[Virginia-American]

I've read that while special relativity was "in the air," so that someone else would have come up with it, general relativity was so theoretical, and so austere, that it might have taken another century for others to piece together all the clues (which came later) and produce that theory. But I'll leave this to those who know more than I do.

In Gravitation by Misner, Thorne, and Wheeler, (IIRC), there is speculation that Riemann might have come up with relativity (he died in 1866 at age 40, two years after Maxwell published his equations). He did, after all, develop the geometry needed by Einstein.

[Virginia-American]

I don't mean to imply that he did and his work is lost; I should have said "Had he lived long enough...". He was quite ill (tuberculosis) the last few years of his life.

[RightWingAtheist]

Poincare also predicted some of the aspects of special relativity late in his career.