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TheCambrian explosion (also known asCambrian radiation[1] orCambrian diversification) is an interval of time beginning approximately538.8 million years ago in theCambrian period of the earlyPaleozoic, when a suddenradiation ofcomplex life occurred and practically all majoranimalphyla started appearing in thefossil record.[2][3][4] It lasted for about 13[5][6][7] to 25[8][9] million years and resulted in thedivergence of most modernmetazoan phyla.[10] The event was accompanied by major diversification in other groups of organisms as well.[a]
Before early Cambrian diversification,[b] most organisms were relatively simple, composed of individual cells or small multicellular organisms, occasionally organized intocolonies. As the rate of diversification subsequently accelerated, the variety of life became much more complex and began to resemble that of today.[12] Almost all present-day animal phyla appeared during this period,[13][14] including theearliest chordates.[15]
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−590 — – −580 — – −570 — – −560 — – −550 — – −540 — – −530 — – −520 — – −510 — – −500 — – −490 — – | "Stage 2" "Stage 3" "Stage 4" "Stage 10" * * * * * * * * * * * * * * * * * * * * * * * * * Baykonur glaciation * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * |
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−4500 — – — – −4000 — – — – −3500 — – — – −3000 — – — – −2500 — – — – −2000 — – — – −1500 — – — – −1000 — – — – −500 — – — – 0 — |
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The seemingly rapid appearance of fossils in the "Primordial Strata" was noted byWilliam Buckland in the 1840s.[16]Charles Darwin in his 1859 bookOn the Origin of Species discussed the then inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification throughnatural selection.[17] The long-running puzzlement about the seemingly sudden appearance of the Cambrianfauna 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 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 weretrilobites, described byEdward Lhuyd, the curator of theOxford Museum, in 1698.[18] Although their evolutionary importance 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.[16] Nineteenth-century geologists such asAdam Sedgwick andRoderick Murchison used the fossils for dating rock strata, specifically for establishing theCambrian andSilurian periods.[19] By 1859, leading geologists including Roderick Murchison were convinced that what was then called the lowest Silurian stratum showed the origin of life on Earth, though others, includingCharles Lyell, differed. InOn 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 the imperfections of the fossil record.[17] In the sixth edition of his book, he stressed his problem further as:[20]
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.
The American paleontologistCharles Walcott, who studied theBurgess Shale fauna, proposed that an interval of time, 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.[21]
Earlier fossil evidence has since been found. The earliest claim is that the history of life on Earth goes back3,850 million years:[22] Rocks of that age atWarrawoona, Australia, were claimed to contain fossilstromatolites, stubby pillars formed by colonies ofmicroorganisms. Fossils (Grypania) of more complexeukaryotic cells, from which all animals, plants and fungi are built, have been found in rocks from1,400 million years ago, inChina andMontana. Rocks dating from580 to 543 million years ago contain fossils of theEdiacaran biota, organisms so large that they are likely multicelled, but very unlike any modern organism.[23] In 1948,Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian,[24] but as recently as the 1970s, no sign was seen of how the 'relatively' modern-looking organisms of the Middle and LateCambrian arose.[23]
The intense modern interest in this "Cambrian explosion" was sparked by the work ofHarry 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.[25][26] The most common organism,Marrella, was clearly anarthropod, but not a member of any known arthropodclass. Organisms such as the five-eyedOpabinia and spiny slug-likeWiwaxia 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,[27] brought 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 ofpunctuated equilibrium, whichEldredge and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change.[28]
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.[29][30][31]
Radiometric dates for much of the Cambrian, obtained by analysis of radioactive elements contained within rocks, have only recently become available, and for only a few regions.
