The ancient Babylonians had a complex number system that included a placeholder marking a placeholder number (e.g. the zero in 60.) This did cause some ambiguity in the written representation of numbers, and the quantity had to be discerned partially by context in some cases. Thus, this was not a true zero. The ancient Chinese understood the concept of both negative numbers and zero, but they did not have a symbol for zero.
The ancient greeks at least puzzled over the meaning of a zero. They got hung up on the philosophical question of 'how can a something be a nothing?'
The idea of the zero as a number and not a philosophical construct, or merely a symbol for separation came from India arround 800 AD. The symbol for zero most like our own came from Ptolemy-- a circle with a bar over it. The Romans also used the symbol N for zero, a shorthand for the word nihil-- nil, or nothing.