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129-567: The Precambrian is an informal unit of geologic time, subdivided into three eons ( Hadean , Archean , Proterozoic ) of the geologic time scale . It spans from the formation of Earth about 4.6 billion years ago ( Ga ) to the beginning of the Cambrian Period, about 538.8 million years ago ( Ma ), when hard-shelled creatures first appeared in abundance. Relatively little is known about the Precambrian, despite it making up roughly seven-eighths of

258-690: A supereon , but this is also an informal term, not defined by the ICS in its chronostratigraphic guide. Eozoic (from eo- "earliest") was a synonym for pre-Cambrian , or more specifically Archean . A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks (Archean Eon) from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia . Probable fossils 100 million years older have been found in

387-667: A "stunning" find of lagerstätten, known as the Qingjiang biota , was reported from the Danshui river in Hubei province, China . More than 20,000 fossil specimens were collected, including many soft bodied animals such as jellyfish, sea anemones and worms, as well as sponges, arthropods and algae. In some specimens the internal body structures were sufficiently preserved that soft tissues, including muscles, gills, mouths, guts and eyes, can be seen. The remains were dated to around 518 Mya and around half of

516-604: A coelom—for example flatworms , whose organs are surrounded by unspecialized tissues . Changes in the abundance and diversity of some types of fossil have been interpreted as evidence for "attacks" by animals or other organisms. Stromatolites, stubby pillars built by colonies of microorganisms , are a major constituent of the fossil record from about 2,700  million years ago , but their abundance and diversity declined steeply after about 1,250  million years ago . This decline has been attributed to disruption by grazing and burrowing animals. Precambrian marine diversity

645-408: A few regions. Relative dating ( A was before B ) is often assumed sufficient for studying processes of evolution, but this, too, has been difficult, because of the problems involved in matching up rocks of the same age across different continents . Therefore, dates or descriptions of sequences of events should be regarded with some caution until better data become available. In 2004, the start of

774-531: A formal proposal to the ICS for the establishment of the Anthropocene Series/Epoch. Nevertheless, the definition of the Anthropocene as a geologic time period rather than a geologic event remains controversial and difficult. An international working group of the ICS on pre-Cryogenian chronostratigraphic subdivision have outlined a template to improve the pre-Cryogenian geologic time scale based on

903-461: A geochronologic unit can be changed (and is more often subject to change) when refined by geochronometry while the equivalent chronostratigraphic unit (the revision of which is less frequent) remains unchanged. For example, in early 2022, the boundary between the Ediacaran and Cambrian periods (geochronologic units) was revised from 541 Ma to 538.8 Ma but the rock definition of the boundary (GSSP) at

1032-484: A head and a tail, the burrowers probably had bilateral symmetry—which would in all probability make them bilaterian animals. They fed above the sediment surface, but were forced to burrow to avoid predators. Trace fossils (burrows, etc.) are a reliable indicator of what life was around, and indicate a diversification of life around the start of the Cambrian, with the freshwater realm colonized by animals almost as quickly as

1161-637: A known geological context. The geological history of Mars has been divided into two alternate time scales. The first time scale for Mars was developed by studying the impact crater densities on the Martian surface. Through this method four periods have been defined, the Pre-Noachian (~4,500–4,100 Ma), Noachian (~4,100–3,700 Ma), Hesperian (~3,700–3,000 Ma), and Amazonian (~3,000 Ma to present). Cambrian explosion The Cambrian explosion (also known as Cambrian radiation or Cambrian diversification )

1290-541: A large (about 30 cm) organism, and would have provided great potential for diversification: Specialised feeding apparatus allows a number of different approaches to feeding and development, and creates a number of different approaches to avoid being eaten. The earliest generally accepted echinoderm fossils appeared in the Late Atdabanian ; unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical. These provide firm data points for

1419-403: A lesser degree, they serve as armor for protection against predators and adverse conditions of environment. Some Cloudina fossils show small holes in shells. The holes possibly are evidence of boring by predators sufficiently advanced to penetrate shells. A possible " evolutionary arms race " between predators and prey is one of the hypotheses that attempt to explain the Cambrian explosion. In

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1548-617: A machine-readable Resource Description Framework / Web Ontology Language representation of the time scale, which is available through the Commission for the Management and Application of Geoscience Information GeoSciML project as a service and at a SPARQL end-point. Some other planets and satellites in the Solar System have sufficiently rigid structures to have preserved records of their own histories, for example, Venus , Mars and

1677-707: A possible 2450 Ma red alga from the Kola Peninsula , 1650 Ma carbonaceous biosignatures in north China, the 1600 Ma Rafatazmia , and a possible 1047 Ma Bangiomorpha red alga from the Canadian Arctic. The earliest fossils widely accepted as complex multicellular organisms date from the Ediacaran Period. A very diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma. These are referred to as Ediacaran or Vendian biota . Hard-shelled creatures appeared toward

1806-471: A relative interval of geologic time. A chronostratigraphic unit is a body of rock, layered or unlayered, that is defined between specified stratigraphic horizons which represent specified intervals of geologic time. They include all rocks representative of a specific interval of geologic time, and only this time span. Eonothem, erathem, system, series, subseries, stage, and substage are the hierarchical chronostratigraphic units. A geochronologic unit

1935-861: A single-celled egg to a larva or juvenile form. The innermost layer forms the digestive tract (gut); the outermost forms skin; and the middle one forms muscles and all the internal organs except the digestive system. Most types of living animal are triploblastic—the best-known exceptions are Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.). The bilaterians are animals that have right and left sides at some point in their life histories. This implies that they have top and bottom surfaces and, importantly, distinct front and back ends. All known bilaterian animals are triploblastic, and all known triploblastic animals are bilaterian. Living echinoderms ( sea stars , sea urchins , sea cucumbers , etc.) 'look' radially symmetrical (like wheels) rather than bilaterian, but their larvae exhibit bilateral symmetry and some of

