Millions of years ago,the universe began with a “Big Bang”.At first,the earth was a cloud of dust.After a long time ,the temperature of air began to drop down,then the earth became a solid ball.When the temperature got to a relatively staple level,water appeared on the earth,so the earth became a place where was suitable for life to be live.About two hundred millions of years ago,small plants began to grow in the water,insects and amphibians appeared.Then reptiles ,dinosaurs and mammals appeared ,too.All these creatures flourished on the earth and the lives became to be diverse.Finally clever animals with hands and feet appeared.