Persian empire.

If that's the case, why do people still call Sumeria the cradle of civilization when Egypt developed further?

I haven't heard of any Ancient Egyptian cylinder seals- all the reading that I've read seems to indicate that there wasn't that much contact between them ligusitically - Sumerian and Egyptian were both completely different. However, on a larger, architectural basis, there do seem to be a lot of links. Djoser's step pyramid is reminicent of the contempary Zigguarats of the time, and it's evolution from Mastabas piled up on top of each other is remarkably similar to the evolution of the Ziggurat from tiers being added to a temple over time. In fact, regarding that, several Jemdat Nasr period stepped temples have collinades and friezes which are very similar to Egyptian ones. The same gladually inclining slits in the walls and tall, fluted edges. It's in fact amazing when you consider the large amounts of evidence of Mesopotamian and Levantine influence in Egypt that it remained so remarkably static culturally and socially for centuries- professor W.B.Emery gave massive amounts of evidence for Syrian invasions into lower Egypt after the old kingdom, and there are loads of records of extensive trading between the nations. It seems that Sumer took more of Egypt culturally that Egypt did of Sumer.