2064-429: A specific and reliable order. This allows for a correlation of strata even when the horizon between them is not continuous. The geologic time scale is divided into chronostratigraphic units and their corresponding geochronologic units. The subdivisions Early and Late are used as the geochronologic equivalents of the chronostratigraphic Lower and Upper , e.g., Early Triassic Period (geochronologic unit)

2193-463: A successful analysis is a hierarchy of clades —groups whose members are believed to share a common ancestor. The cladistic technique is sometimes problematic, as some features, such as wings or camera eyes , evolved more than once, convergently —this must be taken into account in analyses. From the relationships, it may be possible to constrain the date that lineages first appeared. For instance, if fossils of B or C date to X million years ago and

2322-481: A system/series (early/middle/late); however, the International Commission on Stratigraphy advocates for all new series and subseries to be named for a geographic feature in the vicinity of its stratotype or type locality . The name of stages should also be derived from a geographic feature in the locality of its stratotype or type locality. Informally, the time before the Cambrian is often referred to as

2451-458: A wider sense, correlating strata across national and continental boundaries based on their similarity to each other. Many of the names below erathem/era rank in use on the modern ICC/GTS were determined during the early to mid-19th century. During the 19th century, the debate regarding Earth's age was renewed, with geologists estimating ages based on denudation rates and sedimentary thicknesses or ocean chemistry, and physicists determining ages for

2580-494: Is a numeric-only, chronologic reference point used to define the base of geochronologic units prior to the Cryogenian. These points are arbitrarily defined. They are used where GSSPs have not yet been established. Research is ongoing to define GSSPs for the base of all units that are currently defined by GSSAs. The standard international units of the geologic time scale are published by the International Commission on Stratigraphy on

2709-479: Is a subdivision of geologic time. It is a numeric representation of an intangible property (time). These units are arranged in a hierarchy: eon, era, period, epoch, subepoch, age, and subage. Geochronology is the scientific branch of geology that aims to determine the age of rocks, fossils, and sediments either through absolute (e.g., radiometric dating ) or relative means (e.g., stratigraphic position , paleomagnetism , stable isotope ratios ). Geochronometry

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2838-475: Is a way of representing deep time based on events that have occurred throughout Earth's history , a time span of about 4.54 ± 0.05 Ga (4.54 billion years). It chronologically organises strata, and subsequently time, by observing fundamental changes in stratigraphy that correspond to major geological or paleontological events. For example, the Cretaceous–Paleogene extinction event , marks

2967-713: Is an interval of time beginning approximately 538.8  million years ago in the Cambrian period of the early Paleozoic , when a sudden radiation of complex life occurred and practically all major animal phyla started appearing in the fossil record . It lasted for about 13 to 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well. Before early Cambrian diversification, most organisms were relatively simple, composed of individual cells or small multicellular organisms, occasionally organized into colonies . As

3096-411: Is based on living organisms, it accommodates extinct organisms poorly, if at all. The concept of stem groups was introduced to cover evolutionary "aunts" and "cousins" of living groups, and have been hypothesized based on this scientific theory. A crown group is a group of closely related living animals plus their last common ancestor plus all its descendants. A stem group is a set of offshoots from

3225-407: Is controversial. If it does represent a cnidarian larva, Eolarva would represent the first fossil evidence of indirect development in metazoans in the earliest Cambrian. Medusozoans developed complex life cycles with a medusa stage during the Cambrian explosion, as evidenced by the discovery of Burgessomedusa phasmiformis . The earliest trilobite fossils are about 530 million years old, but

3354-596: Is known to occur during the RNA replication of extant coronaviruses . Evidence of the details of plate motions and other tectonic activity in the Precambrian is difficult to interpret. It is generally believed that small proto-continents existed before 4280 Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia , broke up around 750 Ma. A number of glacial periods have been identified going as far back as

3483-513: Is left behind when the rock is dissolved with hydrofluoric acid . The diversity of this assemblage is similar to that of modern crustacean faunas. Analysis of fragments of feeding machinery found in the formation shows that it was adapted to feed in a very precise and refined fashion. This contrasts with most other early Cambrian arthropods, which fed messily by shovelling anything they could get their feeding appendages on into their mouths. This sophisticated and specialised feeding machinery belonged to

3612-419: Is not a fundamental division of nature, such as the difference between electrons and protons . It is simply a very high-level grouping in a classification system created to describe all currently living organisms. This system is imperfect, even for modern animals: different books quote different numbers of phyla, mainly because they disagree about the classification of a huge number of worm-like species. As it

3741-401: Is the field of geochronology that numerically quantifies geologic time. A Global Boundary Stratotype Section and Point (GSSP) is an internationally agreed-upon reference point on a stratigraphic section that defines the lower boundaries of stages on the geologic time scale. (Recently this has been used to define the base of a system) A Global Standard Stratigraphic Age (GSSA)

3870-663: Is the responsibility of the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS), whose primary objective is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC) that are used to define divisions of geologic time. The chronostratigraphic divisions are in turn used to define geochronologic units. The geologic time scale

3999-512: Is thought that the Earth coalesced from material in orbit around the Sun at roughly 4,543 Ma, and may have been struck by another planet called Theia shortly after it formed, splitting off material that formed the Moon (see Giant-impact hypothesis ). A stable crust was apparently in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma. The term "Precambrian"

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4128-524: Is thought to have formed about 1300-900 Ma, to have included most or all of Earth's continents and to have broken up into eight continents around 750–600 million years ago. Eon (geology) The geologic time scale or geological time scale ( GTS ) is a representation of time based on the rock record of Earth . It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronology (a scientific branch of geology that aims to determine

4257-625: Is used by geologists and paleontologists for general discussions not requiring a more specific eon name. However, both the United States Geological Survey and the International Commission on Stratigraphy regard the term as informal. Because the span of time falling under the Precambrian consists of three eons (the Hadean , the Archean , and the Proterozoic ), it is sometimes described as

