The CAMBRIAN EXPLOSION or CAMBRIAN RADIATION was the relatively
short span event, occurring approximately 541 million years ago in
Cambrian period , during which most major animal phyla appeared,
as indicated by the fossil record. Lasting for about the next 20
–25 million years, it resulted in the divergence of most modern
metazoan phyla. Additionally, the event was accompanied by major
diversification of other organisms. Prior to the
most organisms were simple, composed of individual cells occasionally
organized into colonies . Over the following 70 to 80 million years,
the rate of diversification accelerated by an order of magnitude and
the diversity of life began to resemble that of today. Almost all
present animal phyla appeared during this period. There is strong
evidence for species of
Porifera existing in the
Ediacaran and possible members of
Porifera even before that during
Cryogenian . Bryozoans don't appear in the fossil record until
after the Cambrian, in the Lower
Cambrian explosion has generated extensive scientific debate. The
seemingly rapid appearance of fossils in the "Primordial Strata" was
William Buckland in the 1840s, and in 1859 Charles Darwin
discussed it as one of the main objections that could be made against
the theory of evolution by natural selection . The long-running
puzzlement about the appearance of the
Cambrian fauna , seemingly
abruptly, without precursor, centers on three key points: whether
there really was a mass diversification of complex organisms over a
relatively short period of time during the early Cambrian; what might
have caused such rapid change; and what it would imply about the
origin of animal life. Interpretation is difficult due to a limited
supply of evidence, based mainly on an incomplete fossil record and
chemical signatures remaining in
Phylogenetic analysis has been used to support the view that during
Cambrian explosion, metazoans (multi-celled animals) evolved
monophyletically from a single common ancestor: flagellated colonial
protists similar to modern choanoflagellates . Key
events view • discuss • edit -590 — – -580 — – -570 —
– -560 — – -550 — – -540 — – -530 — – -520 — –
-510 — – -500 — – -490 — – N
c EDIACARAN C
n ORDOVICIAN T
"Stage 2 " "Stage 3 " "Stage 4 " "Stage 5 "
Jiangshanian "Stage 10 "
Biota * * * * * * * * * * * * * * * * * * * * * * * * * Baykonur
glaciation * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * * * * * * *
* * * * * * * * * * * * * * * * * * * * * * * * * ← Orsten
Burgess Shale ←
Kaili biota ← Archaeocyatha
extinction ← Emu Bay Shale ←
Sirius Passet biota ←
Chengjiang biota ← First arthropods with mineralized carapace
Trilobites ) ← SSF diversification, first brachiopods
font-size:100%; position:absolute; text-align:center;
margin-top:32.196261682243em;"> ← First halkieriids , mollusсs ,
hyoliths SSF ← _
Treptichnus pedum _
Large negative peak δ 13Ccarb excursion ← First _
font-size:100%; position:absolute; text-align:center;
Mollusc -like _
Kimberella _ and
its trace fossils ← Gaskiers glaciation
_Archaeonassa_-type trace fossils
* 1 History and significance
* 1.1 Dating the
* 1.2 Body fossils
* 1.3 Trace fossils
* 1.5 Phylogenetic techniques
* 2 Explanation of key scientific terms
* 3 Precambrian life
* 3.1 Evidence of animals around 1 billion years ago
* 3.2 Fossils of the
* 3.3 Burrows
* 3.5 Beck Spring Dolomite
* 4 Ediacaran–Early
* 5.1 Trace fossils
Small shelly fauna
* 5.3 Early
Cambrian trilobites and echinoderms
Burgess Shale type faunas
* 5.5 Early
* 5.6 Early
* 6 Stages
* 7 How real was the explosion?
