There aren't any stoneage artifacts (?)
not so sure but I think that's the answer anyways.
How do you know that Sumerian society developed after the Stone Age???
2 answers
Sumerians developed writing and left written records and literature, had a complex society, and used metal tools.
http://history-world.org/sumeria.htm
http://www.theology.edu/sumer.htm
http://history-world.org/sumeria.htm
http://www.theology.edu/sumer.htm