Cambrian explosion

The Cambrian explosion or Cambrian radiation describes the seemingly rapid appearance of most major groups of complex animals in the fossil record, around 530 million years ago. This is accompanied by a major diversification of other organisms. Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organised into colonies. In the following 70 million to 80 million years, the rate of evolution accelerated by an order of magnitude, and the diversity of life began to resemble today’s.

The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the mid 19th century, and Charles Darwin saw it as one of the main objections that could be made against his theory of evolution by natural selection.

The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was an “explosion” of complex organisms in the early Cambrian; what might have caused such rapid evolution; and what it can tell us about the origin and possible evolution of animals. A limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures left in Cambrian rocks, makes interpretation difficult.

History and significance

Geologists as long ago as Buckland (1784–1856) realised that a dramatic step change in the fossil record occurred around the base of what we now call the Cambrian. Charles Darwin considered this sudden appearance of many animal groups with few or no antecedents to be the greatest single objection to his theory of evolution: indeed, he devoted a substantial chapter of The Origin of Species to this problem.

American palæontologist Charles Walcott, who extensively studied North American fossil animals, 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.

The intense modern interest in the subject was sparked by the work of Harry B. Whittington and colleagues, who in the 1970s re-analysed many fossils from the Burgess Shale (see below) and concluded that several were complex but very different from any living animals. Stephen Jay Gould’s popular 1989 account of this work, Wonderful Life, 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 phylum had appeared rather suddenly. But 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.

Difficulty of dating the Cambrian

It has been difficult to work out the chronology of the early Cambrian. Absolute radiometric dates for much of the Cambrian, obtained by detailed analysis of radioactive elements contained within rocks, have only rather recently become available, and for only a few regions.

Relative dating (A was before B) is often good enough for studying processes of evolution, but this has also been difficult, because of the problems involved in matching up rocks of the same age across different continents, particularly around the internationally-defined Precambrian/Cambrian boundary section. (the most common technique uses widespread but short-lived fossil species to identify rocks of similar ages)

So any dates or descriptions of sequences of events should be regarded with caution until better data become available.

Types of evidence

Body fossils

Body fossils preserve all or significant parts of organisms and are therefore the most informative type of evidence. Unfortunately they are increasingly rare as one looks further back in time, among other reasons because the rocks in which they are buried are usually covered by more recent rocks. One recent study concluded that “parts of the fossil record are clearly incomplete, but they can be regarded as adequate to illustrate the broad patterns of the history of life.” But there is evidence that some types of animals or parts of animals are relatively likely to be preserved as fossils in some environments and times, and extremely unlikely to be preserved in other environments and times. Part of this is due to changes in the chemistry of the oceans, which were partly caused by the on-going evolution of life, and these changes were most significant before the start of the Cambrian – for example any increase in the marine biomass would reduce the concentration of carbon, and the appearance of sponges reduced the concentration of silicon.

Another limitation in the discovery and use of body fossils is the lack of preservation of large portions of the body. In most cases the sole anatomical features that are fossilized are the highly mineralised body parts containing high proportions of silica (sponges' skeletons), calcium carbonate (the shells of bivalves, gastropods and ammonites and exoskeletons of most trilobites and some crustaceans) or calcium phosphate (the bones of vertebrates). The majority of animal species living now are unlikely ever to leave fossils, since they are soft-bodied invertebrates such as worms and slugs. Of the more than 30 phyla of living animals, two-thirds of these have never been found as fossils.

File:Marella200x155.png

A fossil of Marrella from the Burgess Shale lagerstätte. The animal was under 2 cm long but the fine-grained shale has preserved a very detailed image of it.

The Cambrian fossil record includes an unusually high number of lagerstätten which preserved the fossils' soft tissues in extremely fine detail, allowing a very informative study of animals that normally would not have left fossils. The fine detail of the deposits have allowed paleontologists to examine the internal workings of animals which in other sediments are only represented by shells, spines, claws, etc. The most significant Cambrian lagerstätten are: the early Cambrian Maotianshan shale beds of Chengjiang (Yunnan, China) and Sirius Passet (Greenland); the middle Cambrian Burgess Shale (British Columbia, Canada); and the Upper Cambrian Orsten (Sweden) fossil beds.

While lagerstätten are superior to most fossil beds in preserving fine anatomical detail, they are far from perfect. The majority of then-living animals are probably not represented because lagerstätten are restricted to a narrow range of environments (e.g. where soft-bodied organisms can be preserved very quickly such as by mudslides), and the exceptional events that cause quick burial make it difficult to study the normal environments of the animals. In addition, the known lagerstätten cover only a very limited period of time within the Cambrian, and none covers the crucial period just before the start of the Cambrian. Because normal fossil beds are very rare and lagerstätten even rarer, both are very unlikely to show the first occurrence of any type of organism.

Trace fossils

Trace fossil of the type called Cruziana, possibly made by a trilobite.

Trace fossils consist mainly of tracks and burrows on and under what was then the seabed.

Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them. Whilst exact assignment of trace fossils to their makers is generally impossible, traces may provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).

Geochemical observations

The ratios of three major isotopes, ⁸⁷Sr / ⁸⁶Sr, ³⁴S / ³²S and ¹³C / ¹²C, undergo dramatic fluctuations around the beginning of the Cambrian. This chemical signature in the rocks of the Precambrian/Cambrian boundary is difficult to interpret, and may be related to continental break-up, the end of a “global glaciation”, or a catastrophic drop in productivity caused by a mass extinction just before the beginning of the Cambrian.

Carbon has 2 stable isotopes, carbon-12 (¹²C) and carbon-13 (¹³C). Causes often suggested for changes in the ratio of ¹³C to ¹²C found in rocks include:

• A mass extinction. Chemistry is largely driven by electro-magnetic forces, and lighter isotopes such as ¹²C respond to these more quickly than heavier ones such as ¹³C. So living organisms generally contain a disproportionate amount of ¹²C. A mass extinction would increase the amount of ¹²C available to be included in rocks and therefore reduce the ratio of ¹³C to ¹²C.
• A methane “burp”. In permafrosts and continental shelves methane produced by bacteria gets trapped in “cages” of water molecules, forming a mixture called a clathrate. This methane is very rich in ¹²C because it has been produced by organisms. Clathrates may dissociate (break up) suddenly if the temperature rises or the pressure on them drops. Such dissociations release the ¹²C-rich methane and thus reduce the ratio of ¹³C to ¹²C as this carbon is gradually incorporated into rocks (methane in the atmosphere breaks down into carbon dioxide and water; carbon dioxide reacts with minerals to form carbonate rocks).

Comparative anatomy

Cladistics is a technique for working out the “family tree” of a set of organisms, and has most often applied to evidence from comparative anatomy (features of the bodies of organisms). In this kind of analysis it is possible to include both living and fossilized organisms and work out their evolutionary relationships. Sometimes one can conclude that group A must have evolved before groups B and C, because B and C have more similarities to each other than either has to A. On its own this method can say nothing about when A evolved, but if there are fossils of B or C dating from X million years ago, then A must have evolved more than X million years ago.

