Disclaimer:
I am asking this question as an improvement to this question, which should be community wiki. This is in line with the actions taken by Andy Putman in a similar case (cf. meta).
See the relevant meta thread about the previous question.
Edit: If it wasn't already obvious, I only asked this question to prevent the other one (which was not made community wiki) from being reopened.
Question:
The scope of mathematics has grown immensely since ancient times. At what point in time did it become impossible for a single person to understand the majority of mathematics enough to keep current with contemporary research?
Edit: Clarified the wording.