Billions of years ago, a chance combination of chemicals somewhere on Earth’s surface created a substance that could soak up energy and reproduce itself - the first living organism. This was the beginning of the amazing story of life on Earth.
Planet Earth was formed from a cloud of space rock, dust, and gas. For millions of years it was a mass of hot molten rock with a poisonous atmosphere. But eventually its crust cooled to the point where water could form vast oceans. The shallow fringes of these oceans were probably where life began, about 3.8 billion years ago, in a series of chemical reactions that assembled the first living cells.
As soon as life began, it started to change. Living things thrive by making copies of themselves, but the copies are not exact. Over time, the differences generate new forms of life. This process of change, called evolution, has created the diversity of life on Earth.
Every living thing is slightly different from its parents. If the difference helps it to survive, it is likely to pass on the advantage to its own young. This is the basis of evolution. Many years later, it may lead to a change that is large enough for the result to be called a different species.
The main mechanism of evolution is called natural selection. Life in the natural world is a competition, with losers and winners. Those that survive and breed happen to have a combination of qualities that helps them thrive in their habitat. But if conditions change, the winners may turn into losers.
Life has existed on planet Earth for 3.8 billion years. Over that time, it has evolved into a dazzling diversity of forms, from microscopic bacteria to giant dinosaurs.
For most of Earth’s long history, the only living things were single-celled microbes that lived in the oceans. But around 600 million years ago, the first multicellular animals appeared. This led to an evolutionary explosion of complex oceanic life forms during the Cambrian Period. And once the first simple plants had evolved, life was able to colonize the continents as well as the oceans.
The history of life is recorded by fossils preserved in rock layers. Older rocks lie beneath more recent ones, so each layer represents a span of time. Scientists divide this vast stretch of time into eras, and these are further divided into smaller time spans called periods. This geological timescale is the basis of the timeline, measured in millions of years ago (MYA).