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The Earth is thought to have been formed about 4.5 billion years ago by collisions in the giant disc-shaped cloud of material that also formed the Sun. Gravity slowly gathered this gas and dust together into clumps that became asteroids and small early planets called planetesimals.
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Recent work has suggested that deep-sea hydrothermal vents, such as those found along mid-ocean spreading centers today, may have been the cradle of Earth's life. These environments contain the chemicals and the source of energy needed to synthesize more complex organic structures. Although scientists have not succeeded in creating life from organic molecules in the laboratory, they have reproduced many of the intermediate steps.
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Approximately 3.5 billion years ago There were tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (known as chloroplasts) to do their photosynthesis for them down to this day.
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Some of the earliest known single-celled eukaryote fossils are acritarchs, which become conspicuous at about 2.1 billion years ago. In fact, acritarchs are the most common fossils of the late Proterozoic.
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More complex forms of life took longer to evolve, with the first multicellular animals not appearing until about 600 million years ago.
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The third largest extinction in Earth's history, the Ordovician-Silurian mass extinction had two peak dying times separated by hundreds of thousands of years. During the Ordovician, most life was in the sea, so it was sea creatures such as trilobites, brachiopods and graptolites that were drastically reduced in number.
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Three quarters of all species on Earth died out in the Late Devonian mass extinction, though it may have been a series of extinctions over several million years, rather than a single event. Life in the shallow seas were the worst affected, and reefs took a hammering, not returning to their former glory until new types of coral evolved over 100 million years later.
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The Permian mass extinction has been nicknamed The Great Dying, since a staggering 96% of species died out. All life on Earth today is descended from the 4% of species that survived.
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The most recent of a series of supercontinents on Earth, known as Pangea, formed about 270 million years ago. At this time most of the dry land on Earth was joined into one huge landmass that covered nearly a third of the planet's surface.
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The second major phase in the break-up of Pangaea began in the Early Cretaceous (150–140 Ma), when the minor supercontinent of Gondwana separated into multiple continents (Africa, South America, India, Antarctica, and Australia). The fist consisted of the break up of North America, Asia, and Europe around 200 million years ago
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During the final 18 million years of the Triassic period, there were two or three phases of extinction whose combined effects created the Triassic-Jurassic mass extinction event. Climate change, flood basalt eruptions and an asteroid impact have all been blamed for this loss of life.
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The Cretaceous-Tertiary mass extinction - also known as the K/T extinction - is famed for the death of the dinosaurs. However, many other organisms perished at the end of the Cretaceous including the ammonites, many flowering plants and the last of the pterosaurs.
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So far, the earliest finds of modern Homo sapiens skeletons come from Africa. They date to nearly 200,000 years ago on that continent. They appear in Southwest Asia around 100,000 years ago and elsewhere in the Old World by 60,000-40,000 years ago.