Molecular phylogenetics

Molecular phylogenetics attempts to reconstruct the relationships between organisms by comparing details of their biochemistry, such as their DNA. In other words, it applies the analysis techniques of cladistics to biochemical rather than anatomical features. It provides an alternative line of evidence about evolution in the Cambrian and Precambrian, although the need for calibration against the fossil record means it is not entirely independent. Further, since the “clocks” measure molecular evolution, a period of rapid evolution is indistinguishable from a longer period of slow change, so it is unwise to rely on molecular phylogeny for estimates of dates.

Although this rapidly developing science must be treated with a degree of caution, it has yielded some useful results. For example, it provides evidence that the three major animal groups diverged some time before the Cambrian, then independently underwent a rapid Cambrian diversification – although the reliability and implications of this apparent finding are still being debated. Generally, it can be stated that the current knowledge of molecular evolution seems not to support the cambrian explosion theory, but a considerably earlier radiation.

Evidence in rocks

This lists the main items in order of the time when the relevant rocks were formed, because timing is the central issue in the Cambrian explosion – but remember that dating rocks from the Cambrian and earlier rocks is very difficult. The survey also starts well before the start of the Cambrian and finishes in the early Ordovician, because some scientists think that the diversification of animal life started before and finished after the Cambrian.

It covers body fossils, trace fossils and geochemical evidence, because these are all found in rocks which can be dated at least approximately. Arguments based on molecular phylogenetics will appear in a separate section, because this type of evidence is much harder to date with confidence.

Explanation of a few scientific terms

To avoid becoming even longer this article uses some scientific terms, and this is a good place for some simple explanations.

Triploblastic means consisting of 3 layers, which are formed in the embryo (before the animal is born / hatched). 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.).

Bilaterian means having 2 sides; this implies that they also have top and bottom surfaces and, perhaps more importantly, distinct front and back ends. All known bilaterian animals are triploblastic, and all known triploblastic animals are bilaterian except for echinoderms (but sea cucumbers do have distinct front and back ends; and echinoderm larvae have 2 sides). Porifera (sponges) and Cnidaria (jellyfish, sea anemones, etc.) are radially symmetrical (like wheels).

Most of the phyla in the debate about the Cambrian explosion are coelomates; (priapulids are an important exception). Coelomate means having a body cavity (coelom) which contains the internal organs. Arthropods, [annelid] worms, molluscs, echniderms (starfish, sea urchins, sea cucumbers, sea lilies) and chordates (including us vertebrates) are coelomates. All coelomate animals are triploblastic, but some triploblastic animals do not have a coelom (e.g. flatworms; their organs are surrounded by unspecialized tissues). Some bilaterian animals are not coelomates (e.g. flatworms). Echinoderms are coelomates but not completely bilaterian.

Decline of stromatolites over 1 billion years ago

Modern stromatolites in Shark Bay, Western Australia.

Stromatolites are not organisms, they are stubby pillars of sediment built by photosynthesizing microorganisms, especially cyanobacteria. They are now restricted to hostile environments such as extremely salty lagoons, because in less hostile environments they are eliminated by grazing and burrowing invertebrates.

Stromatolites are an important part of the fossil record for about the first 3 billion years of life on earth, peaking about 1250 million years ago, but after then they declined in abundance and diversity, and by the start of the Cambrian had fallen to 20% of their peak. The most widely-supported explanation is that stromatolite-building organisms were the victims of grazing animals, which would imply that sufficiently complex animals were common over 1 billion years ago. This connection is supported by the facts that: stromatolites declined again when the abundance and diversity of marine animals increased in the Ordovician Radiation; and stromatolite abundance increased after the end-Ordovician and end-Permian extinctions decimated marine animals, but fell back to earlier levels as marine animals recovered.

Increase in abundance and spininess of acritarchs

Acritarchs include the remains of a wide range of quite different kinds of organisms - ranging from the egg cases of small metazoans to resting cysts of many different kinds of chlorophyta (green algae). They first appear in rocks about 2 billion years old, but about 1 billion years they started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during the Snowball Earth episodes, but they reached their highest diversity in the Paleozoic era. Their increasingly spiny forms in the last 1 billion years is probably due to the need for defense against predators, especially predators large enough to swallow them or tear them apart. Other groups of small organisms from the Neoproterozoic era also show signs of anti-predator defenses.

Trace fossils 1 billion years ago?

Trace fossils found in rocks about 1 billion years old in India may represent marks of creatures moving across and below soft surfaces. The organisms making the traces were clearly not exploiting deep sediments, but only the layers immediately below the mat of cyanobacteria that covered the seabed. The researchers concluded that the burrows were produced by the peristaltic action of triploblastic metazoans up to 5 mm wide—in other words by animals about the diameter of earthworms, about as complex and possibly coelomates. But other researchers have dismissed this and other purported finds of trace fossils older than about 600 million years ago, usually on the grounds that they were produced by physical processes rather than by organisms.

Cryogenian glaciations

The Cryogenian Period between 750 and 600 million years ago was cold, with a few major glaciations:

• The Sturtian, for which evidence was found in South Australian deposits, occurred about 720 million years old.
• The Changan (glacial deposits found in China)
• The Tiesiao (glacial deposits found in China) ended before 633 million years ago.
• The Nantuo (glacial deposits found in China) began later than 633 million years ago and is probably equivalent to the Marinoan glaciation in South Australia, which is dated at 630 million years ago.

Doushantuo Formation

The Doushantuo Formation in China contains one of the oldest known lagerstätten. These rocks range from about 635 million to about 551 million years ago, but their animal fossils are mostly less than 580 million years old, predating by perhaps 5 million years the earliest of the 'classical' Ediacaran faunas (see below) from Mistaken Point, Newfoundland. Doushantuo fossils are all marine, microscopic and highly preserved. They include algae, giant acritarchs and what may be phosphatised embryos of bilaterian animals; but some scientists think the “embryos” are fossils of giant sulfur-metabolising bacteria like Thiomargarita, which is so large that it is visible to the naked eye.

Vernanimalcula interpreted as an early coelomate. Note that some paleontologists think this “fossil” is a result of purely mineral processes.

One Doushantuo fossil from about 580M years ago, Vernanimalcula (0.1 to 0.2 mm in diameter), has been described as a possible adult triploblastic coelomate bilaterian, in other words about as complex as an earthworm or a mollusc; others think it was more probably created by non-biological rock-forming processes; but the team that discovered Vernanimalcula have defended their conclusion that it was an animal, pointing out that they found 10 specimens of the same size and configuration, and stating that non-biological processes would be very unlikely to produce so many specimens that were so alike.

The Gaskiers glaciation, known from glacial deposits in Newfoundland and Massachusetts, is later than the earliest Doushantuo fossils although it is regarded as the last of the Cryogenian series of glaciations.

The most recent Doushantuo rocks show a sharp decrease in the ¹³C/¹²C carbon istope ratio. Since this change appears to be worldwide but its timing does not match that of any other known major event such as a mass extinction, it may represent “possible feedback relationships between evolutionary innovation and seawater chemistry” in which metazoans (multi-celled organisms) removed carbon from the water, this increased the concentration of oxygen, and the increased oxygen level made possible the evolution of new metazoans such as Namapoikia (see below).

Ediacaran organisms

Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance.

