The Changing earthThe Earth formed from a cloud of dust and gas drifting through space about 4600 million years ago. Dense minerals sank to the centre while lighter ones formed the thin crust. However the first known life forms-bacteria and blue green algae-did not appear until about 3400 million years ago, and it was only about 700 million years ago that more complex plants and animals began to develop. Since then thousands of life forms have evolved such as the dinosaurs survived for many millions of years, while others died quickly. The Earth itself is continually changing. Although continents neared their present locations about 50 million years ago, they are still drifting slowly over the planet’s surface and mountain ranges such as the Himalayas-which began to form 40 million years ago-are continually being built up and worn away. Climate is also subject to change; the earth has undergone a series of ice ages interspersed with warmer periods.