From molten rock to living planet: Earth's first billion years
Earth took shape about 4.54 billion years ago, roughly a third of the universe's age, and spent its youth molten, battered and nearly oxygen-free. Yet zircon crystals 4.4 billion years old already show contact with liquid water, and recognizable humans have existed for at most 2 million years of that span.
The Solar System began as a rotating cloud of gas and dust. About 4.5 billion years ago it started contracting, perhaps nudged by the shock wave of a nearby supernova, and flattened into a disk. Its dense centre heated until hydrogen fusion ignited and a young T Tauri star grew into the Sun. Farther out, dust and debris clumped into ever larger fragments through runaway accretion. Earth was largely assembled within 10 to 20 million years, though evidence reported in June 2023 suggests it may have taken only three million.
As the young planet heated, dense metals melted and sank through the lighter silicates. This iron catastrophe split Earth into a mantle and a metallic core within roughly 10 million years, setting up the magnetic field. Geophysicist J.A. Jacobs later proposed that the solid inner core is still freezing out of the liquid outer core as the interior cools by about 100 degrees Celsius per billion years.
Then came the Moon. The widely accepted giant impact hypothesis holds that a Mars-sized body, sometimes called Theia, struck the proto-Earth a glancing blow, releasing roughly 100 million times the energy of the Chicxulub impact that doomed the non-avian dinosaurs. Mantle debris flung into orbit may have gathered into a single body within weeks. That explains why the Moon has a small core, a density just 3.3 times that of water against Earth's 5.5, and the same oxygen isotope signature as Earth. Apollo samples date it to about 4.53 billion years, though newer work suggests 4.48.
The Hadean eon ended about 4.03 billion years ago. A spell of heavy meteorite strikes, the Late Heavy Bombardment, ran from about 4.1 to 3.8 billion years ago, and together with fast early plate tectonics it erased the original basaltic crust entirely. Small continents appeared near 4.0 billion years ago; their surviving remnants, cratons, are the cores around which today's continents grew. Stromatolites in 3.48-billion-year-old Western Australian sandstone are among the earliest firm signs of life, and photosynthesizers began adding oxygen to the air between 3.2 and 2.4 billion years ago.
Source: History of Earth
