THE CHANGING EARTH

The Earth formed from a cloud of dust and gas drifting through space about 4,600 million years ago. Dense minerals sank to the center while lighter ones formed a thin rocky crust. How ever, the first  known life forms bacteria and blue green algae did not appear until about 3,400 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 animal and plant species have evolved; some, such as the dinosaurs, survived for many million of years, while others died out 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 begin built up and worn away. Climate is also subject to change: the Earth has under gone a series of ice ages interspersed with warmer periods (the most recent glacial period was at its height about 20,000 years ago)