* 8 Possible causes
* 8.1 Changes in the environment
* 8.1.1 Increase in oxygen levels
* 8.1.2 Ozone formation
* 8.1.4 Increase in the calcium concentration of the Cambrian
* 8.2 Developmental explanations
* 8.3 Ecological explanations
* 8.3.1 End-
Ediacaran mass extinction
* 8.3.2 Evolution of eyes
* 8.3.3 Arms races between predators and prey
* 8.3.4 Increase in size and diversity of planktonic animals
* 8.4 Ecosystem engineering
* 8.5 Complexity threshold
* 9 Uniqueness of the explosion
* 10 See also
* 11 Notes
* 11.1 Footnotes
* 11.2 Citations
* 12 Further reading
* 13 External links
HISTORY AND SIGNIFICANCE
Life timeline view • discuss • edit -4500 — – -4000 —
– -3500 — – -3000 — – -2500 — – -2000 — – -1500 —
– -1000 — – -500 — – 0 — _WATER _ Single-celled
life _PHOTOSYNTHESIS _ EUKARYOTES Multicellular
life LAND LIFE DINOSAURS MAMMALS FLOWERS ←
Earliest Earth (−4540 ) ← Earliest water ← Earliest
life ← LHB meteorites ← Earliest oxygen ←
Atmospheric oxygen ←
Oxygen crisis ← Earliest sexual
Ediacara biota ←
← Earliest humans P
n Pongola Huronian
Cryogenian Andean Karoo Quaternary
Axis scale : millions of years .
Orange labels: known _ICE AGES_.
Also see: _
Human timeline _ and _Nature timeline _ Main article:
Evolutionary history of life
The first discovered
Cambrian fossils were trilobites , described by
Edward Lhuyd , the curator of
Oxford Museum , in 1698. Although their
evolutionary importance was not known, on the basis of their old age,
William Buckland (1784–1856) realised that a dramatic step-change in
the fossil record had occurred around the base of what we now call the
Cambrian. Nineteenth-century geologists such as
Adam Sedgwick and
Roderick Murchison used the fossils for dating rock strata,
specifically for establishing the
Silurian periods. By
1859, leading geologists including Roderick Murchison, were convinced
that what was then called the lowest
Silurian stratum showed the
origin of life on Earth, though others, including
Charles Lyell ,
differed. In _
On the Origin of Species
On the Origin of Species _,
Charles Darwin considered
this sudden appearance of a solitary group of trilobites, with no
apparent antecedents, and absence of other fossils, to be "undoubtedly
of the gravest nature" among the difficulties in his theory of natural
selection. He reasoned that earlier seas had swarmed with living
creatures, but that their fossils had not been found due to the
imperfections of the fossil record. In the sixth edition of his book,
he stressed his problem further as:
To the question why we do not find rich fossiliferous deposits
belonging to these assumed earliest periods prior to the Cambrian
system, I can give no satisfactory answer.
American paleontologist Charles Walcott , who studied the Burgess
Shale fauna , proposed that an interval of time, the "Lipalian", was
not represented in the fossil record or did not preserve fossils, and
that the ancestors of the
Cambrian animals evolved during this time.
Earlier fossil evidence has since been found. The earliest claim is
that the history of life on earth goes back 3,850 million years:
Rocks of that age at
Warrawoona, Australia , were claimed to contain
fossil stromatolites , stubby pillars formed by colonies of
microorganisms . Fossils (_
Grypania _) of more complex eukaryotic
cells, from which all animals, plants, and fungi are built, have been
found in rocks from 1,400 million years ago , in
Rocks dating from 580 to 543 million years ago contain fossils of the
Ediacara biota , organisms so large that they are likely multicelled,
but very unlike any modern organism. In 1948,
Preston Cloud argued
that a period of "eruptive" evolution occurred in the Early Cambrian,
but as recently as the 1970s, no sign was seen of how the 'relatively'
modern-looking organisms of the Middle and Late
Cambrian arose. _
Opabinia _ made the largest single contribution to modern interest in
The intense modern interest in this "
Cambrian explosion" was sparked
by the work of
Harry B. Whittington and colleagues, who, in the 1970s,
reanalysed many fossils from the
Burgess Shale and concluded that
several were complex, but different from any living animals. The
most common organism, _
Marrella _, was clearly an arthropod , but not
a member of any known arthropod class . Organisms such as the
Opabinia _ and spiny slug-like _
Wiwaxia _ were so different
from anything else known that Whittington's team assumed they must
represent different phyla, seemingly unrelated to anything known
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 almost simultaneously in a rather
short span of geological period. This view led to the modernization of
Darwin's tree of life and the theory of punctuated equilibrium , which
Eldredge and Gould developed in the early 1970s and which views
evolution as long intervals of near-stasis "punctuated" by short
periods of rapid change.
