Evolution; people and the animal world

The Earth formed about 4.5 billion years ago. At first it was very hot, with erupting volcanoes and falling meteorites. When the planet cooled, water appeared and formed the ocean. The first traces of life appeared in the ocean as microscopic creatures. Algae and special bacteria made food from light and released oxygen, which changed the atmosphere. Later plants and then animals moved onto land. About 230 million years ago dinosaurs appeared — they were reptiles, and some were enormous. About 66 million years ago most of them died out; scientists think the main cause was a huge asteroid striking the Earth and a sudden change of climate. After that mammals flourished. Today’s birds are descendants of dinosaurs. Humans appeared much later, roughly 300,000 years ago. Paleontologists study ancient creatures through fossil bones and tracks.