4386-573: Is used in place of Lower Triassic System (chronostratigraphic unit). Rocks representing a given chronostratigraphic unit are that chronostratigraphic unit, and the time they were laid down in is the geochronologic unit, e.g., the rocks that represent the Silurian System are the Silurian System and they were deposited during the Silurian Period. This definition means the numeric age of

4515-598: The Anthropocene is a proposed epoch/series for the most recent time in Earth's history. While still informal, it is a widely used term to denote the present geologic time interval, in which many conditions and processes on Earth are profoundly altered by human impact. As of April 2022 the Anthropocene has not been ratified by the ICS; however, in May 2019 the Anthropocene Working Group voted in favour of submitting

4644-529: The Brothers of Purity , who wrote on the processes of stratification over the passage of time in their treatises . Their work likely inspired that of the 11th-century Persian polymath Avicenna (Ibn Sînâ, 980–1037) who wrote in The Book of Healing (1027) on the concept of stratification and superposition, pre-dating Nicolas Steno by more than six centuries. Avicenna also recognised fossils as "petrifications of

4773-518: The Earth's history , and what is known has largely been discovered from the 1960s onwards. The Precambrian fossil record is poorer than that of the succeeding Phanerozoic , and fossils from the Precambrian (e.g. stromatolites ) are of limited biostratigraphic use. This is because many Precambrian rocks have been heavily metamorphosed , obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata. It

4902-568: The Ediacara biota , organisms so large that they are likely multicelled, but very unlike any modern organism. In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian, but as recently as the 1970s, no sign was seen of how the 'relatively' modern-looking organisms of the Middle and Late Cambrian arose. The intense modern interest in this "Cambrian explosion"

5031-520: The Huronian epoch, roughly 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a " Snowball Earth ". The atmosphere of the early Earth is not well understood. Most geologists believe it was composed primarily of nitrogen, carbon dioxide, and other relatively inert gases, and

5160-530: The Precambrian or pre-Cambrian (Supereon). While a modern geological time scale was not formulated until 1911 by Arthur Holmes , the broader concept that rocks and time are related can be traced back to (at least) the philosophers of Ancient Greece . Xenophanes of Colophon (c. 570–487  BCE ) observed rock beds with fossils of shells located above the sea-level, viewed them as once living organisms, and used this to imply an unstable relationship in which

5289-512: The Tommotian has historically been understood to mark an explosive increase of the number and variety of fossils of molluscs, hyoliths and sponges , along with a rich complex of skeletal elements of unknown animals, the first archaeocyathids , brachiopods , tommotiids and others. Also soft-bodied extant phyla such as comb jellies , scalidophorans , entoproctans , horseshoe worms and lobopodians had armored forms. This sudden increase

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5418-482: The oxygen catastrophe . At first, oxygen would have quickly combined with other elements in Earth's crust, primarily iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, and the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides. A terminology has evolved covering

5547-776: The "Lipalian", was not represented in the fossil record or did not preserve fossils, and that the ancestors of the Cambrian animals evolved during this time. Earlier fossil evidence has since been found. The earliest claim is that the history of life on Earth goes back 3,850 million years : Rocks of that age at Warrawoona, Australia , were claimed to contain fossil stromatolites , stubby pillars formed by colonies of microorganisms . Fossils ( Grypania ) of more complex eukaryotic cells, from which all animals, plants and fungi are built, have been found in rocks from 1,400  million years ago , in China and Montana . Rocks dating from 580 to 543 million years ago contain fossils of

5676-860: The "end" of the explosion, or at least indications that the crown groups of modern phyla were represented. Around the start of the Cambrian (about 539  million years ago ), many new types of traces first appear, including well-known vertical burrows such as Diplocraterion and Skolithos , and traces normally attributed to arthropods, such as Cruziana and Rusophycus . The vertical burrows indicate that worm-like animals acquired new behaviours, and possibly new physical capabilities. Some Cambrian trace fossils indicate that their makers possessed hard exoskeletons , although they were not necessarily mineralised. Meiofaunal as well as macrofaunal bilaterians participated in this invasion of infaunal niches. Burrows provide firm evidence of complex organisms; they are also much more readily preserved than body fossils, to

5805-662: The "explosion" looks a lot less sudden than if all these organisms represent an unrelated "experiment", and were replaced by the animal kingdom fairly soon thereafter (40 million years is "soon" by evolutionary and geological standards). The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the Ediacaran period, about 565  million years ago . They were probably made by organisms resembling earthworms in shape, size and how they moved. The burrow-makers have never been found preserved, but, because they would need

5934-499: The Cambrian was dated to 542 Ma . In 2012, it was revised to 541 Ma then in 2022 it was changed again to 538.8 Ma. Some theory suggest Cambrian explosion occurred during the last stages of Gondwanan assembly, which is formed following Rodinia splitting, overlapped with the opening of the Iapetus Ocean between Laurentia and western Gondwana. The largest Cambrian faunal province is located around Gondwana, which extended from

6063-459: The Cambrian. The markers are consistent with a mass extinction, or with a massive warming resulting from the release of methane ice . Such changes may reflect a cause of the Cambrian explosion, although they may also have resulted from an increased level of biological activity—a possible result of the explosion. Despite these uncertainties, the geochemical evidence helps by making scientists focus on theories that are consistent with at least one of

6192-668: The Commission on Stratigraphy (applied in 1965) to become a member commission of IUGS led to the founding of the ICS. One of the primary objectives of the ICS is "the establishment, publication and revision of the ICS International Chronostratigraphic Chart which is the standard, reference global Geological Time Scale to include the ratified Commission decisions". Following on from Holmes, several A Geological Time Scale books were published in 1982, 1989, 2004, 2008, 2012, 2016, and 2020. However, since 2013,

