Did cousin marriage cause us to lose our leadership in world civilization?

First and foremost, I am not trying to trigger any religion related debate here.

Being Arab nowadays includes the majority of people from MENA region so I will refer to all these people as Arabs in history for the sake of simplicity. This region (MENA) is where civilization literally started between Mesopotamia and ancient Egypt, the majority of things we use nowadays from Mathematics, the wheel, writing, irrigation system, law and government,..hell, even fashion products like high heels were made in ancient Egypt.

The westerners try make it perceived as the civilization started with the ancient Greeks which is a lie. The Greeks themselves learned how write from Phoenicians (todays Lebnan) and didn't really start being civilized until around 800BC, 3000 years after arabs. Even Sabae in Yemen had complex civilization that predated the ancient Greeks by centuries.

Now I am not going to bring everything from MENA because our history is so rich that it cannot be brought in a single post.

What I am asking is what's the reason of our decline and as the title suggests, could it be a biological issue since we adopted cousin marriage? I say that because there are countless strong studies suggesting that cousin marriage lowers IQ as well as leading to other genetic defects.
I believe that is the only explanation considering we did not only predate the ancients greeks by 3000 years, but also the self-proclaimed genetically superior North Europeans who really did not even know how to write up until medieval ages. So we had the upper hand for a very long time.

What do you think?