Fossil of Spriggina, one of the Ediacaran biota and possibly a trilobite

Main article: Ediacaran biota

Strange-looking fossils were found first in the Ediacara Hills in Australia and then in marine sediments from many parts of the world including Charnwood Forest (England) and the Avalon Peninsula (Canada), with dates between 610 million and 543 million years ago (right up to the start of the Cambrian). Most of the Ediacaran biota were at least a few centimeters long, significantly larger than previous finds.

Many were unlike anything that appeared before or since, resembling discs, mud-filled bags, or quilted mattresses – one palæontologist proposed that the strangest organisms should be classified as a separate kingdom, Vendozoa. The earliest known body fossils of complex organisms are of one of these strange organisms, Charnia, from about 580 million years ago.

But some were possibly early forms of the phyla at the heart of the debate about the "Cambrian explosion": Kimberella was possibly a mollusc (see below), and is one of the rare Ediacaran fossils whose mode of feeding may be known, enabling easier comparison with Cambrian forms; Arkarua was possibly an echinoderm, although it lacked a feature present in later echinoderms (stereom, a unique crystalline form of calcium carbonate from which their skeletons are built); Spriggina was possibly a trilobite and therefore an arthropod. However, such fossils lack any evidence of legs or a complex digestive system.

Cloudina is a small animal (diameter 0.3 mm to 6.5 mm; length 8 mm to 150 mm) which looks like a rather loose, wobbly stack of cones, sharp end downwards. It has been suggested that Cloudina is a stem group polychaete worm, but there is still much debate about how to classify it. More importantly it was one of the earliest animals to have a calcareous shell, i.e. hard parts in the palæontologists’ sense. In some locations, up to 20% of Cloudina fossils contain predatory borings ranging from 15 to 400 µm in diameter. Some tubes had been bored multiple times, indicating that Cloudina could survive attacks (predators do not attack empty shells). The rather similar shelly fossil Sinotubulites, which appears in the same fossil beds, was not affected by borings. In addition, the distribution of borings suggests selection for size. This evidence of predator selectivity shows the possibility of speciation in response to predation, which is often suggested as a potential cause of the Cambrian explosion.

In 2002 another mineralized metazoan, Namapoikia, was found in rocks about 549 million years old, i.e. about 6 million years before the start of the Cambrian. Namapoikia was up to 1m (39in) in diameter and was probably a cnidarian (group which includes jellyfish and sea anemones) or a poriferan (i.e. a sponge).

It is generally agreed that at least the vast majority and possibly all of the "classic" Ediacaran biota (the organisms that looked most different from any of to-day’s animals) became extinct some time before the start of the Cambrian. One Cambrian discovery may be a fossil of Swartpuntia, a genuine "Vendobiont". Other finds have been reported as "Vendobionts" that survived into the Cambrian, but it appears that these are not "Vendobionts" after all and some are probably colonies of microbes.

Mollusc-like animals 555 million years ago

Fossil of Kimberella, a triploblastic bilaterian and possibly a mollusc.

A fossil bed in Russia contains a few layers of volcanic ash which have been dated by radiometric methods (uranium-lead ratios in zircons) to a little over 555 million years ago. The fossils found there include Kimberella, the oldest well-documented triploblastic bilaterian. Kimberella was 3 mm to 100 mm long and very like a mollusc: its body was metameric (built as a series of repeated “modules”) but without visible segmentation; it had a single broad, muscular foot and a single shell (not mineralized but fairly stiff). So far Kimberella fossils show no sign of a radula (toothed chitinous “tongue”, which is the signature feature of modern molluscs except bivalves), but radulae are very rarely preserved in any fossil molluscs. However the rocks around the Kimberella fossils bear scratches which are very similar those made by the radulas of grazing molluscs. Researchers concluded that “This is important evidence for the existence of large triploblastic metazoans in the Precambrian and indicates that the origin of the higher groups of protostomes lies well back in the Precambrian.”

Change in carbon isotope ratios at Ediacaran-Cambrian boundary

Carbon has 2 stable isotopes, carbon-12 (¹²C) and carbon-13 (¹³C). At the boundary between the Ediacaran and Cambrian periods the ratio of ¹³C to ¹²C drops sharply, and then is unusually erratic until the mid-Cambrian. There is no easy explanation for the rapid variation of the ratio in the first half of the Cambrian, and at present it is impossible to decide between the two widely-supported explanations for the sharp drop at the Ediacaran-Cambrian boundary, a mass extinction or a methane “burp”.

Early Cambrian diversification of trace fossils

Around the start of the Cambrian (about 543 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 behaviors and possibly new physical capabilities. If traces such as Cruziana and Rusophycus were produced by arthropods, that would indicate that arthropods or their immediate predecessors had developed exoskeletons, although not necessarily as hard as they became later in the Cambrian.

Small shelly fauna

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.

Early Cambrian trilobites and echinoderms

Fossilized trilobite, an ancient type of arthropod

The earliest Cambrian trilobite fossils are about 530 million years old, but even then they were quite diverse and world-wide, which suggests that these arthropods had been around for quite some time.

The earliest generally-accepted echinoderms appeared at about the same time, although it has been suggested that some fossils from the Ediacaran period were echinoderms (see above). The early Cambrian Helicoplacus was a cigar-shaped creature up to 7 cm long that stood upright on one end. Unlike modern echinoderms it was not radially symmetrical with the mouth at the center, but had a spiral food groove on the outside along which food was moved to a mouth that is thought to be located on the side.

Sirius Passet fauna

Sirius Passet is a lagerstätte in Greenland which was formed about 527 million years ago. Its most common fossils are arthropods, but there is only a handful of trilobite species. There are also very few species with hard (mineralized) parts: trilobites, hyoliths, sponges, brachiopods, and no echinoderms or molluscs.

One of the arthropods, Pauloterminus, has a bivalve-like carapace.

Halkieria has features associated with more than one phylum, and is discussed below.

File:Kerygmachela dorsal 193x70.png

Reconstruction of Kerygmachela from Sirius Passet, viewed from the top, with the head to the right. The shaded areas on the lobes (flaps on the sides) are thought to have functioned as gills.

The strangest-looking animals from Sirius Passet are Pambdelurion and Kerygmachela. They are generally regarded as anomalocarids because they have long, soft, segmented bodies with a pair of broad fin-like flaps on most segments and a pair of segmented appendages at the rear. The outer parts of the top surfaces of the flaps have grooved areas which are thought to have acted as gills. Under each flap there is a short, fleshy leg. This arrangement suggests the animals are related to biramous arthropods. Both were apparently blind, as the fossils show no trace of eyes. Kerygmachela had a small conical mouth flanked by robust, unsegmented appendages which had short spines on the front edge and were tipped with longer spines. The spiny front limbs suggest that it may have been a predator, but its small mouth suggests it would have been restricted to very small prey. Pambdelurion lacked trailing appendages but had a more typically anomalocarid-style mouth, a relatively large ring of crushing plates under the front of its head. Its mouth was flanked by a pair of thick, segmented appendages slightly longer than the swimming flaps and equipped with a flexible spine on each segment.

Chengjiang fauna

There are several Cambrian fossil sites in the Chengjiang county of China’s Yunnan province. The most significant is the Maotianshan shale, a lagerstätte which preserves soft tissues very well. The Chengjiang fauna date to between 525 million and 520 million years ago, about the middle of the early Cambrian epoch, a few million years after Sirius Passet and at least 10 million years earlier than the Burgess Shale.

