>""They are useful only in particular types of war. ""<

This is true.

...... and then there's the thought that the earliest shields were likely as simple as hide or even tree-bark drawn over willow-type/sapling frames (ref: North and South American, African, and some Asian, South Asian and Pacific Islander indigenous populations as recently as the 20th century).

Such shields/artifacts are within the technical abilities of any population capable of making awls, needles, clothes and/or certain weapons, such as stone-bladed spears and hafted stone knives and axes, and would NOT likely survive the way other implements- like those with flint or volcanic glass (obsidian) or even bronze fittings/nails/rivets/etc. would. Hide and wood shields could have been used for as long as Mousterian or even Acheulean technology has been available, and we wouldn't have evidence of them unless some enterprising person were to paint images of them in use on a cave wall for us to find (there ARE petroglyphs of shields in use, but from what era???)

Just a thought........