I really wonder how zero came to be, was it a need to homogenize the algebraic tools (we have nice operations and want to reify the 'nothing' dead end into something that can be kept chugging along) or something else /
I have to imagine that the multiplicative identity (1) and additive identity (0) were also known at that time. They really figured out everything else there is to integer-based math.
They failed at irrational numbers (like sqrt(2)). But their mastery of integers was outstanding. Perhaps 0 wasn't used on the number-representation yet, but the concept had to have been known.
(mods I promise I'm not here to shill this book but people keep wondering about the history and that's a great resource that I like XD)