The Chengjiang sediments provide what are currently the oldest known chordates, the phylum to which all vertebrates belong. The 8 chordate species include Myllokunmingia, possibly a very primitive agnathid (jawless fish) and Haikouichthys, which may be related to lampreys. Yunnanozoon may be the oldest known hemichordate (a phylum closely related to chordates).

Vetulicola is a small swimming animal with a carapace covering the front half of its body. Its classification is uncertain: it has paired openings connecting the pharynx to the outside, which may be primitive gill slits; because of these, some researchers argue that it is a deuterostome (“super-phylum” which includes chordates) and possibly even a larvacean (urochordate which remains free-swimming throughout its life); but others classify it as an arthropod.

File:Anomalocaris Saron 200x59.png

Reconstruction of Anomalocaris saron, viewed from the top with the head to the right. The shaded patches at the bases of the flaps are thought to have acted as gills.

Anomalocaris was a mainly soft-bodied swimming predator which was gigantic for its time (up to 70 cm = 2¼ feet long; some later species were 3 times as long); the soft, segmented body had a pair of broad fin-like flaps along each side, except that the last 3 segments had a pair of “fans” arranged in a “V” shape. Unlike Kerygmachela and Pambdelurion (see above), Anomalocaris apparently had no legs, and the grooved patches which are thought to have acted as gills were at the bases of the flaps, or even overlapping on to its back. The two eyes were on relatively long horizontal stalks; the mouth lay under the head and was a round-cornered square of plates which could not close completely; and in front of the mouth were two jointed appendages which were shaped like a shrimp’s body, curved backwards and with short spines on the inside of the curve. Amplectobelua, also found at Chengjiang, was similar, smaller than Anomalocaris but considerably larger than most other Chengjiang animals. Both are thought to have been powerful predators.

Hallucigenia looks like a long-legged caterpillar with spines on its back, and almost certainly crawled on the seabed.

Nearly half of the Chengjiang fossil species are arthropods, few of which had the hard, mineral-reinforced exoskeletons found in most later marine arthropods; only about 3% of the organisms known from Chengjiang have hard shells, and most of those are trilobites (although Misszhouia is a soft-bodied trilobite). Many other phyla are found there: Porifera (sponges) and Priapulida (burrowing “worms” which were ambush predators), Brachiopoda (these had bivalve-like shells, but fed by means of a lophophore, a fan-like filter which occupied about of half of the internal space), Chaetognatha (arrow worms), Cnidaria (jellyfish, sea anemones), Ctenophora (comb jellies), Echinodermata (starfish, sea urchins, etc.), Hyolitha (enigmatic animals with small conical shells), Nematomorpha (horse hair worms, parasites which are typically about 1 m long and 1 mm to 3 mm in diameter), Phoronida (horseshoe worms which live in chitinous tubes and feed by means of a lophophore), and Protista (single-celled animals).

Early Cambrian crustaceans

Crustaceans are one of the three great modern groups of arthropods – the others are chelicerates (spiders, scorpions, horseshoe crabs) and uniramia (the most important members are insects, millipedes, centipedes). Ercaia is a small crustacean from 520 million years ago, found in the Maotianshan shale (a lagerstätte described above). Small phosphatocopid crustaceans (a group known only in the Cambrian) have been found in the Protolenus Limestone (early Cambrian) of Shropshire, England.

Burgess Shale

The Burgess Shale was the first of the Cambrian lagerstätten to be discovered (by Walcott in 1909), and the re-analysis of the Burgess Shale by Whittington and others in the 1970s was the basis of Gould’s book Wonderful Life, which was largely responsible for non-scientists' awareness of the Cambrian explosion. The fossils date from the mid Cambrian, about 515 million years ago and 10 million years later than the Chengjiang fauna.

The most common Burgess Shale fossils are arthropods, but many of them are unusual and difficult to classify, for example:

• Marrella is the most common fossil (see picture above), but Whittington’s re-analysis showed that it belonged to none of the known marine arthropod groups (trilobites, crustaceans, chelicerates; well-known modern chelicerates include spiders and scorpions).
• Yohoia was a tiny animal (7 mm to 23 mm long) with: a head shield; a slim, segmented body covered on top by armor plates; a paddle-like tail; 3 pairs of legs under the head shield; a single flap-like appendage fringed with setae (bristles) under each body segment, probably used for swimming and/or respiration; a pair of relatively large appendages at the front of the head shield, each with a pronounced “elbow” and ending in four long spines which may have functioned as “fingers”. Yohoia is assumed to been a mainly benthic (bottom-dwelling) creature that swam just above the ocean floor and used its appendages to scavenge or capture prey. It may be a member of the arachnomorphs, a group of arthropods that includes the chelicerates and trilobites.
• Naraoia was a soft-bodied animal (no mineralized parts) which is classified as a trilobite because its appendages (legs, mouth-parts) are very similar.
• Waptia, Canadaspis and Plenocaris had bivalve-like carapaces. It is uncertain whether these animals are related or acquired bivalve-like carapaces by convergent evolution.

Pikaia resembled the modern lancelet, and was the earliest known chordate until the discovery of the fish-like Myllokunmingia and Haikouichthys among the Chengjiang fauna.

Reconstruction of Opabinia, one of the strangest animals from the Burgess Shale

But the “weird wonders”, creatures that resembled nothing known in the 1970s, attracted the most publicity, for example:

• Whittington’s first presentation about Opabinia made the audience laugh. The reconstruction showed a soft-bodied animal with: a slim, segmented body; a pair of flap-like appendages on each segment with gills above the flaps, except that the last 3 segments had no gills and the flaps formed a tail; five stalked eyes; a backward-facing mouth under the head; a long, flexible, hose-like proboscis which extended from under the front of the head and ended in a “claw” fringed with spines. Subsequent research has concluded that Opabinia is a lobopod, closely related to the arthropods and possibly even closer to ancestors of the arthropods.
• Anomalocaris and Hallucigenia were first found in the Burgess Shale, but older specimens have been found in the Chengjiang fauna. They are now regarded as lobopods, and Anomalocaris is very similar to Opabinia in most respects (except the eyes and feeding mechanisms) – see above.
• Odontogriphus is currently regarded as either a mollusc or a lophotrochozoan, i.e. fairly closely related to the ancestors of molluscs (see above).

Molluscs, annelids or brachiopods?

Fossil of Halkieria

Wiwaxia, found so far only in the Burgess Shale, had chitinous armor consisting of long vertical spines and short overlapping horizontal spines. It also had what looked like a radula (chitinous toothed “tongue”), a feature which is otherwise only known in molluscs. Some researchers think the pattern of its scales links its closely to the annelids (worms) or more specifically to the polychaetes (“many bristles”; marine annelids with leg-like appendages); but others disagree.

Orthrozanclus, also discovered in the Burgess Shale, had long spines like those of the wiwaxiids, and small armor plates plus a cap of shell at the front end like those of the halkieriids. The scientists who described it say it may have been closely related to the halkieriids and the wiwaxiids.

