They could quarry perfect slabs of granite, marble and limestone and build well engineered structures, but for some reason couldn't come up with a complex symbol to represent their society a mere 5000 years ago?
"Ugg! me make cross" said Omar while he sitting on the edge of the marble walls of aquaduct which brought water from the glacier on the mountain 5 miles away, and supplied the entire city with indoor water service, as well as powered the city gates, the city flour mill, and sewage system... Anyu number of societies could have used simple shapes like a cross for any number of reasons in their daily lives.
I’ve never been one for progressivist history. Man has been very sophisticated for a very long time.