A fascinating question and one which archaeologists have discussed ad nauseum. Are the periods or stages we use to divide history and prehistory "real" in some basic sense or are they mostly items of convenience for modern scholars? I tend to fall in the latter category, and so do many others in the historical fields, since we see much less use of these concepts in the technical literature than we used to
My belief is that how we divide human history up tells us more about the attitudes of historians and archaeologists than historical reality. Since humans by nature like to name and divide things, I suspect few scholarly traditions would last long with a "seamless web" approach, plus there are some real discontinuities in regional history which need to be identified (such as pre-Columbian and post-Columbian America). However, continued use of schemes such as "paleolithic, mesolithic, neolithic, bronze age, and iron age" has more to do with eurocentrism than anything else, and the use of "ancient, medieval, rennaissance, enlightenment" etc probably are a completely arbitrary and value-laden way of characterizing history.