6321-578: The Doushantou Formation in China and from deposits of the same age in Mongolia, although the interpretation of these fossils as spicules has been challenged. In the late Ediacaran-lowest Cambrian, numerous tube dwellings of enigmatic organisms appeared. It was organic-walled tubes (e.g. Saarina ) and chitinous tubes of the sabelliditids (e.g. Sokoloviina , Sabellidites , Paleolina ) that prospered up to

6450-464: The Earth's Moon . Dominantly fluid planets, such as the giant planets , do not comparably preserve their history. Apart from the Late Heavy Bombardment , events on other planets probably had little direct influence on the Earth, and events on Earth had correspondingly little effect on those planets. Construction of a time scale that links the planets is, therefore, of only limited relevance to

6579-515: The Earth's time scale, except in a Solar System context. The existence, timing, and terrestrial effects of the Late Heavy Bombardment are still a matter of debate. The geologic history of Earth's Moon has been divided into a time scale based on geomorphological markers, namely impact cratering , volcanism , and erosion . This process of dividing the Moon's history in this manner means that

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6708-447: The ICS has taken responsibility for producing and distributing the ICC citing the commercial nature, independent creation, and lack of oversight by the ICS on the prior published GTS versions (GTS books prior to 2013) although these versions were published in close association with the ICS. Subsequent Geologic Time Scale books (2016 and 2020 ) are commercial publications with no oversight from

6837-404: The ICS, and do not entirely conform to the chart produced by the ICS. The ICS produced GTS charts are versioned (year/month) beginning at v2013/01. At least one new version is published each year incorporating any changes ratified by the ICS since the prior version. The following five timelines show the geologic time scale to scale. The first shows the entire time from the formation of the Earth to

6966-415: The ICS. While some regional terms are still in use, the table of geologic time conforms to the nomenclature , ages, and colour codes set forth by the International Commission on Stratigraphy in the official International Chronostratigraphic Chart. The International Commission on Stratigraphy also provide an online interactive version of this chart. The interactive version is based on a service delivering

7095-665: The International Chronostratigraphic Chart; however, regional terms are still in use in some areas. The numeric values on the International Chronostratigrahpic Chart are represented by the unit Ma (megaannum, for 'million years '). For example, 201.4 ± 0.2 Ma, the lower boundary of the Jurassic Period, is defined as 201,400,000 years old with an uncertainty of 200,000 years. Other SI prefix units commonly used by geologists are Ga (gigaannum, billion years), and ka (kiloannum, thousand years), with

7224-445: The action of gravity. However, it is now known that not all sedimentary layers are deposited purely horizontally, but this principle is still a useful concept. The principle of lateral continuity that states layers of sediments extend laterally in all directions until either thinning out or being cut off by a different rock layer, i.e. they are laterally continuous. Layers do not extend indefinitely; their limits are controlled by

7353-505: The age of rocks). It is used primarily by Earth scientists (including geologists , paleontologists , geophysicists , geochemists , and paleoclimatologists ) to describe the timing and relationships of events in geologic history. The time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies , paleomagnetic properties, and fossils . The definition of standardised international units of geologic time

7482-409: The amount and type of sediment in a sedimentary basin , and the geometry of that basin. The principle of cross-cutting relationships that states a rock that cuts across another rock must be younger than the rock it cuts across. The law of included fragments that states small fragments of one type of rock that are embedded in a second type of rock must have formed first, and were included when

7611-456: The base of the Cambrian, and thus the boundary between the Ediacaran and Cambrian systems (chronostratigraphic units) has not been changed; rather, the absolute age has merely been refined. Chronostratigraphy is the element of stratigraphy that deals with the relation between rock bodies and the relative measurement of geological time. It is the process where distinct strata between defined stratigraphic horizons are assigned to represent

7740-480: The beginning of the Tommotian . The mineralized tubes of Cloudina , Namacalathus , Sinotubulites and a dozen more of the other organisms from carbonate rocks formed near the end of the Ediacaran period from 549 to 542 million years ago , as well as the triradially symmetrical mineralized tubes of anabaritids (e.g. Anabarites , Cambrotubulus ) from uppermost Ediacaran and lower Cambrian. Ediacaran mineralized tubes are often found in carbonates of

7869-518: The bodies of plants and animals", with the 13th-century Dominican bishop Albertus Magnus (c. 1200–1280) extending this into a theory of a petrifying fluid. These works appeared to have little influence on scholars in Medieval Europe who looked to the Bible to explain the origins of fossils and sea-level changes, often attributing these to the ' Deluge ', including Ristoro d'Arezzo in 1282. It

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7998-400: The body fossils of animals that are thought to have been capable of making them. While exact assignment of trace fossils to their makers is generally impossible, traces may, for example, provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms ). Several chemical markers indicate a drastic change in the environment around the start of

8127-491: The calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago. It is also possible to estimate how long ago two living clades diverged—i.e. about how long ago their last common ancestor must have lived—by assuming that DNA mutations accumulate at a constant rate. These " molecular clocks ", however, are fallible, and provide only a very approximate timing: they are not sufficiently precise and reliable for estimating when

8256-550: The class was already quite diverse and cosmopolitan , suggesting they had been around for quite some time. The fossil record of trilobites began with the appearance of trilobites with mineral exoskeletons—not from the time of their origin. Crustaceans, one of the four great modern groups of arthropods, are very rare throughout the Cambrian. Convincing crustaceans were once thought to be common in Burgess Shale-type biotas, but none of these individuals can be shown to fall into

8385-520: The cooling of the Earth or the Sun using basic thermodynamics or orbital physics. These estimations varied from 15,000 million years to 0.075 million years depending on method and author, but the estimations of Lord Kelvin and Clarence King were held in high regard at the time due to their pre-eminence in physics and geology. All of these early geochronometric determinations would later prove to be incorrect. The discovery of radioactive decay by Henri Becquerel , Marie Curie , and Pierre Curie laid

