The Greek philosoper Hero of Alexandria actually invented something close to a steam engine in the 1st century BE [1]. Whenever I think about "obvious" inventions which happened quite late in our civilization, I remember "The road not taken", a fantastic short story that plays with this idea. [2]
[1] https://en.wikipedia.org/wiki/Aeolipile [2] https://en.wikipedia.org/wiki/The_Road_Not_Taken_(short_stor...