The Earth was formed at the same time as the rest of the solar system by the coming together of materials that were not incorporated into the sun. The Earth was very hot (perhaps molten) at first, but quickly cooled off. Oceans appeared very early and have been on the surface continuously ever since. Life in the form of single-celled algae was present when the Earth was 3.5 billion years old. Animals with hard skeletons and shells appeared and formed fossils 540 million years ago. The period since then is usually broken up into the Paleozoic Era (“old life”) from 540 to 225 million years ago, the Mesozoic Era (“middle life”) from 225 million to 65 million years ago, and the Cenozoic Era (“modern life”) from 65 million years ago to the present. Dinosaurs flourished in the Mesozoic, whereas human beings have been present for only a few million years.