Relative dating (A was beforeB) 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 differentcontinents.[32]
Therefore, dates or descriptions of sequences of events should be regarded with some caution until better data become available. In 2004, the start of the Cambrian was dated to 542Ma.[33] In 2012, it was revised to 541 Ma[34] then in 2022 it was changed again to 538.8 Ma.[2]
Some theory suggest Cambrian explosion occurred during the last stages ofGondwanan assembly, which is formed followingRodinia splitting, overlapped with the opening of theIapetus Ocean betweenLaurentia and western Gondwana.[35][36] The largest Cambrian faunal province is located around Gondwana, which extended from 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 andSiberia on their separate ways.[37]
Fossils of organisms' bodies are usually the most informative type of evidence. Fossilization is a rare event, and most fossils are destroyed byerosion ormetamorphism before they can be observed. Hence, the fossil record is very incomplete, increasingly so as earlier times are considered. Despite this, it is often adequate to illustrate the broader patterns of life's history.[38] Also, biases exist in the fossil record: different environments are more favourable to the preservation of different types of organism or parts of organisms.[39] Further, only the parts of organisms that were alreadymineralised are usually preserved, such as the shells ofmolluscs. 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.[23]
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The Cambrian fossil record includes an unusually high number oflagerstätten, which preserve soft tissues. These allowpaleontologists 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 the early CambrianMaotianshan shale beds of Chengjiang (Yunnan,China) andSirius Passet (Greenland),[40] the middle CambrianBurgess Shale (British Columbia,Canada)[41] and the late CambrianOrsten (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, the exceptional conditions that create lagerstätten probably do not represent normal living conditions.[42] In addition, the known Cambrian lagerstätten are rare and difficult to date, whilePrecambrian 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 theSignor–Lipps effect.[43]
In 2019, a "stunning" find of lagerstätten, known as theQingjiang biota, was reported from theDanshui river inHubei 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 the species identified at the time of reporting were previously unknown.[44][45][46]
Trace fossils consist mainly of tracks and burrows, but also includecoprolites (fossilfeces) and marks left by feeding.[47][48] 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 the body fossils of animals that are thought to have been capable of making them.[49] 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 toearthworms).[48]
Severalchemical markers indicate a drastic change in the environment around the start of the Cambrian. The markers are consistent with a mass extinction,[50][51] or with a massive warming resulting from the release ofmethane ice.[52]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.[52] Despite these uncertainties, the geochemical evidence helps by making scientists focus on theories that are consistent with at least one of 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 beanatomical, such as the presence of anotochord, ormolecular, by comparing sequences ofDNA orprotein. The result of a successful analysis is a hierarchy ofclades—groups whose members are believed to share a common ancestor. The cladistic technique is sometimes problematic, as some features, such as wings orcamera 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 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 DNAmutations 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 the groups that feature in the Cambrian explosion first evolved,[53] and estimates produced by different techniques vary by a factor of two.[54] 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.[55]
Aphylum is the highest level in theLinnaean system for classifying organisms. Phyla can be thought of as groupings of animals based on general body plan.[57] Despite the seemingly different external appearances of organisms, they are classified into phyla based on their internal and developmental organizations.[58] For example, despite their obvious differences,spiders andbarnacles both belong to the phylum Arthropoda, butearthworms andtapeworms, 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 is not a fundamental division of nature, such as the difference betweenelectrons andprotons. It is simply a very high-level grouping in aclassification 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 is based on living organisms, it accommodates extinct organisms poorly, if at all.[23][59]
The concept ofstem groups was introduced to cover evolutionary "aunts" and "cousins" of living groups, and have been hypothesized based on this scientific theory. Acrown 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 the lineage at a point earlier than the last common ancestor of the crown group; it is a relative concept, for exampletardigrades 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.[56][60]
The termTriploblastic means consisting of three layers, which are formed in theembryo, quite early in the animal's development from a single-celled egg to a larva or juvenile form. The innermost layer forms thedigestive 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 arePorifera (sponges) andCnidaria (jellyfish, sea anemones, etc.).
Thebilaterians 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. Livingechinoderms (sea stars,sea urchins,sea cucumbers, etc.) 'look' radially symmetrical (like wheels) rather than bilaterian, but their larvae exhibit bilateral symmetry and some of the earliest echinoderms may have been bilaterally symmetrical.[61] Porifera and Cnidaria are radially symmetrical, not bilaterian, and not triploblastic (but the common Bilateria-Cnidaria ancestor'splanula larva is suspected to be bilaterally symmetric).