Halkieria resembled a rather long slug, but had a small cap of shell at each end and overlapping armor plates covering the rest of its upper surface – the shell caps and armor plates were made of calcium carbonate. Its fossils are found on almost every continent in early to mid Cambrian deposits, and the “small shelly fauna” deposits contain many fragments which are now recognized as parts of Halkieria’s armor. Some researchers have suggested that halkieriids were closely related to the ancestors of brachiopods (the structure of halkieriids' front and rear shell caps resembles that of brachiopod shells) and to the wiwaxiids (the pattern of the scale armor over most of their bodies is very similar). Others think the halkieriids are closely related to molluscs and have a particularly strong resemblance to chitons.

Odontogriphus is known from almost 200 specimens in the Burgess Shale. It was a flattened bilaterian up to 12 cm (5 in) long, oval in shape, with a ventral U-shaped mouth surrounded by small protrusions. The most recently found specimens are very well preserved and show what may be a radula, which led those who described these specimens to propose that it was a mollusc. But others disputed the finding of a radula and suggested Odontogriphus was a jawed segmented worm belonging to the Lophotrochozoa (a “super-phylum” which contains the annelids, brachiopods, molluscs and all other descendants of their last common ancestor).

Late Cambrian and early Ordovician organisms

File:OilShaleFossilsEstonia.JPG

Bryozoan fossils in an Ordovician oil shale, northern Estonia.

Right up to the end of the Cambrian there were high levels of “disparity” (sets of organisms with significantly different “designs”) but low levels of diversity (total numbers of species or genera; variations on the main “design” themes); and as a result Cambrian ecosystems are much simpler than those from later in the Paleozoic era. There was a mass extinction at the Cambrian-Ordovician boundary, and typical Paleozoic marine diversity and ecosystems only appear during the recovery from the extinction. It is also worth noting that the earliest fossils of one phylum, the Bryozoa, first appear in the Ordovician period.

How real was the explosion?

How fast did the main metazoan groups evolve?

In Darwin’s time what was known of the fossil record 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. But more recently-discovered fossil evidence suggests that at least some triploblastic bilaterians were present before the start of the Cambrian: Kimberella left the kind of fossils one would expect of an early mollusc, and the scratches on the rocks near these fossils suggest a mollusc-like method of feeding; and if Vernanimalcula was a triploblastic bilaterian coelomate, it would prove that moderately complex animals appeared even earlier. And the presence of borings in shells of Cloudina suggests there were sufficiently advanced predators in the late Ediacaran period. Further back in time, the long decline of stromatolites after about 1250 million years ago suggests that animals sufficiently complex to graze on bacterial mats were abundant well before the Ediacaran period; and the increase in abundance, diversity and spininess of acritarchs in the same period suggests that there were sufficient predators large enough to make such defenses necessary.

At the other end of the critical time range, several major modern types of animal did not appear until the late Cambrian, while typical Paleozoic ecosystems did not appear until the Ordovician.

So the evidence no longer appears to support the view that the major metazoan groups appeared in a few million years of the early to mid-Cambrian. But the rise in "disparity" (wide range of animals with significantly different "designs") seems to have occurred mostly in the early Cambrian.

Was there a “riot of disparity” in the early Cambrian?

In this context “disparity” means a wide range of animals with significantly different “designs”; while “diversity” means total number of genera or species and says nothing about the number of different basic “designs” (there could be many variations on the same few designs). There is little doubt that disparity rose sharply in the early Cambrian and was exceptionally high for the rest of the Cambrian – we see both modern-looking animals such as crustaceans, echinoderms, and fish at about the same time and often in the same fossil beds as creatures like Anomalocaris and Halkieria, which are currently regarded as “aunts” or “great-aunts” of modern groups.

On closer examination we find another surprise – some modern-looking animals, e.g. the early Cambrian crustaceans, trilobites and echinoderms, appear earlier in the fossil record than some of the “aunts” or “great-aunts” of modern groups. This could be a result of gaps in the fossil record or of preservational biases in different environments; or it could mean that the ancestors of various modern groups evolved at different times and possibly at different speeds.

Possible causes of the “explosion”

Despite the evidence that moderately complex animals (triploblastic bilaterians) 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. Naturally there has been a lot of discussion about why this should have happened.

Changes in the environment

Increase in oxygen levels

Earth’s earliest atmosphere contained no free oxygen; the oxygen that animals breathe today, both in the air and dissolved in water, is the product of billions of years of photosynthesis, mainly by microorganisms such as cyanobacteria. The concentration of oxygen in the atmosphere has risen gradually (with a few ups and downs) over about the last 2.5 billion years (before that oxygen-hungry elements such as iron reacted with all the oxygen that was produced).

Shortage of oxygen might well have prevented the rise of large, complex animals for a long time. 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); but 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 reduce or remove this difficulty. But apparently there was already enough oxygen to support reasonably large “Vendobionta” in the Ediacaran period. Perhaps a further increase in oxygen concentration was required to give animals the energy to produce substances such as collagen which are needed for the construction of complex structures, particularly those used in predation and defense against predation.

Snowball Earths

There is plenty of evidence that in the late Neoproterozoic (up to the start of the Ediacaran period) the Earth suffered massive glaciations in which most of its surface was covered by ice and temperatures were around freezing even at the Equator. Some researchers argue that these may have been an important factor in the Cambrian explosion, since the earliest known fossils of animals appear shortly after the last "Snowball Earth" episode.

But it is hard to see how such catastrophes could have led to increases in the size and complexity of animals without clear evidence of a causal mechanism. Perhaps the cold temperatures increased the concentration of oxygen in the oceans—the solubility of oxygen nearly doubles as seawater cools from 30 °C to 0 °C. On the other hand they may have delayed the evolution of existing metazoans to larger sizes.

Carbon isotope fluctuations

As we've already seen, there was a very sharp decrease in the ¹³C/¹²C ratio at the Ediacaran-Cambrian boundary, followed by unusually strong fluctuations throughout the early Cambrian. Many scientists assume that the initial sharp drop represents a mass extinction at the start of the Cambrian. It might even have caused a mass extinction – the Permian–Triassic extinction event is associated with a similar sharp decrease in the ¹³C/¹²C ratio; this is usually explained as due to massive dissociation of methane clathrates, and it is widely thought that the resulting methane emissions triggered severe global warming and other environmental catastrophes. And the ¹³C/¹²C fluctuations in the early Cambrian resemble those of the early Triassic, when life was struggling to recover from the Permian-Triassic extinction.

But it’s difficult to see how a mass extinction could have triggered a sharp increase in disparity and diversity. Mass extinctions such as the Permian-Triassic and Cretaceous–Tertiary raised existing animals from insignificance to “dominance”, but these replaced different but similarly complex animals that were dominant before these extinctions, and there was no increase in disparity or diversity.

Others have suggested that each short-term decrease in the ¹³C/¹²C ratio through out the early Cambrian represents a methane “burp” which, by raising global temperatures, triggered an increase in diversity. But this hypothesis also fails explain the increase in disparity.

Developmental Explanations

Some theories are based on the idea that relatively small changes in the way in which animals develop from embryo to adult may have produced very rapid evolution of body forms. Unfortunately such theories do not explain why the origin of such a development system should by itself lead to increased diversity or disparity. In fact if at least one Ediacaran is a bilaterian (for example Kimberella, Spriggina or Arkarua), then the bilaterian developmental system existed at least a few tens of millions of years before the Cambrian "explosion", which suggests that something else might be needed to account for the "explosion".

