Since its part of basic human instinct to move close together in case of danger, I'd say that close formations has always been part of warfare. The trick is to maintain such a formation when going into combat as that requires some discipline and most likely some dedicated file closers.

One might even argue that the Spartans were the only army that really managed that in the early days of Greek warfare.

But I'm sure it would have been used earlier as in the case of Mesopotamia. Although I would say that not only the overlapping shield but also height/size of the shield could suggest a formation meant for protection against enemy missiles. Perhaps working a bit like the Italian militias who had pavise equipped spearmen in front and crossbows behind.


CBR