History of Earth

• 1. Introduction

  ›

  Earth formed around 4.54 billion years ago, roughly one-third the age of the universe. It began as nothing but debris pulled together from a rotating cloud of dust and gas. Much of the young planet was molten, battered by frequent collisions, its surface a hellish landscape of volcanic activity. There was no oxygen worth speaking of. There was no life. From that violent start to the present day, the planet has never stopped changing. Its crust shifts, its oceans open and close, and the life it carries splits, evolves, and disappears. So how does a ball of molten rock become a living world? Where did its oceans come from, and why did the first breath of oxygen poison most of the life that made it? How did a small African ape come to walk upright and remember its own past? The answers are written in rock, in fossils, and in the slow arithmetic of deep time. This is the story of a planet measured not in years but in millions and billions of them.

• 2. The Clock In The Rocks

  ›

  Geologists measure the past in Ma, a unit standing for one million years ago. Across that enormous span, the history of Earth is divided into four great eons, each marked by the deepest changes in the planet's composition, climate, and life. Each eon breaks down into eras, eras into periods, and periods into epochs.

  The Hadean takes its name from Hades, and the name fits. Running from 4,540 to 4,000 million years ago, it was a time of extreme heat, constant volcanism, and a nebular atmosphere, with no life at all. The Archean followed, from 4,000 to 2,500 million years ago, when the first prokaryotic life emerged through a process called abiogenesis. The Proterozoic, from 2,500 to 538.8 million years ago, means early life, the era when more complex eukaryotes appeared and bacteria began producing oxygen. The current eon is the Phanerozoic, meaning visible life, stretching from 538.8 million years ago to today.

  This framework is the geologic time scale, defined by international convention and split into intervals through stratigraphic analysis of rock layers. It is the spine of everything that follows. The oldest detrital zircon crystals found in rocks date to about 4.4 billion years ago, and those tiny grains hold a secret about water that the next chapter will open.

• 3. Birth From A Spinning Cloud

  ›

  About 4.5 billion years ago, a vast rotating cloud of interstellar dust and gas, called the solar nebula, began to contract. The trigger may have been a shock wave from a nearby supernova, which would also have set the cloud spinning. As it accelerated, the cloud flattened into a protoplanetary disk. Its center, holding little angular momentum, collapsed and heated until hydrogen fused into helium, and a T Tauri star ignited and became the Sun.

  In the outer disk, dust and debris clumped through runaway accretion, with larger and larger fragments gathering into planets. Earth grew this way and was largely complete within 10 to 20 million years. In June 2023, scientists reported evidence that Earth may have come together in just three million years, far faster than once believed. The solar wind of the new star then swept away most of the leftover material that had not yet condensed.

  The proto-Earth kept growing until its interior melted the heavy, iron-loving metals. Denser than the surrounding silicates, these metals sank in an event called the iron catastrophe. Within only 10 million years of Earth's beginning, this separated a metallic core from a primitive mantle and set up the planet's magnetic field. The scientist J.A. Jacobs was first to propose that Earth's solid inner core is still freezing and growing out of the liquid outer core, as the interior cools by about 100 degrees Celsius every billion years.

• 4. A Collision That Made The Moon

  ›

  Earth's only natural satellite is larger relative to its planet than any other moon in the Solar System. During the Apollo program, astronauts carried rocks from the Moon's surface back to Earth, and radiometric dating put the Moon at 4.53 billion years old, give or take 10 million. Newer evidence suggests it formed even later, around 4.48 billion years ago, some 70 to 110 million years after the Solar System began.

  The Moon poses puzzles. Its density is only 3.3 times that of water, against 5.5 for Earth, and it carries a small metallic core. Yet Earth and Moon share the same oxygen isotopic signature. One explanation has won wide acceptance. The giant impact hypothesis holds that a body the size of Mars, sometimes named Theia, struck the proto-Earth a glancing blow.

  That collision released about 100 million times more energy than the later Chicxulub impact linked to the extinction of the non-avian dinosaurs. It was enough to vaporize Earth's outer layers and melt both bodies. Mantle material was flung into orbit, and because it carried little metal, the giant impact hypothesis neatly explains the Moon's odd composition. The ejected debris could have condensed into a single body within a couple of weeks, and under its own gravity it rounded into the Moon we see now.

• 5. Cratons And The First Land

  ›

  During the early Archean, around 3.0 billion years ago, the mantle ran roughly 1600 degrees Celsius hotter than today. Convection moved faster, so plate tectonics worked faster too, with smaller plates and more common subduction zones. The original crust, basaltic like today's ocean floor, vanished entirely, erased by this rapid tectonic churn and the pounding of the Late Heavy Bombardment.

  The first sizable pieces of continental crust appeared at the end of the Hadean, about 4.0 billion years ago, formed from lighter elements that floated upward during partial melting. What remains of these first small continents are called cratons, the ancient cores around which today's continents grew. The oldest rocks on Earth sit in the North American craton of Canada. They are tonalites from about 4.0 billion years ago, bearing both signs of high-temperature metamorphism and sedimentary grains rounded by water, proof that rivers and seas already flowed.