8514-618: The crown group of "true crustaceans". The Cambrian record of crown-group crustaceans comes from microfossils . The Swedish Orsten horizons contain later Cambrian crustaceans, but only organisms smaller than 2 mm are preserved. This restricts the data set to juveniles and miniaturised adults. A more informative data source is the organic microfossils of the Mount Cap formation , Mackenzie Mountains, Canada. This late Early Cambrian assemblage ( 510 to 515 million years ago ) consists of microscopic fragments of arthropods' cuticle, which

8643-400: The current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs . The Precambrian could be divided into five "natural" eons, characterized as follows: The movement of Earth's plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent containing most or all of

8772-453: The developments in mass spectrometry pioneered by Francis William Aston , Arthur Jeffrey Dempster , and Alfred O. C. Nier during the early to mid- 20th century would finally allow for the accurate determination of radiometric ages, with Holmes publishing several revisions to his geological time-scale with his final version in 1960. The establishment of the IUGS in 1961 and acceptance of

8901-412: The diagnostic features that allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the ediacarans. However, there seems little doubt that Kimberella was at least a triploblastic bilaterian animal. These organisms are central to the debate about how abrupt the Cambrian explosion was. If some were early members of the animal phyla seen today,

9030-404: The different layers of stone unless they had been upon the shore and had been covered over by earth newly thrown up by the sea which then became petrified? And if the above-mentioned Deluge had carried them to these places from the sea, you would find the shells at the edge of one layer of rock only, not at the edge of many where may be counted the winters of the years during which the sea multiplied

9159-673: The earliest echinoderms may have been bilaterally symmetrical. Porifera and Cnidaria are radially symmetrical, not bilaterian, and not triploblastic (but the common Bilateria-Cnidaria ancestor's planula larva is suspected to be bilaterally symmetric). The term Coelomate means having a body cavity (coelom) containing the internal organs. Most of the phyla featured in the debate about the Cambrian explosion are coelomates: arthropods, annelid worms, molluscs, echinoderms and chordates —the noncoelomate priapulids are an important exception. All known coelomate animals are triploblastic bilaterians, but some triploblastic bilaterian animals do not have

9288-460: The early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change. Other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian. Radiometric dates for much of the Cambrian, obtained by analysis of radioactive elements contained within rocks, have only recently become available, and for only

9417-542: The early Cambrian Maotianshan shale beds of Chengjiang ( Yunnan , China ) and Sirius Passet ( Greenland ), the middle Cambrian Burgess Shale ( British Columbia , Canada ) and the late Cambrian Orsten ( Sweden ) fossil beds. While lagerstätten preserve far more than the conventional fossil record, they are far from complete. Because lagerstätten are restricted to a narrow range of environments (where soft-bodied organisms can be preserved very quickly, e.g. by mudslides), most animals are probably not represented; further,

9546-451: The early Cambrian, what might have caused such rapid change, and what it would imply about the origin of animal life. Interpretation is difficult, owing to a limited supply of evidence based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks. The first discovered Cambrian fossils were trilobites , described by Edward Lhuyd , the curator of Oxford Museum , in 1698. Although their evolutionary importance

9675-452: The early years of the Earth's existence, as radiometric dating has allowed absolute dates to be assigned to specific formations and features. The Precambrian is divided into three eons: the Hadean (4567.3–4031 Ma), Archean (4031-2500 Ma) and Proterozoic (2500-538.8 Ma). See Timetable of the Precambrian . It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than

9804-688: The end of that time span, marking the beginning of the Phanerozoic Eon. By the middle of the following Cambrian Period, a very diverse fauna is recorded in the Burgess Shale , including some which may represent stem groups of modern taxa. The increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life. While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic mats that covered terrestrial areas. Tracks from an animal with leg-like appendages have been found in what

9933-477: The exceptional conditions that create lagerstätten probably do not represent normal living conditions. In addition, the known Cambrian lagerstätten are rare and difficult to date, while Precambrian lagerstätten have yet to be studied in detail. The sparseness of the fossil record means that organisms usually exist long before they are found in the fossil record—this is known as the Signor–Lipps effect . In 2019,

10062-519: The extent that the absence of trace fossils has been used to imply the genuine absence of large, motile, bottom-dwelling organisms. They provide a further line of evidence to show that the Cambrian explosion represents a real diversification, and is not a preservational artifact. The first Ediacaran and lowest Cambrian ( Nemakit-Daldynian ) skeletal fossils represent tubes and problematic sponge spicules. The oldest sponge spicules are monaxon siliceous, aged around 580  million years ago , known from

10191-438: The fossil record is very incomplete, increasingly so as earlier times are considered. Despite this, they are often adequate to illustrate the broader patterns of life's history. Also, biases exist in the fossil record: different environments are more favourable to the preservation of different types of organism or parts of organisms. Further, only the parts of organisms that were already mineralised are usually preserved, such as

10320-414: The foundational principles of determining the correlation of strata relative to geologic time. Over the course of the 18th-century geologists realised that: The apparent, earliest formal division of the geologic record with respect to time was introduced during the era of Biblical models by Thomas Burnet who applied a two-fold terminology to mountains by identifying " montes primarii " for rock formed at

10449-458: The geologic time scale of Earth. This table is arranged with the most recent geologic periods at the top, and the oldest at the bottom. The height of each table entry does not correspond to the duration of each subdivision of time. As such, this table is not to scale and does not accurately represent the relative time-spans of each geochronologic unit. While the Phanerozoic Eon looks longer than

10578-492: The ground work for radiometric dating, but the knowledge and tools required for accurate determination of radiometric ages would not be in place until the mid-1950s. Early attempts at determining ages of uranium minerals and rocks by Ernest Rutherford , Bertram Boltwood , Robert Strutt , and Arthur Holmes, would culminate in what are considered the first international geological time scales by Holmes in 1911 and 1913. The discovery of isotopes in 1913 by Frederick Soddy , and

