Why started civilization when it started?

The first civilizations were the Summerians, the Egyptians, the Indus river culture and afaik some culture in China. They all began within a few millenia. The Americas also saw the beginning of their civilizations in what we call antiquity. But the human species has been existing for much longer. Modern humans exist for about 300.000 years and have spread around the world for 100.000 years. Also the first humans reached the Americas more than 10.000 years ago. So why began human civilization in many different places within a time frame that is much shorter than the time scale of human evolution? And why began civilization in the Americas not that much later than in the old world even though there was no contact between the two hemispheres?