Everything on this universe kept on changing. That's a universal truth. That's why..our earth changed to a model suiatable for the human existence. Many things lead to the change. Now the trees number are decreasing and this will bring another change on earth. It was because of that dinosaurs got disappeared from this earth. Due to the revolution and rotation of earth and due to the movement of other planets...we got iceage and mammoths like things got disappeared. At that time earth was frozened, But later it changed.