Everything about Cambrian Explosion totally explained
The
Cambrian explosion or
Cambrian radiation was the seemingly rapid appearance of most major groups of complex
animals around, as evidenced by the
fossil record. This was accompanied by a major diversification of other organisms. Before about, 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 theory 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 implies about the origin and possible evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures left in Cambrian rocks.
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.
American palæontologist
Charles Walcott, who extensively studied
North American fossil animals, proposed that an interval of time, the “Lipalian”, wasn't represented in the fossil record or didn't 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
phyla 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.
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)
Therefore 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 significant parts of organisms and are therefore the most informative type of evidence. Unfortunately they're increasingly rare as one looks further back in time, among other reasons because the rocks in which they're buried are usually covered by more recent rocks and because they may have been
eroded before being covered by later 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's 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.
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're 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.
The Cambrian fossil record includes an unusually high number of
lagerstätten which preserved the fossils' soft tissues in extremely fine detail. This has 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're far from perfect. The majority of then-living animals are probably not represented because lagerstätten are restricted to a narrow range of environments (for example where soft-bodied organisms can be preserved very quickly by processes such as 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 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 isn't 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:.
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.
Phylum is the highest level in the
Linnean system for classifying animals. Phyla can be thought of as groupings of animals based on general body plan. Despite the seemingly different
external appearances of organisms, they're classified into phyla based on their
internal organizations. For example despite their obvious differences
spiders and
crabs both belong to the phylum
Arthropoda; but
earthworms and
tapeworms, although similar in shape, are members of the
Annelida and
Platyhelminthes respectively.
But the word "phylum" doesn't describe a fundamental division of nature (not like the difference between electrons and protons). It simply refers to a very high level in the classification system created by Linnaeus in the 18th century to describe all the animals which are alive to-day. This system isn't perfect 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. Classification systems based on living organisms, including Linneus', do not accommodate extinct organisms well, or even at all.
Triploblastic means consisting of 3 layers, which are formed
in the
embryo (quite early in the animal's development from a single-celled egg to a larva or juvenile form). The innermost layer forms the
digestive tract (gut); the outermost forms skin; and the middle one forms muscles and all the internal organs except the digestive system. Most types of living animal are triploblastic – the best-known exceptions are
Porifera (sponges) and
Cnidaria (jellyfish, sea anemones, etc.).
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).
Coelomate means having a body cavity (coelom) which contains the internal organs. Most of the phyla featured in the debate about the Cambrian explosion are coelomates:
arthropods,
annelid worms,
molluscs,
echinoderms and
chordates (which includes us vertebrates) - the non-coelomate
priapulids are an important exception. All coelomate animals are triploblastic, but some triploblastic animals don't have a coelom (for example
flatworms; their organs are surrounded by
unspecialized tissues). Some bilaterian animals are not coelomates (for example
flatworms). Echinoderms are coelomates; living species don't look bilaterian (they are radially symmetrical, although
sea cucumbers) have distinct front and rear ends), but the earliest echinoderms are still poorly understood and some may have been bilaterally symmetrical.
Decline of stromatolites over 1 billion years ago
Stromatolites are not organisms, they're 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're 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.
Increase in size 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, when all or very nearly all of the Earth's surface was covered by ice or snow, but they reached their highest diversity in the
Paleozoic era. Their increasingly spiny forms in the last 1 billion years possibly resulted from 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.
Further evidence that predation, or at least herbivory, on plankton first appeared around this time comes from a consideration of taxon longevity. The abundance of planktonic organimsms that evolved between 1,700 and 1,400 million years ago were limited by nutrient availability - a situation which limits the origination of new species because the existing organisms are so specialised to their niches, and no other niches are available for occupation. Around about 1,000 million years ago, species longevity fell sharply, suggesting that predation pressure, probably by protist herbivores, became an important factor. Predation would have kept populations in check, meaning that some nutrients were left unused, and new niches were available for new species to occupy.
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.
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's visible to the naked eye.
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 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 doesn't 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.
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 may have been a
mollusc (see below),
Spriggina was possibly a
trilobite and therefore an
arthropod, but its body segments seem to be offset across the midline rather than being symmetrically paired as as they're in all known arthropods;
Parvancorina is perhaps a more promising example of an early 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's still much debate about how to classify it.
More importantly it was one of the earliest animals to have a calcareous shell,
for example 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 don't attack empty shells). The rather similar shelly fossil
Sinotubulites, which appears in the same fossil beds, wasn't affected by borings. In addition, the distribution of borings suggests selection for size. This evidence of selective attacks by predators shows the possibility of speciation in response to predation, which is often suggested as a potential cause of the Cambrian explosion.
Mollusc-like animals 555 million years ago
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's 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”.
Ediacaran and Early Cambrian diversification of trace fossils
The earliest Ediacaran fossils (Assemblage 1 above), 610-600M years ago, contain only
cnidarian resting traces. Around 565M years ago (Ediacaran Assemblage 2 above) more complex trace fossils appear, which require a body plan with a
hydrostatic skeleton against which muscles pull, for example more complex body structures than those of cnidarians or
flatworms.
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
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 wasn't 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's 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's 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.
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).
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 couldn't 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 uniramians 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.
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, for example fairly closely related to the ancestors of molluscs (see above).
Molluscs, annelids or brachiopods?
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
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).
Indeed, Cambrian and Ordovician arthropod communities were no less disparate than today's.
There was a mass extinction at the Cambrian-Ordovician boundary, and typical Paleozoic marine diversity and ecosystems appear during the recovery from the extinction. A later study in 1998 found flaws in the first one and concluded that protostomes diverged from deuterostomes about 670M years ago and that chordates diverged from echinoderms about 600M years ago.
There is still debate about the interpretation of data from molecular phylogenetics. For example: one analysis in 2003 concluded that protostomes and deuterostomes diverged 582 ± 112 M years ago (note the wide margin of uncertainty; for example 582-112 = 470M years ago, after the end of the Cambrian); another in April 2004 concluded that the last common ancestor of bilaterians arose between 573M and 656M years ago, for example around the start of the Ediacaran period; and a third in November 2004 concluded that the 2 previous ones were faulty and that protostomes and deuterostomes diverged 786M to 1,166M years ago, for example well before the start of the Ediacaran 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. 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 (extending into the early 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's 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. On the other hand they may have delayed the evolution of existing metazoans to larger sizes. 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's 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. But this hypothesis also fails explain the increase in disparity. 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 doesn't 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'll be very difficult to find fossils of the last common ancestor of all bilaterians. (rather like the way you can deal a larger number of unique hands if you increase the number of different 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" isn't 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.
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.
But there's 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's unlikely that predation triggered the Cambrian "explosion", although it very likely had a strong influence on the body forms that the "explosion" produced. But it's now thought that "cropping" arose before 1 billion years ago, as stromatolites began to decline about 1.25 billion years ago.
Theoretical explanations
Several scientists have produced theoretical models of what might have caused the Cambrian explosion. Of course these models can't 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 little or no competition for the ecological niche that it's 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.
Further Information
Get more info on 'Cambrian Explosion'.
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