When was punctuation introduced into Chinese?
There were no punctuation marks such as comma in ancient Chinese writings. It's stated in "Punctuations in Chinese Language" and in this forum's thread, "Punctuation Marks in Ancient Written Chinese", where there are two links to some ancient writings. As it's said there, having no punctuation at all, ancient scholars knew the meaning thanks to some structure and "rhythm" in the writing, but I haven't found any official source for this statement.

Regarding its introduction, according to Wikipedia, it seems that punctuation in Chinese became a part of its system in the 20th Century, so it's very recent.

The first book to be printed with modern punctuation was "Outline of the History of Chinese Philosophy (中國哲學史大綱)" by Hu Shi (胡適) in 1919.