory. 
1971 


Supersymmetry is invented in two contexts
at once: in ordinary particle field theory and as a consequence of introducing
fermions into string theory. It holds the promise of resolving many problems
in particle theory, but requires equal numbers of fermions and bosons,
so it cannot be an exact symmetry of Nature. 
1974 


String theory using closed strings fails
to describe hadronic physics because the spin 2 excitation has zero mass.
Oops, that makes it an ideal candidate for the missing theory of quantum
gravity!! This marks the advent of string theory as a proposed unified
theory of all four observed forces in Nature. 
1976 


Supersymmetry is added to gravity, making
supergravity. This progress is especially important to string theory,
where gravity can't be separated from the spectrum of excitations. 
1980 


String theory plus supersymmetry yields
an excitation spectrum that has equal numbers of fermions and bosons,
showing that string theory can be made totally supersymmetric. The resulting
objects are called superstrings. 
1984 


This was the year for string theory!
Deadly anomalies that threatened to make the theory senseless were discovered
to cancel each other when the underlying symmetries in the theory belong
two special groups. Finally string theory is accepted by the mainstream
physics community as an actual candidate theory uniting quantum mechanics,
particle physics and gravity. 
1991
1995 


Interesting work on stringy black holes
in higher dimensions leads to a revolution in understanding how different
versions of string theory are related through duality transformations.
This unlocks a surge of progress towards a deeper nonperturbative picture
of string theory. 
1996 


Using Einstein relativity and Hawking
radiation, there were hints in the past that black holes have thermodynamic
properties that need to be understood microscopically. A microscopic origin
for black hole thermodynamics is finally achieved in string theory. String
theory sheds amazing light on the entire perplexing subject of black hole
quantum mechanics. 