How was the Fundamental Theorem of Calculus discovered?
The FTC is at once simple enough that Math.SE is full of questions asking "why is it such a big deal" and yet avoided discovery for perhaps centuries. While rigorous proofs took time, simple arguments from velocity and distance are easy.
How was it discovered? What were the obstacles that needed to be solved? Was it done using the simple arguments from velocity that I linked, from geometry, or from some other method?
Or, to put it differently:
What aspects of the FTC are non-obvious such that they eluded discovery for so long? And how were these obstacles to discovery eventually overcome?
One potential source for this: If anyone has access to the original writings introducing the FTC to the world (e.g. by Newton or Leibniz), these might explain the breakthrough.
Note: History of the Fundamental Theorem of Calculus asks what was done before the FTC. I'm asking specifically how the FTC was discovered.