It has been two millennia since Christ. Two millennia is 20 centuries. When one talks about the history of man or when man started to create art, the numbers in years get too big to be comprehensible. The museum uses the convention of expressing things that happed before Christ in number of centuries before Christ. That is, 40c is about 6,000 years ago.
Art is symbolic communication practiced by humans using icons.
But this definition is too broad in some sense since that would include all language. And intuitively one senses that not all language is art. But if we want to talk about art, especially primitive art, we need to defined it somehow. So let's assume that in addition to being symbolic communication, to be art the object must a special communication above the level of ordinary discourse. This means that a laundry list, or accounting records are not art.