This timeline of Earth's history summarizes significant geological and biological events from the formation of the Earth to the arrival of modern humans. Times are listed in millions of years, or megaanni (Ma).
Dating of the geologic record The geologic record is the strata (layers) of rock in the planet's crust and the science of geology is much concerned with the age and origin of all rocks to determine the history and formation of Earth and to understand the forces that have acted upon it. Geologic time is the timescale used to calculate dates in the planet's geologic history from its origin (currently estimated to have been some 4,600 million years ago) to the present day.
Radiometric dating measures the steady decay of radioactive elements in an object to determine its age. It is used to calculate dates for the older part of the planet's geological record. The theory is very complicated but, in essence, the radioactive elements within an object decay to form isotopes of each chemical element. Isotopes are atoms of the element that differ in mass but share the same general properties. Geologists are most interested in the decay of isotopes carbon-14 (into nitrogen-14) and potassium-40 (into argon-40). Carbon-14 aka radiocarbon dating works for organic materials that are less than about 50,000 years old. For older periods, the potassium-argon dating process is more accurate.
Radiocarbon dating is carried out by measuring how much of the carbon-14 and nitrogen-14 isotopes are found in a material. The ratio between the two is used to estimate the material's age. Suitable materials include wood, charcoal, paper, fabrics, fossils and shells. It is assumed that rock exists in layers according to age, with older beds below later ones. This is the basis of stratigraphy.
The ages of more recent layers are calculated primarily by the study of fossils, which are remains of ancient life preserved in the rock. These occur consistently and so a theory is feasible. Most of the boundaries in recent geologic time coincide with extinctions (e.g., the dinosaurs) and with the appearances of new species (e.g., hominids).
The earliest Solar System
In the earliest Solar System history, the Sun, the planetesimals and the giant planets were formed. The inner Solar System aggregated more slowly than the outer, so the terrestrial planets, including Earth and Moon, formed later than the outer ones. c. 4,570 Ma – A supernova explosion that caused a shock wave in a dense region of the Milky Way galaxy (known as the primal supernova) seeds our galactic neighborhood with heavy elements that will be incorporated into the Earth. The Ca-Al-rich inclusions, which formed 2 million years before chondrules,Amelin, Yuri, Alexander N. Krot, Ian D. Hutcheon, & Alexander A. Ulyanov, "Lead Isotopic Ages of Chondrules and Calcium-Aluminum-Rich Inclusions" (Science, 6 September 2002: Vol. 297. no. 5587, pp. 1678–83) are a key signature of a supernova explosion. c. 4,567 ±3 Ma – the rapid collapse of a hydrogen molecular cloud formed a third-generation Population I star, the Sun, in a region of the Galactic Habitable Zone (GHZ), about 25,000 light years from the center of the Milky Way Galaxy.According to isotopicAges , the Ca-Al-I's (= Ca-Al-rich inclusions) here formed in a proplyd (= protoplanetary disk]). c. 4,566 ±2 Ma – A protoplanetary disc (from which Earth eventually forms) emerges around the young Sun, which is in its T Tauri stage. c. 4,560–4,550 Ma – Proto-Earth forms at the outer (cooler) edge of the habitable zone of the Solar System. At this stage the solar constant of the Sun was only about 73% of its current value, but liquid water may have existed on the surface of the Proto-Earth, probably due to the greenhouse warming of high levels of methane and carbon dioxide present in the atmosphere. Early bombardment phase begins: because the solar neighbourhood is rife with large planetoids and debris, Earth experiences a number of giant impacts that help to increase its overall size.
Precambrian Supereon
c. 4,533 Ma – The Precambrian (to c. 539 Ma) "supereon," formerly an era, is split into three geological time intervals called eons: Hadean, Archaean and Proterozoic. The latter two are sub-divided into several eras as currently defined. In total, the Precambrian comprises some 85% of geological time from the formation of Earth to the time when creatures first developed exoskeletons (i.e., hard outer parts) that left abundant fossil remains.
Hadean Eon
c. 4,533 Ma – The Hadean Eon, a Precambrian Supereon and unofficial Cryptic era, start as the Earth–Moon system forms, possibly as a result of a glancing collision between proto-Earth and the hypothetical protoplanet Theia (the Earth was considerably smaller than before this impact). This impact vaporized a large amount of the crust and sent material into orbit around Earth, which lingered as rings, similar to those of Saturn, for a few million years, until they coalesced to become the Moon. The Moon geology pre-Nectarian period starts. Earth was covered by a magmatic ocean deep caused by the energetic impact from this and other planetesimals during the early bombardment phase and energy released by the formation of the planetary core.
