WHEN THE EARTH FORMED about 4.6 billion years ago, its atmosphere consisted of volcanic gases with little oxygen, making it hostile to most forms of life. One large supercontinent, Gondwana, was situated over the southern polar region, while other smaller continents were spread over the rest of the world. The constant movement of the Earth’s crustal plates carried continents across the earth’s surface. The first primitive life-forms emerged around 3.4 billion years ago in shallow, warm seas. The build-up of oxygen began to form a shield of ozone around the Earth, protecting living organisms from the Sun’s harmful rays and helping to establish an atmosphere in which life could sustain itself. The first vertebrates appeared about 470 million years ago, during the Ordovician period (510–439 million years ago), the first land plants appeared around 400 million years ago during the Devonian period (409–363 million years ago), and the first land animals about 30 million years later.