  Cratons hold two alternating kinds of terrane. Greenstone belts are low-grade metamorphosed sediments resembling material found today in oceanic trenches above subduction zones, hinting that subduction had begun in the Archean. The second kind, called TTG-terranes, are complexes of felsic rocks like tonalite, trondhjemite, and granodiorite, seen as relics of the very first continental crust.

• 6. Three Atmospheres And An Ocean

  ›

  Earth is often said to have had three atmospheres. The first was captured straight from the solar nebula, made of light elements like hydrogen and helium, and the solar wind together with Earth's heat drove it off. After the Moon-forming impact, the molten planet released volatile gases, and volcanoes added more, building a second atmosphere thick with greenhouse gases but nearly free of oxygen.

  Many of those volatiles likely arrived during accretion through impact degassing, as incoming bodies vaporized on impact, so ocean and atmosphere began forming as Earth itself formed. Water, though, posed a problem. At Earth's distance of one astronomical unit, the nebula was too hot for ice, so the water had to come from meteorites in the outer asteroid belt beyond 2.5 astronomical units, with comets possibly helping. Computer simulations show comets were once far more common in the inner Solar System than they are now.

  As Earth cooled, clouds gathered and rain filled the oceans, which may have begun forming as early as 4.4 billion years ago. This raises the faint young Sun paradox. The Sun has grown 30 percent brighter since its formation, so the early Earth should have frozen over. The likely fix is a strong greenhouse blanket of carbon dioxide from volcanoes and methane from early microbes, perhaps joined by an organic haze from methane photolysis. The third atmosphere, the oxygen-rich one we breathe, would not arrive until bacteria began producing oxygen about 2.8 billion years ago.

• 7. From Chemistry To The First Cell

  ›

  In 1952, Stanley Miller and Harold Urey ran an experiment that became a landmark. By passing sparks through a mix of water, methane, ammonia, and hydrogen to mimic lightning, they produced organic molecules, including the nucleobases and amino acids that are life's building blocks. Later experiments with more realistic atmospheres did the same, and simulations suggest such molecules could even have formed in the protoplanetary disk before Earth existed.

  Getting from those chemicals to life remains contested, with many models and little consensus. Complexity could have begun from self-replication, from metabolism, or from external cell membranes. The replication-first idea points to RNA. A ribozyme can catalyze both its own copying and the building of proteins, suggesting an RNA world of individuals without species, where mutations and gene transfers gave each generation new genomes. RNA later yielded to the more stable DNA. In 2003, researchers proposed that porous metal sulfide precipitates near hydrothermal vents could help RNA form at about 100 degrees Celsius under ocean-bottom pressure.

  Rival ideas compete. The metabolism-first iron-sulfur world, tested in experiments starting in 1997, showed amino acids and peptides forming with iron sulfide and nickel sulfide as catalysts, again near hydrothermal vents. A membranes-first lipid world proposes that double-walled bubbles came first. The clay theory points to montmorillonite, which self-replicates its crystal pattern and can catalyze RNA. From this crowd of protocells, phylogenetic evidence suggests only one line survived. The last universal ancestor lived in the early Archean, perhaps 3.5 billion years ago, a prokaryote with a membrane and ribosomes but no nucleus, ancestor of everything alive today.

• 8. Oxygen, Ice, And The Rise Of Complex Cells

  ›

  The earliest cells lived by fermentation in an oxygen-free world, until photosynthesis let them draw energy from the Sun. The simpler anoxygenic form arose about 3.8 billion years ago, used by organisms like purple bacteria and green sulfur bacteria that rely on hydrogen sulfide, sulfur, or iron instead of water. Oxygenic photosynthesis, which strips hydrogen from water and leaves oxygen as waste, had certainly appeared by about 2.4 billion years ago, perhaps as far back as 3.2 billion. At first the released oxygen bound to iron, leaving the red bands of banded iron formations laid down during the Siderian period, between 2,500 and 2,300 million years ago.

  When the easy minerals were used up, oxygen built up in the air, forming a high ozone layer that screened ultraviolet radiation and let cells reach the surface and eventually the land. But oxygen was toxic, and much life died in the oxygen catastrophe. Around the same time, falling methane levels weakened the greenhouse, helping trigger the Huronian glaciation, whose glacial deposits in South Africa date to 2.2 billion years ago and once sat near the equator. Later, in the Cryogenian period, came the harshest Snowball Earths, around 716.5 and 635 million years ago, when ice may have reached the tropics and average temperatures fell near minus 50 degrees Celsius.

  Life grew more complex through partnership. Around 2 billion years ago the Neomura split into Archaea and Eukaryota. A bacterium related to Rickettsia entered a larger cell and, instead of being digested, survived to become the mitochondrion. A similar event turned cyanobacteria into chloroplasts. By about 1.1 billion years ago the plant, animal, and fungi lines had split, and the first multicellular plants, probably green algae, appeared around a billion years ago. After the last Snowball Earth near 600 million years ago, the Ediacara biota appeared, larger and more diverse than anything before, with the first muscular and neural cells. None of them yet had hard skeletons.