The termCoelomate means having a body cavity (coelom) containing the internal organs. Most of the phyla featured in the debate about the Cambrian explosion[clarification needed] are coelomates: arthropods,annelid worms, molluscs, echinoderms andchordates—the noncoelomatepriapulids are an important exception. All known coelomate animals are triploblastic bilaterians, but some triploblastic bilaterian animals do not have a coelom—for exampleflatworms, whose organs are surrounded byunspecialized 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 ofmicroorganisms, are a major constituent of the fossil record from about2,700 million years ago, but their abundance and diversity declined steeply after about1,250 million years ago. This decline has been attributed to disruption by grazing and burrowing animals.[29][30][62]
Precambrian marine diversity was dominated by small fossils known asacritarchs. This term describes almost any small organic-walled fossil—from the egg cases of smallmetazoans to restingcysts of many different kinds ofgreen algae. After appearing around2,000 million years ago, acritarchs underwent a boom around1,000 million years ago, increasing in abundance, diversity, size, complexity of shape, and especially size and number of spines. Their increasingly spiny forms in the last 1 billion years may indicate an increased need for defence against predation. Other groups of small organisms from theNeoproterozoic era also show signs of antipredator defenses.[62] A consideration of taxon longevity appears to support an increase in predation pressure around this time.[63]
In general, the fossil record shows a very slow appearance of these lifeforms in the Precambrian, with manycyanobacterial species making up much of the underlying sediment.[64]
At the start of the Ediacaran period, much of theacritarch 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.[64] This radiation, the first in the fossil record,[64] is followed soon after by an array of unfamiliar, large fossils dubbed the Ediacara biota,[65] which flourished for 40 million years until the start of the Cambrian.[66] 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.[67]
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 the strangest organisms should be classified as a separatekingdom, Vendozoa.[68]
At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate,[clarification needed] having been interpreted as early molluscs (Kimberella),[31][69] echinoderms (Arkarua)[70] and arthropods (Spriggina,[71]Parvancorina,[72]Yilingia). Still, debate exists about the classification of these specimens, mainly because the diagnostic features that allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the ediacarans.[73] However, there seems little doubt thatKimberella was at least a triploblastic bilaterian animal.[73] These organisms are central to the debate about how abrupt the Cambrian explosion was.[citation needed] If some were early members of the animal phyla seen today, 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, about565 million years ago.[c] They were probably made by organisms resemblingearthworms in shape, size and how they moved. The burrow-makers have never been found preserved, but, because they would need a head and a tail, the burrowers probably had bilateral symmetry—which would in all probability make them bilaterian animals.[76] They fed above the sediment surface, but were forced to burrow to avoid predators.[77]
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 the oceans.[78]
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 (theNemakit-Daldynian andTommotian ages; seetimeline). These are a very mixed collection of fossils: spines,sclerites (armor plates), tubes,archeocyathids (sponge-like animals) and small shells very like those ofbrachiopods and snail-like molluscs—but all tiny, mostly 1 to 2 mm long.[79]
_Marjum_biota.png)
While small, these fossils are far more common than complete fossils of 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 genusEolarva, appeared in the Cambrian, although the identity ofEolarva as such 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.[80]
Medusozoans developed complex life cycles with a medusa stage during the Cambrian explosion, as evidenced by the discovery ofBurgessomedusa phasmiformis.[81]
The earliesttrilobite fossils are about 530 million years old, but the class was already quite diverse andcosmopolitan, suggesting they had been around for quite some time.[82]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. Convincingcrustaceans were once thought to be common in Burgess Shale-type biotas, but none of these individuals can be shown to fall into the crown group of "true crustaceans".[83] The Cambrian record of crown-group crustaceans comes frommicrofossils. The SwedishOrsten 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 theMount Cap formation, Mackenzie Mountains, Canada. This late Early Cambrian assemblage (510 to 515 million years ago) consists of microscopic fragments of arthropods' cuticle, which is left behind when the rock is dissolved withhydrofluoric 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 a large (about 30 cm)[84] 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.[83]
The earliest generally accepted echinoderm fossils appeared in the LateAtdabanian; unlike modern echinoderms, these early Cambrian echinoderms were not all radially symmetrical.[85] These provide firm data points for the "end" of the explosion, or at least indications that the crown groups of modern phyla were represented.
Around the start of the Cambrian (about539 million years ago), many new types of traces first appear, including well-known vertical burrows such asDiplocraterion andSkolithos, and traces normally attributed to arthropods, such asCruziana andRusophycus. 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 hardexoskeletons, although they were not necessarily mineralised.[75] Meiofaunal as well as macrofaunal bilaterians participated in this invasion of infaunal niches.[86]
