What marks them both is their willingness to abandon the established dogma when it didn't match the facts, and to set off exploring alternative ideas. Both of them are iconoclasts: Einstein because he proposed time wasn't absolute, which is a seriously batty suggestion to just about everyone unfamiliar with his work because we are very familiar with how time works here on earth. To me Darwin the more impressive in this regard because his idea wasn't just seriously batty (no one had seen evolution at work when he proposed it, despite living in it's midst) but just putting it out there was enough to get others burned at the stake.
Another similarity is neither was your typical extroverted iconoclast. Quite the reverse in fact: they doggedly followed their favoured theories because the felt they did a better job of explaining the facts. Both seemingly didn't care that others would think less of them for their ideas.
You can point to others in the same mould: Galileo for instance. The doctor that endured years of ridicule from his peers for suggest gastric ulcers were the result of a bacterial infection is another.
Nonetheless Einstein stands out for second reason: he did it more than once.
Einstein’s iconoclastic idea was his questioning of Newton’s occult force. That was Einstein’s starting point. He could not accept that an occult notion acting instantaneously at a distance can exist in nature. He set out to fix this situation by trying to explain gravity without the Newtonian force. Did he succeed in this?
Also when we look back, we see that Einstein was chosen as a product to be marketed as genius to unite the post-war Europe. A German Jew whose theories proved by the observations of a British team etc.
I think the idea of evolution was in the air. Lamarck and Wallace wrote about similar things. I don’t know if Darwin knew about them.
The book is of course GR bible Gravitation by MTW.
https://www.amazon.com/Gravitation-Charles-W-Misner/dp/07167...
Of course he dashed off another three articles in the same edition of Annalen der Physik, all on the same level of comprehensibility, one of which scored him a Nobel prize.
The difficulty of relativity was and is a silly meme. The basic principles are straightforward. I understood them as a boy, and I am - by a regrettably enormous stretch - no genius.