Why is the arrival of Islam a good beginning point for a new historical age, the Post Classical Axel?

Post-classical history (also called the post-antiquity era, post-ancient era, or pre-modern era) is a periodization commonly used by the school of world history instead of Medieval or Middle Ages, which is roughly synonymous.[1] The period runs from about 500 AD to 1500 AD, though there may be regional differences and debates. The era was globally characterized by the expansion of civilizations geographically and development of networks of trade between civilizations.[2][1]