Burrows provide firm evidence of complex organisms; they are also much more readily preserved than body fossils, to the extent that the absence of trace fossils has been used to imply the genuine absence of large, motile, bottom-dwelling organisms.[citation needed] They provide a further line of evidence to show that the Cambrian explosion represents a real diversification, and is not a preservational artefact.[87]
The first Ediacaran and lowest Cambrian (Nemakit-Daldynian) skeletal fossils represent tubes and problematic sponge spicules.[88] The oldest sponge spicules are monaxon siliceous, aged around580 million years ago, known from theDoushantuo Formation in China and from deposits of the same age inMongolia, although the interpretation of these fossils as spicules has been challenged.[89] 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)[90][91] that prospered up to the beginning of theTommotian. The mineralized tubes ofCloudina,Namacalathus,Sinotubulites and a dozen more of the other organisms from carbonate rocks formed near the end of the Ediacaran period from549 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.[92] Ediacaran mineralized tubes are often found in carbonates of the stromatolite reefs andthrombolites,[93][94] 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 fromcalcium carbonate).[94][95][96] Secondly, these tubes are a device to rise over a substrate and competitors for effective feeding and, to a lesser degree, they serve as armor for protection against predators and adverse conditions of environment. SomeCloudina fossils show small holes in shells. The holes possibly are evidence of boring by predators sufficiently advanced to penetrate shells.[97] A possible "evolutionary arms race" between predators and prey is one of the hypotheses that attempt to explain the Cambrian explosion.[62]
In the lowest Cambrian, the stromatolites were decimated. This allowed animals to begin colonization of warm-water pools with carbonate sedimentation[citation needed]. At first, it wasanabaritids andProtohertzina (the fossilized grasping spines ofchaetognaths) fossils. Such mineral skeletons as shells, sclerites, thorns and plates appeared in uppermostNemakit-Daldynian; they were the earliest species ofhalkierids,gastropods,hyoliths and other rare organisms. The beginning of theTommotian has historically been understood to mark an explosive increase of the number and variety of fossils of molluscs,hyoliths andsponges, along with a rich complex of skeletal elements of unknown animals, the firstarchaeocyathids,brachiopods,tommotiids and others.[98][99][100][101] Alsosoft-bodied extant phyla such ascomb jellies,scalidophorans,entoproctans,horseshoe worms andlobopodians had armored forms.[102] This sudden increase is partially an artefact of missing strata at the Tommotian-type section, and most of this fauna in fact began to diversify in a series of pulses through the Nemakit-Daldynian and into the Tommotian.[103]
Some animals may already have had sclerites, thorns, and plates in the Ediacaran (e.g.Kimberella had hard sclerites, probably of carbonate), but thin carbonate skeletons cannot be fossilized insiliciclastic deposits.[104] Older (~750 Ma) fossils indicate that mineralization long preceded the Cambrian, probably defending small photosynthetic algae from single-celled eukaryotic predators.[105][106]
The Burgess Shale and similar lagerstätten preserve the soft parts of organisms, which provide a wealth of data to aid in the classification of enigmatic fossils. It often preserved complete specimens of organisms only otherwise known from dispersed parts, such as loose scales or isolated mouthparts. Further, the majority of organisms and taxa in these horizons are entirely soft-bodied, hence absent from the rest of the fossil record.[107] Since a large part of the ecosystem is preserved, the ecology of the community can also be tentatively reconstructed.[verification needed]However, the assemblages may represent a "museum": a deep-water ecosystem that is evolutionarily "behind" the rapidly diversifying fauna of shallower waters.[108]
Because the lagerstätten provide a mode and quality of preservation that is virtually absent outside of the Cambrian, many organisms appear completely different from anything known from the conventional fossil record. This led early workers in the field to attempt to shoehorn the organisms into extant phyla; the shortcomings of this approach led later workers to erect a multitude of new phyla to accommodate all the oddballs. It has since been realised that most oddballs diverged fromlineages before they established the phyla known today[clarification needed]—slightly different designs, which were fated to perish rather than flourish into phyla, as their cousin lineages did.
The preservational mode is rare in the preceding Ediacaran period, but those assemblages known show no trace of animal life—perhaps implying a genuine absence of macroscopic metazoans.[109]
The early Cambrian interval of diversification lasted for about the next 20[6][7]–25[8][9] million years, and its elevated rates of evolution had ended by the base ofCambrian Series 2,521 million years ago, coincident with the first trilobites in the fossil record.[110] Different authors define intervals of diversification during the early Cambrian different ways:
Ed Landing recognizes three stages: Stage 1, spanning the Ediacaran-Cambrian boundary, corresponds to a diversification of biomineralizing animals and of deep and complex burrows; Stage 2, corresponding to the radiation of molluscs and stem-groupBrachiopods (hyoliths andtommotiids), which apparently arose in intertidal waters; and Stage 3, seeing the Atdabanian diversification of trilobites in deeper waters, but little change in the intertidal realm.[111]