Other analyses, some more recent and some dating back to the 1970s,
argue that complex animals similar to modern types evolved well before
the start of the Cambrian.
DATING THE CAMBRIAN
Radiometric dates for much of the Cambrian, obtained by analysis of
radioactive elements contained within rocks, have only recently become
available, and for only a few regions.
Relative dating (_A_ was before _B_) is often assumed sufficient for
studying processes of evolution, but this, too, has been difficult,
because of the problems involved in matching up rocks of the same age
across different continents .
Therefore, dates or descriptions of sequences of events should be
regarded with some caution until better data become available.
Fossils of organisms' bodies are usually the most informative type of
evidence. Fossilization is a rare event, and most fossils are
destroyed by erosion or metamorphism before they can be observed.
Hence, the fossil record is very incomplete, increasingly so as
earlier times are considered. Despite this, they are often adequate to
illustrate the broader patterns of life's history. Also, biases exist
in the fossil record: different environments are more favourable to
the preservation of different types of organism or parts of organisms.
Further, only the parts of organisms that were already mineralised
are usually preserved, such as the shells of molluscs . Since most
animal species are soft-bodied, they decay before they can become
fossilised. As a result, although 30-plus phyla of living animals are
known, two-thirds have never been found as fossils. _ This
Marrella _ specimen illustrates how clear and detailed the fossils
Cambrian fossil record includes an unusually high number of
lagerstätten , which preserve soft tissues. These allow
paleontologists to examine the internal anatomy of animals, which in
other sediments are only represented by shells, spines, claws, etc.
– if they are preserved at all. The most significant Cambrian
lagerstätten are the early
Maotianshan shale beds of
China ) and
Sirius Passet (
Greenland ); the
Burgess Shale (
British Columbia ,
Canada ); and the
Sweden ) fossil beds.
While lagerstätten preserve far more than the conventional fossil
record, they are far from complete. Because lagerstätten are
restricted to a narrow range of environments (where soft-bodied
organisms can be preserved very quickly, e.g. by mudslides), most
animals are probably not represented; further, the exceptional
conditions that create lagerstätten probably do not represent normal
living conditions. In addition, the known
Cambrian lagerstätten are
rare and difficult to date, while Precambrian lagerstätten have yet
to be studied in detail.
The sparseness of the fossil record means that organisms usually
exist long before they are found in the fossil record – this is
known as the
Signor–Lipps effect .
_ Rusophycus_ and other trace fossils from the
Gog Group , Middle
Cambrian , Lake Louise , Alberta,
Trace fossils consist mainly of tracks and burrows, but also include
coprolites (fossil feces ) and marks left by feeding. Trace fossils
are particularly significant because they represent a data source that
is not limited to animals with easily fossilized hard parts, and
reflects organisms' behaviour. Also, many traces date from
significantly earlier than the body fossils of animals that are
thought to have been capable of making them. While exact assignment
of trace fossils to their makers is generally impossible, traces may,
for example, provide the earliest physical evidence of the appearance
of moderately complex animals (comparable to earthworms ).
Main article: Early
Cambrian geochemical fluctuations
Several chemical markers indicate a drastic change in the environment
around the start of the Cambrian. The markers are consistent with a
mass extinction, or with a massive warming resulting from the
release of methane ice . Such changes may reflect a cause of the
Cambrian explosion, although they may also have resulted from an
increased level of biological activity – a possible result of the
explosion. Despite these uncertainties, the geochemical evidence
helps by making scientists focus on theories that are consistent with
at least one of the likely environmental changes.