Origin of the bilaterian developmental system

Hox genes regulate the operation of other genes by switching them on or off in various parts of the body, for example “make an eye here” or “make a leg there”. Very similar Hox genes are found in all animals from Cnidaria (e.g. jellyfish) to humans, although mammals have 4 sets of Hox genes while Cnidaria have only one. Hox genes in different animal groups are so similar that, for example, one can transplant a human “make an eye” Hox gene into a fruitfly embryo and it still causes an eye to form – but it’s a fruitfly eye, because the other genes that the transplanted Hox gene activates are fruitfly genes.

The fact that all animals have such similar Hox genes strongly suggests that the last common ancestor of all bilaterians had similar Hox genes. But this does not mean that the last common ancestor of bilaterians had anatomical features that resembled those of any living animal, since for example the same Hox gene can produce structures as different as a human eye and an insect eye. It’s more likely that the various bilaterian lineages became separate before they were committed to any specific way of building specific organs, and therefore that their last common ancestor was small, very simple, and probably rather delicate. This suggests that it will be very difficult to find fossils of the last common ancestor of all bilaterians.

Small increases in genetic complexity can have large effects

In most organisms that reproduce sexually, each child gets 50% of its genes from each parent. This means that a small increase in the complexity of the genome can produce a wide increase in the range of variations in body form. (rather like the way you can deal a larger number of unique hands if you increase the number of cards in the deck). Much of biological complexity probably arises from the operation of relatively simple rules within large numbers of cells functioning as cellular automata. (a simple example would be Conway's Game of Life, where complex and often surprising patterns are produced by cells that follow very simple rules)

Developmental entrenchment

Several scientists suggest that, as organisms become more complex, the developmental stages that produce the body plans are overlain with "down-stream" genetic mechanisms that produce more specific body components, and that this makes it progressively less likely that modifications of the "up-stream" stages will pass the tests of natural selection. So the developmental stages when the phylum-level body plans are laid down become entrenched and the body plans become frozen in place. Conversely, major modifications are "easier" in the early stages of the evolution of a major clade. But the author of this idea has more recently argued that this "entrenchment" is not a major factor.

The fossil evidence relating to this idea is also ambiguous. It has long been noted that variation within a species is often largest in the earliest members of a clade. For example some Cambrian trilobite species have varying numbers of thoracic segments, but later trilobite species show much less variation in this respect. But a Silurian trilobite species has been found which has as much variation in number of thoracic segments as the Cambrian species. Researchers have suggested that the general decrease in variability was caused by ecological or functional constraints; for example, one might expect a less variable number of segments once trilobites developed rolling up like modern pillbugs as a form of defense.

Ecological Explanations

These focus on the interactions between different types of organism. Some of these hypotheses deal with changes in the food chain; some suggest arms races between predators and prey, which might have driven the evolution of hard body parts in the early Cambrian; and some focus on the more general mechanisms of coevolution (a simple more recent example is the ways in which flowering plants and the insects which pollinate them have adapted to each other). Such theories are well suited to explaining why there was a rapid increase in both disparity and diversity, and the challenge for them is to explain why the "explosion" happened at that particular time.

Arms races between predators and prey

Predation by definition means that the prey dies, so one would expect that it would be one of the strongest components of natural selection. The pressure to adapt should be stronger on the prey than one the predator, because the predator lives to hunt again if it "loses a contest" (this is known as the "life-dinner" principle - the predator only risks losing one meal).

But there is enough evidence of predation well before the start of the Cambrian, for example the increasingly spiny forms of acritarchs and the holes drilled in Cloudina shells. Hence it is unlikely that predation triggered the Cambrian "explosion", although it very likely had a strong influence on the body forms that the "explosion" produced. (but see below for a more complex set of processes that may have been triggered by predation)

The appearance of herbivorous organisms

Stanley (1973) suggested that the appearance about 700 million years ago of protists (single-celled eukaryotes) that "cropped" microbial mats greatly expanded food chains and thus allowed rapid diversification, which led to the Cambrian explosion. But it is now thought that "cropping" arose before 1 billion years ago, as stromatolites began to decline about 1.25 billion years ago.

Increase in size and diversity of planktonic animals

Geochemical evidence strongly indicates that the total mass of plankton has been similar to modern levels since early in the Proterozoic. But before the start of the Cambrian the plankton made no contribution to the food supply of organisms at greater depths, because their corpses and droppings were too small to fall quickly towards the sea-bed (their "drag" was about the same as their weight) and so they were eaten by other plankton or destroyed by chemical processes before they could become food for necktonic and benthic animals (swimmers and sea-bottom crawlers).

Early Cambrian fossils have been found of mesozooplankton (mid-sized planktonic animals, barely large enough to see without magnification) that were well-equipped for filter-feeding on microscopic plankton (mostly phytoplankton, i.e. planktonic "plants"). The new mesozooplankton would have produced droppings and corpses that were large enough to fall fairly quickly; if they were eaten, they provided food for necktonic and benthic animals, which could therefore become larger and more diverse; if the falling particles reached the sea-floor without being eaten, they would be buried and this would increase the concentration of oxygen in the water by reducing the concentration of carbon (carbon is an "oxygen-hungry" element) - in other words, the appearance of mesozooplankton loosened two constraints on the evolution of larger, more diverse necktonic and benthic animals, namely shortage of food and shortage of oxygen. The rise of herbivorous mesozooplankton would also have created an ecological niche for even larger carnivorous mesozooplankton, whose corpses and droppings would have produced a further increase in the food and oxygen available.

The initial herbivorous mesozooplankton were probably larvae of benthic animals, and the evolution of planktonic larvae of benthic animals was probably a consequence of the increasing level of predation at the sea-floor in the Ediacaran period.

Theoretical explanations

Several scientists have produced theoretical models of what might have caused the Cambrian explosion. Of course these models cannot prove what did happen, but a model whose "predictions" match the known fossil evidence may help paleontologists by prompting them to look for evidence that matches the model's assumptions (such evidence may be new, or may be new interpretations of known fossils).

Lots of empty niches

Valentine has argued in several papers that it's reasonable to assume that: significant changes in body form are "difficult"; a new major innovation has much more chance of being successful if it faces at most limited competition for the ecological niche that it is trying to occupy, so that the prospective new type of organism has enough time to adapt well to its new niche (a simple modern analogy would be that golfers who change their swings have a short-term loss of form before they start getting the benefits). This would imply that major innovations are much more likely to succeed during the early stages of the diversification of animals, because that diversification fills almost all the ecological niches. It also implies that there is a wide range of other potential phyla, but the lack of empty niches prevents them from developing. Valentine's model does make it easy to understand why the Cambrian explosion happened only once and why its duration was limited.

Significance of the data

Magnitude (and existence?) of the explosion

The apparent suddenness of the Cambrian radiations led Darwin to propose that the origins of animals actually lies far back in Proterozoic time, and that the Cambrian explosion represents only an “unveiling” of true Proterozoic diversity. Such a view has been sporadically supported through time by the description of purported trace fossils from deep in the Proterozoic.

