Mutations led to variations, some of which couldn't survive but some that could. Life continues Single-celled organisms lived in the oceans, using sunlight and chemical reactions for energy. Organisms doing photosynthesis converted CO 2 to O 2 leading to increase of oxygen in atmosphere. Life altered atmosphere and as a consequence led to diversity of life on both land and in water, and to having both plants and animals. About half a billion years ago, life became very diversified.
Fossil record shows many types of plants and animals.
Competition for food and necessities of life led to complex species. Dinosaurs wiped out 65 million years ago, large mammals luckily survived. We showed up a few million years ago. Better to be safe than sorry, so work is going on to reduce the amount of greenhouse gases we create. The ozone layer is being depleted.