Story of the Earth
Formation of the Earth Planet Earth is one of eight planets orbiting the Sun, one of billions of stars in the Milky Way Galaxy, itself one of billions of galaxies in the Universe. The Universe came into existence 13.7 billion years ago in an incredibly hot, dense explosion called the Big Bang. A few billion years later our Galaxy began to form, but it was not until about 4600 million years ago that the Sun and its family of planets, including the Earth, made their appearance.
Formation of the Earth
A swirling disc of gas and dust, called a solar nebula, gradually merged into a dense ball of hot gas: the Sun. Spinning around this central furnace, particles of dust began to clump together, first into small fragments of rock, then large boulders. Over millions of years, some grew into blocks several kilometres across, called planetesimals. These, in turn started to collide with one another, building up like snowballs to become the four rocky inner planets, one of which was the Earth.
1. In its early days, Earth was a barren planet, rather like the Moon is today. Unprotected by an atmosphere, it was continually bombarded by meteorites, millions of rocky fragments that careered around the youthful Solar System. These crashed to the ground, gouging out craters.
2. The persistent bombardment may have caused the Earth’s rocky surface to melt: the planet became a global sea of extremely hot, molten rock. Eventually the bombardment eased and the surface cooled. But the newly solidifed surface trapped gases beneath it.
3. The pressure built up and hydrogen, carbon dioxide, water vapour and nitrogen burst through the crust in volcanoes. Thousands of eruptions raged all over the globe. The gases collected to form a new atmosphere around the Earth. Water vapour formed clouds that enveloped the planet.
4. Soon, as the Sun’s intense heat began to cool, rain started to fall. It must have been the longest storm the Earth has ever known. Water poured from the sky for millions of years until the basins in the land filled up, becoming the oceans.
The first life on Earth appeared not on land but in the oceans. The atmosphere had a very different composition from today: it lacked oxygen, but probably contained a lot of methane. Early life-forms had to flourish without oxygen in the atmosphere or dissolved in water. At this point, the Earth’s surface would have been lethal because of high levels of ultraviolet radiation.
Life probably arose about 3800 million years ago, although the earliest fossil evidence we have is 3500 million years old. No one knows how life began, but warm-water pools on seashores or cracks in the ocean floor where hot, mineral-rich water shoots up (hydrothermal vents), would both have been ideal places for it to happen.
Somehow, certain substances must have eventually undergone chemical reactions to become the first building blocks of life. The earliest life-forms were the very simplest kinds: bacteria. The oldest fossils are known as stromatolites, bands of blue-green algae (cyanobacteria) that grew in shallow water. It was another 2500 million years before complex life forms, types of seaweed, first appeared.
Consultant: Ian Fairchild
See also in Prehistoric
See also in Space