Graham Budd synthesises various schemes to produce a compatible view of the SSF record of the Cambrian explosion, divided slightly differently into four intervals: a "Tube world", lasting from550 to 536 million years ago, spanning the Ediacaran-Cambrian boundary, dominated by Cloudina, Namacalathus and pseudoconodont-type elements; a "Sclerite world", seeing the rise of halkieriids, tommotiids and hyoliths, lasting to the end of the Fortunian (c. 525 Ma); a brachiopod world, perhaps corresponding to the as yet unratified Cambrian Stage 2; and Trilobite World, kicking off in Stage 3.[112]
Complementary to the shelly fossil record, trace fossils can be divided into five subdivisions: "Flat world" (late Ediacaran), with traces restricted to the sediment surface; Protreozoic III (after Jensen), with increasing complexity;pedum world, initiated at the base of the Cambrian with the base of theT.pedum zone (seeCambrian#Dating the Cambrian);Rusophycus world, spanning536 to 521 million years ago and thus corresponding exactly to the periods of Sclerite World and Brachiopod World under the SSF paradigm; andCruziana world, with an obvious correspondence to Trilobite World.[112]
There is strong evidence for species ofCnidaria andPorifera existing in theEdiacaran[113] and possible members of Porifera even before that during theCryogenian.[114]Bryozoans, once thought to not appear in the fossil record until after the Cambrian, are now known from strata of Cambrian Age 3 from Australia and South China.[115]
The fossil record as Darwin knew it seemed to suggest that the major metazoan groups appeared in a few million years of the early to mid-Cambrian, and even in the 1980s, this still appeared to be the case.[26][27]
However, evidence of Precambrian Metazoa is gradually accumulating. If the EdiacaranKimberella was a mollusc-likeprotostome (one of the two main groups ofcoelomates),[31][69] the protostome anddeuterostome lineages must have split significantly before550 million years ago (deuterostomes are the other main group of coelomates).[116] Even if it is not a protostome, it is widely accepted as a bilaterian.[73][116] Since fossils of rather modern-lookingcnidarians (jellyfish-like organisms) have been found in theDoushantuolagerstätte, the cnidarian and bilaterian lineages must have diverged well over580 million years ago.[116]
Trace fossils[67] and predatory borings inCloudina shells provide further evidence of Ediacaran animals.[117] Some fossils from the Doushantuo formation have been interpreted as embryos and one (Vernanimalcula) as a bilaterian coelomate, although these interpretations are not universally accepted.[118][119][120] Earlier still, predatory pressure has acted on stromatolites and acritarchs since around1,250 million years ago.[62]
Some say that the evolutionary change was accelerated by anorder of magnitude,[d] but the presence of Precambrian animals somewhat dampens the "bang" of the explosion; not only was the appearance of animals gradual, but theirevolutionary radiation ("diversification") may also not have been as rapid as once thought. Indeed, statistical analysis shows that the Cambrian explosion was no faster than any of the other radiations in animals' history.[e] However, it does seem that some innovations linked to the explosion—such as resistant armour—only evolved once in the animal lineage; this makes a lengthy Precambrian animal lineage harder to defend.[122] Further, the conventional view that all the phyla arose in the Cambrian is flawed; while the phyla may have diversified in this time period, representatives of the crown groups of many phyla do not appear until much later in the Phanerozoic.[13] Further, the mineralised phyla that form the basis of the fossil record may not be representative of other phyla, since most mineralised phyla originated in abenthic setting. The fossil record is consistent with a Cambrian explosion that was limited to the benthos, with pelagic phyla evolving much later.[13]
Ecological complexity among marine animals increased in the Cambrian, as well later in the Ordovician.[12] However, recent research has overthrown the once-popular idea that disparity was exceptionally high throughout the Cambrian, before subsequently decreasing.[123] In fact, disparity remains relatively low throughout the Cambrian, with modern levels of disparity only attained after the early Ordovician radiation.[12]
The diversity of many Cambrian assemblages is similar to today's,[83][124] and at a high (class/phylum) level, diversity is thought by some to have risen relatively smoothly through the Cambrian, stabilizing somewhat in the Ordovician.[125] This interpretation, however, glosses over the astonishing and fundamental pattern of basalpolytomy and phylogenetic telescoping at or near the Cambrian boundary, as seen in most major animal lineages.[126] ThusHarry Blackmore Whittington's questions regarding the abrupt nature of the Cambrian explosion remain, and have yet to be satisfactorily answered.[127]
Budd and Mann[128] suggested that the Cambrian explosion was the result of a type ofsurvivorship bias called the "Push of the past". As groups at their origin tend to go extinct, it follows that any long-lived group would have experienced an unusually rapid rate of diversification early on, creating the illusion of a general speed-up in diversification rates. However, rates of diversification could remain at background levels and still generate this sort of effect in the surviving lineages.
Despite the evidence that moderately complex animals (triploblasticbilaterians) existed before and possibly long before the start of the Cambrian, it seems that the pace of evolution was exceptionally fast in the early Cambrian. Possible explanations for this fall into three broad categories: environmental, developmental and ecological changes. Any explanation must explain both the timing and magnitude of the explosion.