10707-579: The groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques vary by a factor of two. However, the clocks can give an indication of branching rate, and when combined with the constraints of the fossil record, recent clocks suggest a sustained period of diversification through the Ediacaran and Cambrian. A phylum is the highest level in the Linnaean system for classifying organisms . Phyla can be thought of as groupings of animals based on general body plan. Despite

10836-403: The imperfections of the fossil record. In the sixth edition of his book, he stressed his problem further as: To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer. American paleontologist Charles Walcott , who studied the Burgess Shale fauna , proposed that an interval of time,

10965-539: The landmass. The earliest known supercontinent was Vaalbara . It formed from proto-continents and was a supercontinent 3.636 billion years ago. Vaalbara broke up c. 2.845–2.803 Ga ago. The supercontinent Kenorland was formed c. 2.72 Ga ago and then broke sometime after 2.45–2.1 Ga into the proto-continent cratons called Laurentia , Baltica , Yilgarn craton and Kalahari . The supercontinent Columbia , or Nuna, formed 2.1–1.8 billion years ago and broke up about 1.3–1.2 billion years ago. The supercontinent Rodinia

11094-506: The last 1 billion years may indicate an increased need for defence against predation. Other groups of small organisms from the Neoproterozoic era also show signs of antipredator defenses. A consideration of taxon longevity appears to support an increase in predation pressure around this time. In general, the fossil record shows a very slow appearance of these lifeforms in the Precambrian, with many cyanobacterial species making up much of

11223-913: The latter often represented in calibrated units ( before present ). The names of geologic time units are defined for chronostratigraphic units with the corresponding geochronologic unit sharing the same name with a change to the suffix (e.g. Phanerozoic Eonothem becomes the Phanerozoic Eon). Names of erathems in the Phanerozoic were chosen to reflect major changes in the history of life on Earth: Paleozoic (old life), Mesozoic (middle life), and Cenozoic (new life). Names of systems are diverse in origin, with some indicating chronologic position (e.g., Paleogene), while others are named for lithology (e.g., Cretaceous), geography (e.g., Permian ), or are tribal (e.g., Ordovician ) in origin. Most currently recognised series and subseries are named for their position within

11352-561: The layers of sand and mud brought down by the neighboring rivers and spread them over its shores. And if you wish to say that there must have been many deluges in order to produce these layers and the shells among them it would then become necessary for you to affirm that such a deluge took place every year. These views of da Vinci remained unpublished, and thus lacked influence at the time; however, questions of fossils and their significance were pursued and, while views against Genesis were not readily accepted and dissent from religious doctrine

11481-474: The likely environmental changes. Cladistics is a technique for working out the "family tree" of a set of organisms. It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characteristics that are compared may be anatomical , such as the presence of a notochord , or molecular , by comparing sequences of DNA or protein . The result of

11610-424: The lineage at a point earlier than the last common ancestor of the crown group; it is a relative concept, for example tardigrades are living animals that form a crown group in their own right, but Budd (1996) regarded them as also being a stem group relative to the arthropods. The term Triploblastic means consisting of three layers, which are formed in the embryo , quite early in the animal's development from

11739-537: The litho- and biostratigraphic differences around the world in time equivalent rocks. The ICS has long worked to reconcile conflicting terminology by standardising globally significant and identifiable stratigraphic horizons that can be used to define the lower boundaries of chronostratigraphic units. Defining chronostratigraphic units in such a manner allows for the use of global, standardised nomenclature. The International Chronostratigraphic Chart represents this ongoing effort. Several key principles are used to determine

11868-507: The low northern latitudes to the high southern latitudes, just short of the South Pole. By the middle and later parts of the Cambrian, continued rifting had sent the paleocontinents of Laurentia, Baltica and Siberia on their separate ways. Fossils of organisms' bodies are usually the most informative type of evidence. Fossilization is a rare event, and most fossils are destroyed by erosion or metamorphism before they can be observed. Hence,

11997-607: The lower boundary of the Paleogene System/Period and thus the boundary between the Cretaceous and Paleogene systems/periods. For divisions prior to the Cryogenian , arbitrary numeric boundary definitions ( Global Standard Stratigraphic Ages , GSSAs) are used to divide geologic time. Proposals have been made to better reconcile these divisions with the rock record. Historically, regional geologic time scales were used due to

12126-482: The lowest Cambrian, the stromatolites were decimated. This allowed animals to begin colonization of warm-water pools with carbonate sedimentation. At first, it was anabaritids and Protohertzina (the fossilized grasping spines of chaetognaths ) fossils. Such mineral skeletons as shells, sclerites, thorns and plates appeared in uppermost Nemakit-Daldynian ; they were the earliest species of halkierids , gastropods , hyoliths and other rare organisms. The beginning of

12255-420: The matter into the public eye and raised questions about what the explosion represented. While differing significantly in details, both Whittington and Gould proposed that all modern animal phyla had appeared almost simultaneously in a rather short span of geological period. This view led to the modernization of Darwin's tree of life and the theory of punctuated equilibrium , which Eldredge and Gould developed in

12384-635: The oceans. Fossils known as " small shelly fauna " have been found in many parts on the world, and date from just before the Cambrian to about 10 million years after the start of the Cambrian (the Nemakit-Daldynian and Tommotian ages; see timeline ). These are a very mixed collection of fossils: spines, sclerites (armor plates), tubes, archeocyathids (sponge-like animals) and small shells very like those of brachiopods and snail-like molluscs—but all tiny, mostly 1 to 2 mm long. While small, these fossils are far more common than complete fossils of

12513-419: The organisms that produced them; crucially, they cover the window from the start of the Cambrian to the first lagerstätten: a period of time otherwise lacking in fossils. Hence, they supplement the conventional fossil record and allow the fossil ranges of many groups to be extended. The first cnidarian larvae, represented by the genus Eolarva , appeared in the Cambrian, although the identity of Eolarva as such