More recently and spectacularly, many molecular clock estimates place the origin of bilaterian animals well before the beginning of the Cambrian, perhaps more than 1 billion years ago Given that Cambrian animals are often large, sometimes had hard parts and could evidently make very abundant and obvious benthic trace fossils, their hypothesised Proterozoic predecessors could probably have none of these attributes without leaving at least some trace in the fossil record. As a result, hypothetical Proterozoic bilaterians are usually thought to be some combination of tiny (planktonic or meiofaunal), immobile in sediment (e.g. sessile or planktonic) and without hard parts. In theory, such hypotheses can be tested by phylogenetic reconstruction of the morphology of the most basal bilaterians. However, this has proven to be fraught with difficulty. They seem at least to have possessed a through-gut and striated musculature – neither of which are compatible with a minute size. Some Proterozoic fossils have been interpreted as coprolites (fossilized faeces), and excreting solid waste requires a through-gut; others have been interpreted as tunnels or burrows, which requires a muscular body with a tube-like shape (which also suggests a through-gut).

Proterozoic predecessors

Dickinsonia Costata, an Ediacaran life-form.

The hunt for Precambrian metazoans has intensified as the Cambrian debate has continued. Over the last decades, a rich and diverse prokaryotic and eukaryotic biota has been documented from Proterozoic rocks around the world. However, larger, more obviously animal-like fossils have been much harder to detect, although some disputed carbonaceous tubes have sometimes been described as annelid- or pogonophoran-like.

The Ediacaran Period, immediately preceding the Cambrian, is host not only to the trace fossils and tubes previously mentioned, but also the highly enigmatic Ediacaran biota, which – despite decades of study and a flurry of recent intense interest – remains very hard to place in the context of animal evolution. Some taxa such as Kimberella are thought by some to represent bilaterians or even more derived forms such as molluscs, but these assignations are by no means generally accepted.

Perhaps the most promising area for study is the Doushantuo Formation of China, spectacular fossils from which are probably around 580 million years old or younger. They preserve a variety of fossils in shales, phosphorites and cherts. Of these, the best known are those from the phosphorites. The Doushantuo fossils include algae, giant acritarchs, and, spectacularly, preserved phosphatised spheres that have been interpretted by some as embryos of non-bilaterian animals such as sponge or cnidarian grade organisms, though others consider these more likely to be bacterial in origin. Other bilateran embryos have also been described, along with a possible adult bilaterian, Vernanimalcula. However, these assignments have been criticised on the grounds that they fail to take into proper account the preservational processes that gave rise to the fossils. For example, it has been suggested on the basis of the mode of preservation of Doushantuo fossils, that Vernanimalcula is largely an artefact created by rock-forming processes. As a result, opinion is split about the age of the first convincing bilaterian fossil: the first universally accepted bilaterian fossils are probably not known until the Cambrian. Clearly, further research is required to clarify the many problematic aspects of Doushantuo diversity.

Early trace fossils

Late Ediacaran trace fossils preserved on a bedding plane

It is fair to say that no convincing trace fossils before the end of the Ediacaran are currently accepted: most of these have turned out to be pseudofossils. A few have been reported, including one from approximately one billion year-old sandstones from India, and some even older structures from the Stirling quartzite in Australia. Of these, the biogenicity of the former has now been abandoned by the original authors, and doubts have been cast on the latter in the literature.. However new putative trace fossils from Stirling rocks have recently been found.

The sum of the evidence, then, suggests that neither large bilateral animals (which would probably have been capable of leaving a body or trace fossil record) nor tiny ones (which would perhaps be expected to be found in the Doushantuo Formation) existed before close to the end of the Proterozoic.

Evolutionary significance

The rapidity of the Cambrian explosion, the lack of precursors in the fossil record, the lack of discovered “new” post-Cambrian species, and the apparent bewildering diversity of the forms displayed by the exceptional faunas, has generated much interest from many students of evolution, including most recently from the field of evolutionary developmental biology (“Evo-Devo”). Stephen Jay Gould’s promulgation of the view that the Cambrian represented an unprecedented riot of disparity, of which only a very few managed to survive until the present day, still represents the most widespread view of the event. However, recent taxonomic and dating revisions also allow a more sober view to be taken.

A limited record

First, as mentioned above, the diversity seen in all other major exceptional faunas is a sample of life well after the beginning of the Cambrian explosion – in the case of the Burgess Shale, which may be as young as 507 million years or so, some 35 million years after the beginning of the Cambrian, as defined by trace fossil proliferation, and even longer after the first reasonable trace fossils. Nevertheless, the older Chengjiang and Sirius Passet faunas both represent a period of time perhaps more than 10 million years earlier. Clearly, animal life had diversified greatly during the Nemakit-Daldynian and Tommotian, periods of time that, crucially, lack exceptionally preserved faunas of Burgess Shale type. The fossil record is thus currently almost silent on one of the most critical periods of animal evolution. In the gap are found instead the largely enigmatic “small shelly fossils”, poorly understood taxa upon which much more work is required.

Appearance of phyla

While the general rapidity of the Cambrian explosion thus seems to remain a reality, attempts have been made to downplay the “amount” of evolution that was required to generate the taxa actually seen in the Cambrian. In particular, the distinction between “crown” and “stem” groups has been applied to claim that many or even most lower-middle Cambrian taxa fall outside the crown groups of the modern phyla. This in some cases somewhat legalistic argument allows the origins of many of the phyla as we see them today to be pushed up into the succeeding Ordovician Period, or even later. Thus, the view that all modern phyla essentially suddenly appear at the base of the Cambrian has come under assault. One aspect of this reassessment is that many or most of the problematic Cambrian fossils have begun to be seen in the light of a stem-group placement to modern phyla or groups of phyla. Rather than being seen as one-off oddities, they can in this view be seen as representing the progressive adaptive stages of the assembly of modern day body plans, albeit ones with their own particular adaptations. An analogy can be drawn with the origin of the tetrapods or mammals, which have also been sequentially mapped out in the fossil record. Of course, many problematica remain, but in at least some of these cases, such as Odontogriphus, not enough has been known until recently about their morphology in order to come to a reasonable conclusion. Williamson (2006) contends (1) that there were no true larvae until after the establishment of classes in the respective phyla, (2) that early animals hybridized to produce chimeras of parts of dissimilar species, (3) that the Cambrian explosion resulted from many such hybridizations, and (4) that modern animal phyla and classes were produced by such early hybridizations, rather than by the gradual accumulation of specific differences. (Williamson, D.I. 2006. Hybridization in the evolution of animal form and life-cycle. Zoological Journal of the Linnean Society 148: 585-602)

Mechanistic basis

If this viewpoint is correct, then unusual genetic or other evolutionary mechanisms might not be needed to explain what the Cambrian fossil record reveals. As added evidence for this viewpoint, most attempts to quantify morphospace occupancy – that is, the proportion of possible modes of life that are exercised – in the Cambrian have suggested that it is certainly not greater than today, and most studies have suggested it to be considerably lesser. However, this area remains a topic of considerable controversy.

Causes of the Cambrian explosion

Understanding why the Cambrian explosion happened when it did revolves around three major themes: i) extrinsic forcing events such as environmental change; ii) intrinsic mechanisms such as the acquisition of complex genomes; and iii) intrinsic mechanisms such as the natural consequences of metazoan ecology.