Outgassing from crustal rocks gives Earth a reducing atmosphere of methane, nitrogen, hydrogen, ammonia, and water vapour, with lesser amounts of hydrogen sulfide, carbon monoxide, and carbon dioxide. The amounts of nitrogen and ammonia eventually decline and comparable amounts of methane, carbon monoxide, carbon dioxide, water vapour, and hydrogen continue to be released by crustal rocks. c. 4,500 Ma – The Sun enters main sequence: a solar wind sweeps the Earth-Moon system clear of debris (mainly dust and gas). This is the end of the Early Bombardment Phase and the Basin Groups Era begins on Earth. c. 4,450 Ma – 100 million years after the Moon formed, the first lunar crust, formed of lunar anorthosite, differentiates from lower magmas. The earliest Earth crust probably forms in like manner out of similar materials. On Earth, the pluvial period starts, in which the Earth's crust cools enough to let oceans form. c. 4,404 Ma – The first known mineral is found at Jack Hills in Western Australia. Detrital zircons demonstrate the presence of a solid crust and liquid water. This is the latest possible date for a secondary atmosphere to form that is produced by the Earth's crust outgassing, which is reinforced by water and possibly organic molecules delivered by comet impacts and carbonaceous chondrites (including type CI shown to be high in a number of amino acids and polycyclic aromatic hydrocarbons (PAH)). c. 4,300 Ma – The Nectarian Era begins on Earth. c. 4,250 Ma – The Earliest evidence for life appears as unusually high amounts of light isotopes of carbon, a common sign of life, are found in Earth's oldest mineral deposits located in the Jack Hills of Western Australia. c. 4,100 Ma – The Early Imbrian Era begins on Earth. A Late Heavy Bombardment of the Moon (and probably of the Earth) by bolides and asteroids is produced, possibly by the planetary migration of Neptune into the Kuiper belt as a result of orbital resonances between Jupiter and Saturn.Taylor, G. Jeffrey (2006), "Wandering Gas Giants and Lunar Bombardment: Outward migration of Saturn might have triggered a dramatic increase in the bombardment rate on the Moon 3.9 billion years ago, an idea testable with lunar samples" "Remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. Early edition, published online before print. According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."
Archean Eon
Eoarchean Era
c. 4,031 Ma – Archean Eon and Eoarchean Era start. Possible first appearance of plate tectonic activity in the Earth's crust as plate structures may have begun appearing. Possible beginning of Napier Mountains Orogeny forces of faulting and folding create first metamorphic rocks. Origins of life. c. 4,030 Ma – Acasta Gneiss of Northwest Territories, Canada, first known oldest rock, or aggregate of minerals. c. 3,930 Ma – Possible stabilization of Canadian Shield begins c. 3,920–3,850 Ma – Final phase of Late Heavy Bombardment c. 3,850 Ma – Greenland apatite shows evidence of 12C enrichment, characteristic of the presence of photosynthetic life.Mojzis, S, et al. (1996), "Evidence for Life on Earth before 3800 million years ago", (Nature, 384) c. 3,850 Ma – Evidence of life: Akilia Island graphite off Western Greenland contains evidence of kerogen, of a type consistent with photosynthesis. c. 3,800 Ma – Oldest banded iron formations found.Czaja, Andrew D.; Johnson, Clark M.; Beard, Brian L.; Roden, Eric E.; Li, Weiqiang; Moorbath, Stephen (February 2013). "Biological Fe oxidation controlled deposition of banded iron formation in the ca. 3770Ma Isua Supracrustal Belt (West Greenland)". Earth and Planetary Science Letters. 363: 192–203. Bibcode:2013E&PSL.363..192C. doi:10.1016/j.epsl.2012.12.025. First complete continental masses or cratons, formed of granite blocks, appear on Earth. Occurrence of initial felsic igneous activity on eastern edge of Antarctic craton as first great continental mass begins to coalesce. East European Craton begins to formfirst rocks of the Ukrainian Shield and Voronezh Massif are laid down c. 3,750 Ma – Nuvvuagittuq Greenstone Belt forms c. 3,700 Ma – Graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland Stabilization of Kaapval craton begins: old tonaltic gneisses laid down
Paleoarchean Era
c. 3,600 Ma – Paleoarchean Era starts. Possible assembly of the Vaalbara supercontinent; oldest cratons on Earth (such as the Canadian Shield, East European Craton and Kaapval) begin growing as a result of crustal disturbances along continents coalescing into VaalbaraPilbara Craton stabilizes. Formation of Barberton greenstone belt: Makhonjwa Mountains uplifts on the eastern edge of Kaapval craton, oldest mountains in Africaarea called the "genesis of life" for exceptional preservation of fossils. Narryer gneiss terrane stabilizes: these gneisses become the "bedrock" for the formation of the Yilgarn craton in Australianoted for the survival of the Jack Hills where the oldest mineral, a zircon was uncovered. c. 3,500 Ma – Lifetime of the Last universal ancestor: split between bacteria and archaea occurs as "tree of life" begins branching outvarieties of Eubacteria begin to radiate out globally. Fossils resembling cyanobacteria, found at Warrawoona, Western Australia. c. 3,480 Ma – Fossils of microbial mat found in 3.48 billion-year-old sandstone discovered in Western Australia. First appearance of stromatolitic organisms that grow at interfaces between different types of material, mostly on submerged or moist surfaces. c. 3,460 Ma – Fossils of bacteria in chert. Zimbabwe Craton stabilizes from the suture of two smaller crustal blocks, the Tokwe Segment to the south and the Rhodesdale Segment or Rhodesdale gneiss to the north. c. 3.400 Ma – Eleven taxa of prokaryotes are preserved in the Apex Chert of the Pilbara craton in Australia. Because chert is fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrious material, it preserves small fossils quite well. Stabilization of Baltic Shield begins. c. 3.340 Ma – Johannesburg Dome forms in South Africa: located in the central part of Kaapvaal craton and consists of trondhjemitic and tonalitic granitic rocks intruded into mafic-ultramafic greenstonethe oldest granitoid phase recognised so far. c. 3,300 Ma – Onset of compressional tectonics. Intrusion of granitic plutons on the Kaapvaal craton.