12642-408: The pertinent time span. As of April 2022 these proposed changes have not been accepted by the ICS. The proposed changes (changes from the current scale [v2023/09]) are italicised: Proposed pre-Cambrian timeline (GTS2012), shown to scale: Current ICC pre-Cambrian timeline (v2023/09), shown to scale: The following table summarises the major events and characteristics of the divisions making up

12771-452: The present, but this gives little space for the most recent eon. The second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, the most recent period is expanded in the fourth timeline, and the most recent epoch is expanded in the fifth timeline. Horizontal scale is Millions of years (above timelines) / Thousands of years (below timeline) First suggested in 2000,

12900-485: The principles of superposition, original horizontality, lateral continuity, and cross-cutting relationships. From this Steno reasoned that strata were laid down in succession and inferred relative time (in Steno's belief, time from Creation ). While Steno's principles were simple and attracted much attention, applying them proved challenging. These basic principles, albeit with improved and more nuanced interpretations, still form

13029-485: The rate of diversification subsequently accelerated, the variety of life became much more complex and began to resemble that of today. Almost all present-day animal phyla appeared during this period, including the earliest chordates . The seemingly rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the 1840s, and in his 1859 book On the Origin of Species , Charles Darwin discussed

13158-521: The relative relationships of rocks and thus their chronostratigraphic position. The law of superposition that states that in undeformed stratigraphic sequences the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface. In practice, this means a younger rock will lie on top of an older rock unless there is evidence to suggest otherwise. The principle of original horizontality that states layers of sediments will originally be deposited horizontally under

13287-467: The rest, it merely spans ~539 million years (~12% of Earth's history), whilst the previous three eons collectively span ~3,461 million years (~76% of Earth's history). This bias toward the most recent eon is in part due to the relative lack of information about events that occurred during the first three eons compared to the current eon (the Phanerozoic). The use of subseries/subepochs has been ratified by

13416-630: The rock record to bring it in line with the post-Tonian geologic time scale. This work assessed the geologic history of the currently defined eons and eras of the pre-Cambrian, and the proposals in the "Geological Time Scale" books 2004, 2012, and 2020. Their recommend revisions of the pre-Cryogenian geologic time scale were (changes from the current scale [v2023/09] are italicised): Proposed pre-Cambrian timeline (Shield et al. 2021, ICS working group on pre-Cryogenian chronostratigraphy), shown to scale: Current ICC pre-Cambrian timeline (v2023/09), shown to scale: The book, Geologic Time Scale 2012,

13545-520: The same area. However, there is evidence that life could have evolved over 4.280 billion years ago. There is a fairly solid record of bacterial life throughout the remainder (Proterozoic Eon) of the Precambrian. Complex multicellular organisms may have appeared as early as 2100 Ma. However, the interpretation of ancient fossils is problematic, and "... some definitions of multicellularity encompass everything from simple bacterial colonies to badgers." Other possible early complex multicellular organisms include

13674-474: The sea had at times transgressed over the land and at other times had regressed . This view was shared by a few of Xenophanes's contemporaries and those that followed, including Aristotle (384–322 BCE) who (with additional observations) reasoned that the positions of land and sea had changed over long periods of time. The concept of deep time was also recognised by Chinese naturalist Shen Kuo (1031–1095) and Islamic scientist -philosophers, notably

13803-544: The second rock was forming. The relationships of unconformities which are geologic features representing a gap in the geologic record. Unconformities are formed during periods of erosion or non-deposition, indicating non-continuous sediment deposition. Observing the type and relationships of unconformities in strata allows geologist to understand the relative timing the strata. The principle of faunal succession (where applicable) that states rock strata contain distinctive sets of fossils that succeed each other vertically in

13932-472: The seemingly different external appearances of organisms, they are classified into phyla based on their internal and developmental organizations. For example, despite their obvious differences, spiders and barnacles both belong to the phylum Arthropoda, but earthworms and tapeworms , although similar in shape, belong to different phyla. As chemical and genetic testing becomes more accurate, previously hypothesised phyla are often entirely reworked. A phylum

14061-564: The shells of molluscs . Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although 30-plus phyla of living animals are known, two-thirds have never been found as fossils. The Cambrian fossil record includes an unusually high number of lagerstätten , which preserve soft tissues. These allow paleontologists to examine the internal anatomy of animals, which in other sediments are only represented by shells, spines, claws, etc.—if they are preserved at all. The most significant Cambrian lagerstätten are

14190-443: The species identified at the time of reporting were previously unknown. Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces ) and marks left by feeding. Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily fossilized hard parts, and reflects organisms' behaviour. Also, many traces date from significantly earlier than

14319-418: The start of the Cambrian. Most of this "Ediacara biota" were at least a few centimeters long, significantly larger than any earlier fossils. The organisms form three distinct assemblages, increasing in size and complexity as time progressed. Many of these organisms were quite unlike anything that appeared before or since, resembling discs, mud-filled bags, or quilted mattresses—one paleontologist proposed that

14448-421: The strangest organisms should be classified as a separate kingdom , Vendozoa. At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, having been interpreted as early molluscs ( Kimberella ), echinoderms ( Arkarua ) and arthropods ( Spriggina , Parvancorina , Yilingia ). Still, debate exists about the classification of these specimens, mainly because

14577-453: The stromatolite reefs and thrombolites , i.e. they could live in an environment adverse to the majority of animals. Although they are as hard to classify as most other Ediacaran organisms, they are important in two other ways. First, they are the earliest known calcifying organisms (organisms that built shells from calcium carbonate ). Secondly, these tubes are a device to rise over a substrate and competitors for effective feeding and, to

14706-411: The then-inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection . The long-running puzzlement about the seemingly-sudden appearance of the Cambrian fauna without evident precursor(s) centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period during