Cladistics is a technique for working out the "family tree" of a set
of organisms. It works by the logic that, if groups B and C have more
similarities to each other than either has to group A, then B and C
are more closely related to each other than either is to A.
Characteristics that are compared may be anatomical , such as the
presence of a notochord , or molecular , by comparing sequences of DNA
or protein . The result of a successful analysis is a hierarchy of
clades – groups whose members are believed to share a common
ancestor. The cladistic technique is sometimes problematic, as some
features, such as wings or camera eyes , evolved more than once,
convergently – this must be taken into account in analyses.
From the relationships, it may be possible to constrain the date that
lineages first appeared. For instance, if fossils of B or C date to X
million years ago and the calculated "family tree" says A was an
ancestor of B and C, then A must have evolved more than X million
It is also possible to estimate how long ago two living clades
diverged – i.e. about how long ago their last common ancestor must
have lived – by assuming that
DNA mutations accumulate at a constant
rate. These "molecular clocks ", however, are fallible, and provide
only a very approximate timing: they are not sufficiently precise and
reliable for estimating when the groups that feature in the Cambrian
explosion first evolved, and estimates produced by different
techniques vary by a factor of two. However, the clocks can give an
indication of branching rate, and when combined with the constraints
of the fossil record, recent clocks suggest a sustained period of
diversification through the
Ediacaran and Cambrian.
EXPLANATION OF KEY SCIENTIFIC TERMS
* — = Lines of descent
* = Basal node
* = Crown node
* = Total group
* = Crown group
* = Stem group
A phylum is the highest level in the Linnaean system for classifying
organisms . Phyla can be thought of as groupings of animals based on
general body plan. Despite the seemingly different external
appearances of organisms, they are classified into phyla based on
their internal and developmental organizations. For example, despite
their obvious differences, spiders and barnacles both belong to the
phylum Arthropoda, but earthworms and tapeworms , although similar in
shape, belong to different phyla. As chemical and genetic testing
becomes more accurate, previously hypothesised phyla are often
A phylum is not a fundamental division of nature, such as the
difference between electrons and protons . It is simply a very
high-level grouping in a classification system created to describe all
currently living organisms. This system is imperfect, even for modern
animals: different books quote different numbers of phyla, mainly
because they disagree about the classification of a huge number of
worm-like species. As it is based on living organisms, it accommodates
extinct organisms poorly, if at all.
The concept of stem groups was introduced to cover evolutionary
"aunts" and "cousins" of living groups, and have been hypothesized
based on this scientific theory. A crown group is a group of closely
related living animals plus their last common ancestor plus all its
descendants. A stem group is a set of offshoots from the lineage at a
point earlier than the last common ancestor of the crown group; it is
a relative concept, for example tardigrades are living animals that
form a crown group in their own right, but Budd (1996) regarded them
as also being a stem group relative to the arthropods. Skin
(endoderm) A coelomate animal is basically a set of concentric
tubes, with a gap between the gut and the outer tubes.
The term _
Triploblastic _ means consisting of three 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.).
The bilaterians are animals that have right and left sides at some
point in their life histories. This implies that they have top and
bottom surfaces and, importantly, distinct front and back ends. All
known bilaterian animals are triploblastic, and all known
triploblastic animals are bilaterian. Living echinoderms (sea stars ,
sea urchins , sea cucumbers , etc.) 'look' radially symmetrical (like
wheels) rather than bilaterian, but their larvae exhibit bilateral
symmetry and some of the earliest echinoderms may have been
Cnidaria are radially
symmetrical, not bilaterian, and not triploblastic.
The term _
Coelomate _ means having a body cavity (coelom) containing
the internal organs. Most of the phyla featured in the debate about
Cambrian explosion are coelomates: arthropods, annelid worms,
molluscs, echinoderms, and chordates – the noncoelomate priapulids
are an important exception. All known coelomate animals are
triploblastic bilaterians, but some triploblastic bilaterian animals
do not have a coelom – for example flatworms , whose organs are
surrounded by unspecialized tissues .