Earth's earliest atmosphere contained no freeoxygen (O2); the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years ofphotosynthesis.Cyanobacteria were the first organisms to evolve the ability to photosynthesize, introducing a steady supply of oxygen into the environment.[129] Initially, oxygen levels did not increase substantially in the atmosphere.[130] The oxygen quickly reacted with iron and other minerals in the surrounding rock and ocean water. Once a saturation point was reached for the reactions in rock and water, oxygen was able to exist as a gas in itsdiatomic form. Oxygen levels in the atmosphere increased substantially afterward.[131] As a general trend, theconcentration of oxygen in the atmosphere has risen gradually over about the last 2.5 billion years.[23]
Oxygen levels seem to have a positive correlation with diversity ineukaryotes well before the Cambrian period.[132] Thelast common ancestor of all extant eukaryotes is thought to have lived around 1.8 billion years ago. Around 800 million years ago, there was a notable increase in the complexity and number of eukaryotes species in the fossil record.[132] Before the spike in diversity, eukaryotes are thought to have lived in highly sulfuric environments.Sulfide interferes withmitochondrial function inaerobic organisms, limiting the amount of oxygen that could be used to drive metabolism. Oceanic sulfide levels decreased around 800 million years ago, which supports the importance of oxygen in eukaryotic diversity.[132] The increased ventilation of the oceans by sponges, which had already evolved and diversified during the late Neoproterozoic, has been proposed to have increased the availability of oxygen and powered the Cambrian's rapid diversification of multicellular life.[133][134]Molybdenum isotopes show that increases in biodiversity were strongly correlated with expansion of oxygenated bottom waters in the Early Cambrian, lending support for oxygen as a driver of the Cambrian evolutionary radiation.[135]
The shortage of oxygen might well have prevented the rise of large, complex animals. The amount of oxygen an animal can absorb is largely determined by the area of its oxygen-absorbing surfaces (lungs and gills in the most complex animals; the skin in less complex ones), while the amount needed is determined by its volume, which grows faster than the oxygen-absorbing area if an animal's size increases equally in all directions. An increase in the concentration of oxygen in air or water would increase the size to which an organism could grow without its tissues becoming starved of oxygen. However, members of the Ediacara biota reached metres in length tens of millions of years before the Cambrian explosion.[50] Other metabolic functions may have been inhibited by lack of oxygen, for example the construction of tissue such ascollagen, which is required for the construction of complex structures,[136] or the biosynthesis of molecules for the construction of a hard exoskeleton.[137] However, animals were not affected when similar oceanographic conditions occurred in the Phanerozoic; therefore, some see no forcing role of the oxygen level on evolution.[138]
The amount ofozone (O3) required to shield Earth from biologically lethalUV radiation, wavelengths from 200 to 300 nanometers (nm), is believed to have been in existence around the Cambrian explosion.[139] The presence of theozone layer may have enabled the development of complex life and life on land, as opposed to life being restricted to the water.
In the lateNeoproterozoic (extending into the earlyEdiacaran period), the Earth sufferedmassive glaciations in which most of its surface was covered by ice. This may have caused a mass extinction, creating a genetic bottleneck; the resulting diversification may have given rise to theEdiacara biota, which appears soon after the last "Snowball Earth" episode.[140]However, the snowball episodes occurred a long time before the start of the Cambrian, and it is difficult to see how so much diversity could have been caused by even a series of bottlenecks;[52] the cold periods may even havedelayed the evolution of large size organisms.[62] Massive rock erosion caused by glaciers during the "Snowball Earth" may have deposited nutrient-rich sediments into the oceans, setting the stage for the Cambrian explosion.[141]
Newer research suggests that volcanically activemid-ocean ridges caused a massive and sudden surge of thecalcium concentration in the oceans, making it possible for marine organisms to build skeletons and hard body parts.[142]Alternatively a high influx of ions could have been provided by the widespread erosion that produced Powell'sGreat Unconformity.[143]
An increase of calcium may also have been caused by erosion of theTransgondwanan Supermountain that existed at the time of the explosion. The roots of the mountain are preserved in present-dayEast Africa as anorogen.[144]
A range of theories are based on the concept that minormodifications to animals' development as they grow fromembryo to adult may have been able to cause very large changes in the final adult form. TheHox genes, for example, control which organs individual regions of an embryo will develop into. For instance, if a certainHox gene is expressed, a region will develop into a limb; if a different Hox gene is expressed in that region (a minor change), it could develop into an eye instead (a phenotypically major change).
Such a system allows a large range of disparity to appear from a limited set of genes, but such theories linking this with the explosion struggle to explain why the origin of such a development system should by itself lead to increased diversity or disparity. Evidence of Precambrian metazoans[52] combines with molecular data[145] to show that much of the genetic architecture that could feasibly have played a role in the explosion was already well established by the Cambrian.
