Why is the period between 2000 and 1000 b. c.e a climacteric in world history? what were the reasons for the disappearance of many civilizations across the world and how did egypt and china manage to sustain their civilization for longer terms?

That time was a critical time in our history because that was when man began to start evolving from hunter-gathering to farming/agriculture. Many civilizations that stayed in hunting-gathering could not grow their civilization and traditions (because agriculture led to a boom in population) therefore some died out  where some civilizations (Egypt and China) adapted to agriculture early and their land was suited for agriculture, helping their civilization to sustain for a longer period of time.