14835-537: The time during which the rocks were laid down, and the collection of rocks themselves (i.e., it was correct to say Tertiary rocks, and Tertiary Period). Only the Quaternary division is retained in the modern geologic time scale, while the Tertiary division was in use until the early 21st century. The Neptunism and Plutonism theories would compete into the early 19th century with a key driver for resolution of this debate being

14964-730: The time of the 'Deluge', and younger " monticulos secundarios" formed later from the debris of the " primarii" . Anton Moro (1687–1784) also used primary and secondary divisions for rock units but his mechanism was volcanic. In this early version of the Plutonism theory, the interior of Earth was seen as hot, and this drove the creation of primary igneous and metamorphic rocks and secondary rocks formed contorted and fossiliferous sediments. These primary and secondary divisions were expanded on by Giovanni Targioni Tozzetti (1712–1783) and Giovanni Arduino (1713–1795) to include tertiary and quaternary divisions. These divisions were used to describe both

15093-562: The time scale boundaries do not imply fundamental changes in geological processes, unlike Earth's geologic time scale. Five geologic systems/periods ( Pre-Nectarian , Nectarian , Imbrian , Eratosthenian , Copernican ), with the Imbrian divided into two series/epochs (Early and Late) were defined in the latest Lunar geologic time scale. The Moon is unique in the Solar System in that it is the only other body from which humans have rock samples with

15222-513: The underlying sediment. At the start of the Ediacaran period, much of the acritarch fauna, which had remained relatively unchanged for hundreds of millions of years, became extinct, to be replaced with a range of new, larger species, which would prove far more ephemeral. This radiation, the first in the fossil record, is followed soon after by an array of unfamiliar, large fossils dubbed the Ediacara biota, which flourished for 40 million years until

15351-526: The work of James Hutton (1726–1797), in particular his Theory of the Earth , first presented before the Royal Society of Edinburgh in 1785. Hutton's theory would later become known as uniformitarianism , popularised by John Playfair (1748–1819) and later Charles Lyell (1797–1875) in his Principles of Geology . Their theories strongly contested the 6,000 year age of the Earth as suggested determined by James Ussher via Biblical chronology that

15480-429: Was accepted at the time by western religion. Instead, using geological evidence, they contested Earth to be much older, cementing the concept of deep time. During the early 19th century William Smith , Georges Cuvier , Jean d'Omalius d'Halloy , and Alexandre Brongniart pioneered the systematic division of rocks by stratigraphy and fossil assemblages. These geologists began to use the local names given to rock units in

15609-478: Was dominated by small fossils known as acritarchs . This term describes almost any small organic-walled fossil—from the egg cases of small metazoans to resting cysts of many different kinds of green algae . After appearing around 2,000  million years ago , acritarchs underwent a boom around 1,000  million years ago , increasing in abundance, diversity, size, complexity of shape, and especially size and number of spines. Their increasingly spiny forms in

15738-415: Was in some places unwise, scholars such as Girolamo Fracastoro shared da Vinci's views, and found the attribution of fossils to the 'Deluge' absurd. Niels Stensen, more commonly known as Nicolas Steno (1638–1686), is credited with establishing four of the guiding principles of stratigraphy. In De solido intra solidum naturaliter contento dissertationis prodromus Steno states: Respectively, these are

15867-481: Was lacking in free oxygen . There is, however, evidence that an oxygen-rich atmosphere existed since the early Archean. At present, it is still believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism . This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis , sometimes called

15996-841: Was mud 551 million years ago. The RNA world hypothesis asserts that RNA evolved before coded proteins and DNA genomes. During the Hadean Eon (4,567–4,031 Ma) abundant geothermal microenvironments were present that may have had the potential to support the synthesis and replication of RNA and thus possibly the evolution of a primitive life form. It was shown that porous rock systems comprising heated air-water interfaces could allow ribozyme - catalyzed RNA replication of sense and antisense strands that could be followed by strand-dissociation, thus enabling combined synthesis, release and folding of active ribozymes. This primitive RNA replicative system also may have been able to undergo template strand switching during replication ( genetic recombination ) as

16125-468: Was not known, on the basis of their old age, William Buckland (1784–1856) realized that a dramatic step-change in the fossil record had occurred around the base of what we now call the Cambrian. Nineteenth-century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata, specifically for establishing the Cambrian and Silurian periods. By 1859, leading geologists including Roderick Murchison were convinced that what

16254-535: Was not until the Italian Renaissance when Leonardo da Vinci (1452–1519) would reinvigorate the relationships between stratification, relative sea-level change, and time, denouncing attribution of fossils to the 'Deluge': Of the stupidity and ignorance of those who imagine that these creatures were carried to such places distant from the sea by the Deluge...Why do we find so many fragments and whole shells between

16383-713: Was sparked by the work of Harry B. Whittington and colleagues, who, in the 1970s, reanalysed many fossils from the Burgess Shale and concluded that several were as complex as, but different from, any living animals. The most common organism, Marrella , was clearly an arthropod , but not a member of any known arthropod class . Organisms such as the five-eyed Opabinia and spiny slug-like Wiwaxia were so different from anything else known that Whittington's team assumed they must represent different phyla, seemingly unrelated to anything known today. Stephen Jay Gould 's popular 1989 account of this work, Wonderful Life , brought

16512-476: Was the last commercial publication of an international chronostratigraphic chart that was closely associated with the ICS. It included a proposal to substantially revise the pre-Cryogenian time scale to reflect important events such as the formation of the Solar System and the Great Oxidation Event , among others, while at the same time maintaining most of the previous chronostratigraphic nomenclature for

16641-513: Was then called the lowest Silurian stratum showed the origin of life on Earth, though others, including Charles Lyell , differed. In On the Origin of Species , Darwin considered this sudden appearance of a solitary group of trilobites, with no apparent antecedents, and absent other fossils, to be "undoubtedly of the gravest nature" among the difficulties in his theory of natural selection. He reasoned that earlier seas had swarmed with living creatures, but that their fossils had not been found because of

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