This apparent paradox is addressed in a theory that focuses on thephysics of development. It is proposed that the emergence of simple multicellular forms provided a changed context and spatial scale in which novel physical processes and effects were mobilized by the products of genes that had previously evolved to serve unicellular functions. Morphological complexity (layers, segments,lumens, appendages) arose, in this view, byself-organization.[146]
Horizontal gene transfer has also been identified as a possible factor in the rapid acquisition of the biochemical capability of biomineralization among organisms during this period, based on evidence that the gene for a critical protein in the process was originally transferred from a bacterium into sponges.[147]
These focus on the interactions between different types of organism. Some of these hypotheses deal with changes in thefood chain; some suggestarms races between predators and prey, and others focus on the more general mechanisms ofcoevolution. Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity, but they do not explain why the "explosion" happened when it did.[52]
Evidence for such an extinction includes the disappearance from the fossil record of the Ediacara biota and shelly fossils such asCloudina, and the accompanying perturbation in theδ13C record. It is suspected that several globalanoxic events were responsible for the extinction.[148][149]
Mass extinctions are often followed byadaptive radiations as existing clades expand to occupy the ecospace emptied by the extinction. However, once the dust had settled, overall disparity and diversity returned to the pre-extinction level in each of the Phanerozoic extinctions.[52]
The late Ediacaran oceans appears to have suffered from ananoxia that covered much of the seafloor, which would have given mobile animals with the ability to seek out more oxygen-rich environments an advantage over sessile forms of life.[150]
Andrew Parker has proposed that predator-prey relationships changed dramatically after eyesight evolved. Prior to that time, hunting and evading were both close-range affairs—smell (chemoreception), vibration and touch were the only senses used. When predators could see their prey from a distance, new defensive strategies were needed. Armor, spines and similar defenses may also have evolved in response to vision. He further observed that, where animals lose vision in unlighted environments such as caves, diversity of animal forms tends to decrease.[151] Nevertheless, many scientists doubt that vision could have caused the explosion. Eyes may well have evolved long before the start of the Cambrian.[152] It is also difficult to understand why the evolution of eyesight would have caused an explosion, since other senses, such as smell and pressure detection, can detect things at a greater distance in the sea than sight can, but the appearance of these other senses apparently did not cause an evolutionary explosion.[52]
One hypothesis posits that the development of increased cognitive abilities during the Cambrian drove diversity increase. This is evidenced by the fact that the novel ecological lifestyles created during the Cambrian required rapid, regular movement, a feature associated with brain-bearing organisms. The increasing complexity of brains, positively correlated with a greater range of motion and sensory abilities, enabled a wider range of novel ecological modes of life to come into being.[153]
The ability to avoid or recover frompredation often makes the difference between life and death, and is therefore one of the strongest components ofnatural selection. The pressure to adapt is stronger on the prey than on the predator in the sense that, if the predator fails to win a contest, it loses a meal; if the prey is the loser, it loses its life.[154]
But, there is evidence that predation was rife long before the start of the Cambrian, for example in the increasingly spiny forms ofacritarchs, the holes drilled inCloudina shells, and traces of burrowing to avoid predators. Hence, it is unlikely that theappearance of predation was the trigger for the Cambrian "explosion", although it may well have exhibited a strong influence on the body forms that the "explosion" produced.[62] However, the intensity of predation does appear to have increased dramatically during the Cambrian[155] as new predatory "tactics" (such as shell-crushing) emerged.[156] This rise of predation during the Cambrian was confirmed by the temporal pattern of the median predator ratio at the scale of genus, in fossil communities covering the Cambrian and Ordovician periods, but this pattern is not correlated to diversification rate.[157] This lack of correlation between predator ratio and diversification over the Cambrian and Ordovician suggests that predators did not trigger the large evolutionary radiation of animals during this interval. Thus the role of predators as triggerers of diversification may have been limited to the very beginning of the "Cambrian explosion".[157]
Geochemical evidence strongly indicates that the total mass ofplankton has been similar to modern levels since early in the Proterozoic. Before the start of the Cambrian, their corpses and droppings were too small to fall quickly towards the seabed, since theirdrag was about the same as their weight. This meant they were destroyed byscavengers or by chemical processes before they reached the sea floor.[42]
Mesozooplankton are plankton of a larger size. Early Cambrian specimensfiltered microscopic plankton from the seawater. These larger organisms would have produced droppings and ultimately corpses large enough to fall fairly quickly. This provided a new supply of energy and nutrients to the mid-levels and bottoms of the seas, which opened up a new range of possible ways of life. If any of these remains sank uneaten to the sea floor they could be buried; this would have taken somecarbon out ofcirculation, resulting in an increase in theconcentration of breathable oxygen in the seas (carbon readilycombines with oxygen).[42]
The initial herbivorous mesozooplankton were probably larvae of benthic (seafloor) animals. A larval stage was probably an evolutionary innovation driven by the increasing level of predation at the seafloor during theEdiacaran period.[11][158]