  In the Cambrian period, the pace of evolution recorded in fossils suddenly quickened. This Cambrian Explosion was an adaptive radiation that filled niches left empty by the vanished Ediacaran biota, and by its end most modern phyla were already present. The development of shells, skeletons, and exoskeletons in molluscs, echinoderms, crinoids, and arthropods made fossilization easier, which is why so much more is known about life from the Cambrian onward. The first vertebrates appeared here, including early fishes, with a creature called Pikaia carrying a primitive notochord. Later, the placoderm Dunkleosteus grew up to 7 meters long. By the late Cambrian, trilobites reached their greatest diversity.

  Life then crawled ashore. The oldest fossils of land fungi and plants date to 480 to 460 million years ago, though molecular evidence pushes fungi back perhaps to 1 billion years and plants to 700 million. The clearest sign of animals on land is arthropods around 450 million years ago. Around 380 to 375 million years ago, the first tetrapods evolved from fish, their fins becoming limbs. The amniotic egg arrived about 340 million years ago, and synapsids split from sauropsids around 310 million years ago. Dinosaurs split from their reptilian ancestors about 230 million years ago, and Archaeopteryx, long considered one of the first birds, lived around 150 million years ago.

  The planet kept rearranging itself. Laurentia and Baltica collided between 450 and 400 million years ago in the Caledonian Orogeny, leaving mountain traces in Scandinavia, Scotland, and the northern Appalachians. The Variscan or Alleghenian Orogeny of the Carboniferous, 359 to 299 million years ago, assembled the last supercontinent, Pangaea, which by 180 million years ago broke into Laurasia and Gondwana. Through all of this ran five great mass extinctions, the deadliest being the Permian-Triassic Great Dying, which killed about 83 percent of all genera.

  In 66 million years ago, a 10 kilometer asteroid struck just off the Yucatan Peninsula, where the Chicxulub crater now lies. The dust and vapor blotted out the Sun and halted photosynthesis, ending 75 percent of all life, including the non-avian dinosaurs, and closing the Mesozoic era. The first true mammals had evolved in the shadow of dinosaurs as tiny, probably nocturnal creatures, and only now did their diversification truly begin. Ambulocetus took to the water on the path to whales, while primates took to the trees.

  The spread of grass reshaped the world. As grasslands opened, mammals grew larger, and giant ungulates like Paraceratherium and Deinotherium ruled them. The first big cats appeared, and grass also brought primates down from the trees, starting human evolution. The formation of Panama, perhaps the most important geological event of the last 60 million years, closed the Atlantic and Pacific currents, created the Gulf Stream that warmed Europe, and let creatures cross between the Americas, bringing llamas, the spectacled bear, kinkajous, and jaguars to South America.

  A small African ape living around 6 million years ago was the last common ancestor of both humans and chimpanzees. Soon after the split, one branch began to walk upright. Brain size grew fast, and by 2 million years ago the first members of the genus Homo had appeared. Control of fire probably began in Homo erectus, perhaps at least 790,000 years ago. Modern Homo sapiens originated around 200,000 years ago or earlier in Africa, with the oldest fossils near 160,000 years old. Neanderthals buried their dead, and by 32,000 years ago Cro-Magnons left cave paintings and figurines like the Venus of Willendorf. By 11,000 years ago, humans had reached the southern tip of South America.

  Between 8500 and 7000 BC, people in the Fertile Crescent of the Middle East began the systematic husbandry of plants and animals, the birth of agriculture. For more than 90 percent of its history, Homo sapiens had lived in small bands as nomadic hunter-gatherers. With language came a new replicator, the meme, and ideas spread faster than genes ever could. Surplus food allowed governing classes and a division of labor, leading to Earth's first civilization at Sumer in the Middle East between 4000 and 3000 BC, soon followed by ancient Egypt, the Indus River valley, and China.

  Writing let complex societies store knowledge and grow. By around 500 BC, advanced civilizations stood in the Middle East, Iran, India, China, and Greece, and in 221 BC China became a single polity that remains the world's most populous nation. Ancient Greece produced the first democratic government, while Ancient Rome advanced law, government, and engineering. The Roman Empire was Christianized by Emperor Constantine in the early 4th century. In 610, Islam was founded, and the House of Wisdom in Abbasid-era Baghdad became a center of the Islamic Golden Age until the Mongol sack of Baghdad in 1258 AD.

  From the 14th century, the Renaissance began in Italy. In 1492, Christopher Columbus reached the Americas. Europe moved through the Scientific Revolution, the Industrial Revolution, and the Enlightenment, and in 1776 the United States declared independence from the British Empire. The 20th century carried humanity from the Wright brothers' first powered flight in 1903 to the Apollo 11 Moon landing in 1969 and the rise of the internet, while the population grew from 1 billion in 1800 to over 8 billion today. Now some scientists argue the planet has entered a sixth mass extinction and a new epoch, the Anthropocene, though both ideas remain disputed.