The role of oxygen

Of the first class of explanation, by far the most popular, dating back at least to the 1950s, is that animals did not evolve before the beginning of the Cambrian because of low atmospheric oxygen. Low oxygen levels could prevent the synthesis of collagen, present in metazoans (and now also known in other eukaryotes) which requires at least 1% of present atmospheric levels (the “Towe limit”); however, it would be more likely to provide a physiological constraint. Animals living in low oxygen environments today tend to have low diversity, thin shells and low metabolic activity. While oxygen levels do certainly have an effect on animal life, it is not currently clear what atmospheric levels of oxygen were during the close of the Proterozoic, to what extent available oxygen was sequestered away by reduced mineral compounds, and what adaptations purported Proterozoic animals had to low oxygen conditions (presumably, they, like many living animals, possessed effective anaerobic metabolic pathways).

Snowball Earth

A present day glacier

Main article: Snowball Earth

A related and currently popular explanation is that of “Snowball Earth”, which ties the severe glaciations towards the end of the Proterozoic to profound changes in oxygen levels and ocean chemistry. The explanatory power of such a hypothesis depends on I) how convincing the evidence for Snowball Earth is and II) providing a clear mechanistic link between what would undoubtedly have been a severe global upheaval and the subsequent radiation of the animals. As well as global cooling, global warming – perhaps as the result of massive methane release into the atmosphere – has been posited, as well as variety of other less exotic mechanisms such as continental breakup together with increased shelf area. Another example is a facilitating change in oceanic chemistry that allowed the formation of hard parts for the first time, although this cannot, of course, explain why some organisms seem to start diversifying before the origin of hard parts.

Developmental mechanisms

Of the second class of explanation, interest has centred on the timing of acquisition of the homeotic genes that all animals seem to possess and use to a greater or lesser extent in laying out their body architecture during development. It has been argued that the radiation of animals could not take place before a certain minimum complexity of such genes had been acquired, to give them the necessary genetic toolbox for subsequent diversification. Clearly, the evolution of development is critical in the history of the animals. However, it is currently difficult to disentangle the origins of bilaterian genetic architectures from their morphological diversification. Recent studies seem to suggest that the genes responsible for bilaterian development were largely present before they radiated, although it is quite possible that they were performing somewhat differing tasks at this time, later being co-opted into the classical patterns of bilaterian development.

Ecological explanations

In addition, several recent examinations of the Cambrian explosion have suggested that ecological diversification is the primary motor for the Cambrian explosion: even that the Cambrian explosion represents nothing more than ecological diversification. Given the evolution of multicellularity in heterotrophic organisms, it could be argued, a dynamic would be set up that would inevitably lead to the familiar food webs consisting of primary and secondary consumers, parasites, and (especially with the advent of mobility) deposit feeding and trophic recuperation. While it has been claimed that certain “key innovations” – most notably the origin of sight, by Parker – were critical in driving the whole process decisively forward, most of these can themselves be seen as products of earlier ecological pressure. In this view, the Cambrian become the first and most spectacular “adaptive radiation” as posited for evolution in general by especially G.G. Simpson.

Timing of the Cambrian Explosion

Assuming that the Cambrian explosion was a real event that occurred broadly as outlined above, there still remains the question of why it occurred precisely when it did. Two broad possibilities exist.

Artist’s impression of an impact event

The first is that the origin of heterotrophic multicellularity was prompted either by climatic change, or by some other trigger. A popular example of the latter would be a meteoritic impact (the Australian Acraman crater, dated to 578 million years old, has been seen as a potential suspect) or some sort of other disastrous ecological collapse. With analogy to the supposed “take-over” by mammals after the extinction of the non-avian dinosaurs at the K-T boundary, the destruction of previous ecological systems allowed the animals to gain the ecological advantage and radiate spectacularly. For a long time, such a view was broadly supported by the evidence that the Ediacaran organisms seemed to go extinct some distance before the base of the Cambrian. More recently, however, this gap has been closed, and indeed surviving Ediacaran taxa have now been reported from the Cambrian itself. Nevertheless, some taxa such as Namacalathus do seem to vanish at this point, and the idea of faunal replacement, as opposed to simple development, cannot be ruled out.

Secondly, there is the view that the Cambrian explosion took place when it did simply because many other events had to take place first. Butterfield, for example, has argued that the presence of animals, with their vigorous ability to move about and prey on other organisms, would have sped up general ecological evolution by a factor of about ten. Indeed, if one shrinks Proterozoic history by this factor, then the time from the origin of the eukaryotes to that of the bilaterian animals then looks like a simple radiation with no undue “delay”. In any event, evolution of complex multicellular heterotrophs clearly massively impacted the biosphere, and a strong, or perhaps even dominant purely ecological component cannot be ruled out in any attempt at explaining this remarkable period in the history of Earth.

See also

• Burgess Shale
• Ediacaran biota

Further reading

• Budd, G. E. & Jensen, J. (2000). A critical reappraisal of the fossil record of the bilaterian phyla. Biological Reviews 75: 253–295.
• Collins, Allen G. “Metazoa: Fossil record”. Retrieved Dec. 14, 2005.
• Conway Morris, S. (1997). The Crucible of Creation: the Burgess Shale and the rise of animals. Oxford University Press. ISBN 0-19-286202-2
• Conway Morris, S. (2006). "Darwin's dilemma: the realities of the Cambrian 'explosion'" (PDF). Philosophical Transactions of the Royal Society B: Biological Sciences. 361 (1470): 1069–1083. An enjoyable account.
• Kennedy, M., M. Droser, L. Mayer., D. Pevear, and D. Mrofka (2006). "Clay and Atmospheric Oxygen". Science. 311 (5766): 1341. doi:10.1126/science.311.5766.1341c.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Knoll,A. H. and Carroll, S. B. (1999). Early Animal Evolution: Emerging Views from Comparative Biology and Geology. Science 284 (5423): 2129 – 2137.
• Alexander V. Markov, and Andrey V. Korotayev (2007) “Phanerozoic marine biodiversity follows a hyperbolic trend” Palaeoworld 16(4): pp. 311-318.
• Parker, A. (2004). In the Blink of an Eye, Free Press, ISBN 0-7432-5733-2.
• Wang, D. Y.-C., S. Kumar and S. B. Hedges (1999). "Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi". Proceedings of the Royal Society of London, Series B, Biological Sciences. 266 (1415): 163–71. doi:10.1098/rspb.1999.0617.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Xiao, S., Y. Zhang, and A. Knoll (1998). "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite". Nature. 391: 553–58. doi:10.1038/35318.{{cite journal}}: CS1 maint: multiple names: authors list (link)

Timeline References:

• Gradstein and Ogg, “A Phanerozoic time scale”, v.19, no.1&2., 1996.
• Martin, M.W.; Grazhdankin, D.V.; Bowring, S.A.; Evans, D.A.D.; Fedonkin, M.A.; Kirschvink, J.L. (2000). "Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution". Science. 288: 841–845.{{cite journal}}: CS1 maint: multiple names: authors list (link)

External links

• The Cambrian “explosion” of metazoans and molecular biology: would Darwin be satisfied?
• On embryos and ancestors by Stephen Jay Gould
• The Cambrian “explosion”: Slow-fuse or megatonnage?
• The Cambrian Explosion – In Our Time, BBC Radio 4 broadcast, 17 February 2005