Metazoans have an amazing ability to increase diversity throughcoevolution.[64] This means that an organism's traits can lead to traits evolving in other organisms; a number of responses are possible, and a different species can potentially emerge from each one. As a simple example, the evolution of predation may have caused one organism to develop a defence, while another developed motion to flee. This would cause the predator lineage to diverge into two species: one that was good at chasing prey, and another that was good at breaking through defences. Actual coevolution is somewhat more subtle, but, in this fashion, great diversity can arise: three quarters of living species are animals, and most of the rest have formed by coevolution with animals.[64]
Evolving organisms inevitably change the environment they evolve in. TheDevoniancolonization of land had planet-wide consequences for sediment cycling and ocean nutrients, and was likely linked to theDevonian mass extinction. A similar process may have occurred on smaller scales in the oceans, with, for example, the sponges filtering particles from the water and depositing them in the mud in a more digestible form; or burrowing organisms making previously unavailable resources available for other organisms.[159]
Increases in burrowing changed the seafloor's geochemistry, and led to decreased oxygen in the ocean and increased CO2 levels in the seas and the atmosphere, resulting in global warming for tens of millions of years, and could be responsible for mass extinctions.[160] But as burrowing became established, it allowed an explosion of its own, for as burrowers disturbed the sea floor, they aerated it, mixing oxygen into the toxic muds. This made the bottom sediments more hospitable, and allowed a wider range of organisms to inhabit them—creating new niches and the scope for higher diversity.[87]
The explosion may not have been a significant evolutionary event. It may represent a threshold being crossed: for example a threshold in genetic complexity that allowed a vast range of morphological forms to be employed.[161] This genetic threshold may have a correlation to the amount of oxygen available to organisms. Using oxygen for metabolism produces much more energy than anaerobic processes. Organisms that use more oxygen have the opportunity to produce more complex proteins, providing a template for further evolution.[130] These proteins translate into larger, more complex structures that allow organisms better to adapt to their environments.[162] With the help of oxygen, genes that code for these proteins could contribute to the expression ofcomplex traits more efficiently. Access to a wider range of structures and functions would allow organisms to evolve in different directions, increasing the number of niches that could be inhabited. Furthermore, organisms had the opportunity to become more specialized in their own niches.[162]
After anextinction at the Cambrian–Ordovician boundary, another radiation occurred, which established the taxa that would dominate the Palaeozoic. This event, known as the Great Ordovician Biodiversification Event (GOBE), has been considered a "follow-up" to the Cambrian explosion.[163] Recent studies have suggested that the Cambrian explosion were not two discrete events but one long evolutionary radiation.[164] Analytical study of the Geobiodiversity Database (GBDB) andPaleobiology Database (PBDB) failed to find a statistical basis for separating the two radiations.[165]
Some researchers have proposed the existence of a biodiversity gap during theFurongian separating the Cambrian explosion and GOBE known as the Furongian Gap.[166] Studies of the Guole Konservat-Lagerstätte and similar fossil sites in South China have instead found the Furongian to instead be a time of rapid biological turnovers though, making the existence of the Furongian Gap highly controversial.[167]
The "Cambrian explosion" can be viewed as two waves of metazoan expansion into empty niches: first, acoevolutionary rise in diversity as animals explored niches on the Ediacaran sea floor, followed by a second expansion in the early Cambrian as they became established in thewater column.[64] The rate of diversification seen in the Cambrian phase of the explosion is unparalleled among marine animals: it affected all metazoanclades of which Cambrian fossils have been found. Laterradiations, such as those offish in theSilurian andDevonian periods, involved fewertaxa, mainly with very similar body plans.[23] Although the recovery from thePermian-Triassic extinction started with about as few animal species as the Cambrian explosion, the recovery produced far fewer significantly new types of animals.[168]
Whatever triggered the early Cambrian diversification opened up an exceptionally wide range of previously unavailableecological niches. When these were all occupied, limited space existed for such wide-ranging diversifications to occur again, because strong competition existed in all niches andincumbents usually had the advantage. If a wide range of empty niches had continued, clades would be able to continue diversifying and become disparate enough for us to recognise them as differentphyla; when niches are filled, lineages will continue to resemble one another long after they diverge, as limited opportunity exists for them to change their life-styles and forms.[169]
There were two similar explosions in theevolution of land plants: after a cryptic history beginning about450 million years ago, land plants underwent a uniquely rapid adaptive radiation during the Devonian period, about400 million years ago.[23] Furthermore, angiosperms (flowering plants) originated and rapidly diversified during theCretaceous period.
The Cambrian radiation was the explosive evolution of marine life that started 550,000,000 years ago. It ranks as one of the most important episodes in Earth history. This key event in thehistory of life on our planet changed the marine biosphere and its sedimentary environment forever, requiring a complex interplay of wide-ranging biologic and nonbiologic processes.
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