Understanding of the
Cambrian explosion relies upon knowing what was
there beforehand – did the event herald the sudden appearance of a
wide range of animals and behaviours, or did such things exist
EVIDENCE OF ANIMALS AROUND 1 BILLION YEARS AGO
_For further information, see
Stromatolites (Pika Formation, Middle Cambrian) near Helen Lake, Banff
National Park ,
Canada Modern stromatolites in Hamelin Pool
Marine Nature Reserve , Western Australia
Changes in the abundance and diversity of some types of fossil have
been interpreted as evidence for "attacks" by animals or other
organisms. Stromatolites, stubby pillars built by colonies of
microorganisms , are a major constituent of the fossil record from
about 2,700 million years ago , but their abundance and diversity
declined steeply after about 1,250 million years ago . This decline
has been attributed to disruption by grazing and burrowing animals.
Our picture of Precambrian marine diversity is dominated by small
fossils known as acritarchs . This term describes almost any small
organic walled fossil – from the egg cases of small metazoans to
resting cysts of many different kinds of green algae . After appearing
around 2,000 million years ago , acritarchs underwent a boom around
1,000 million years ago , increasing in abundance, diversity, size,
complexity of shape, and especially size and number of spines. Their
increasingly spiny forms in the last 1 billion years may indicate an
increased need for defence against predation. Other groups of small
organisms from the
Neoproterozoic era also show signs of antipredator
defenses. A consideration of taxon longevity appears to support an
increase in predation pressure around this time. In general, the
fossil record shows a very slow appearance of these lifeforms in the
Precambrian, with many cyanobacterial species making up much of the
FOSSILS OF THE DOUSHANTUO FORMATION
The layers of the
Doushantuo formation from around 580 million year
old harbour microscopic fossils that may represent early bilaterians.
Some have been described as animal embryos and eggs, although some may
represent the remains of giant bacteria. Another fossil,
Vernanimalcula _, has been interpreted as a coelomate bilaterian,
but may simply be an infilled bubble.
These fossils form the earliest hard-and-fast evidence of animals, as
opposed to other predators.
Cambrian substrate revolution An
fossil, made when an organism burrowed below a microbial mat .
The traces of organisms moving on and directly underneath the
microbial mats that covered the
Ediacaran sea floor are preserved from
Ediacaran period, about 565 million years ago . They were
probably made by organisms resembling earthworms in shape, size, and
how they moved. The burrow-makers have never been found preserved,
but, because they would need a head and a tail, the burrowers probably
had bilateral symmetry – which would in all probability make them
bilaterian animals. They fed above the sediment surface, but were
forced to burrow to avoid predators.
Around the start of the
Cambrian (about 542 million years ago ), many
new types of traces first appear, including well-known vertical
burrows such as _
Diplocraterion _ and _
Skolithos _, and traces
normally attributed to arthropods, such as _
Cruziana _ and _Rusophycus
_. The vertical burrows indicate that worm-like animals acquired new
behaviours, and possibly new physical capabilities. Some Cambrian
trace fossils indicate that their makers possessed hard exoskeletons ,
although they were not necessarily mineralised.
Burrows provide firm evidence of complex organisms; they are also
much more readily preserved than body fossils, to the extent that the
absence of trace fossils has been used to imply the genuine absence of
large, motile, bottom-dwelling organisms. They provide a further line
of evidence to show that the
Cambrian explosion represents a real
diversification, and is not a preservational artefact.
Indeed, as burrowing became established, it allowed an explosion of
its own, for as burrowers disturbed the sea floor, they aerated it,
mixing oxygen into the toxic muds. This made the bottom sediments more
hospitable, and allowed a wider range of organisms to inhabit them –
creating new niches and the scope for higher diversity.
Dickinsonia costata _, an
Ediacaran organism of unknown
affinity, with a quilted appearance Main articles:
Ediacara biota ,
Kimberella , and
At the start of the
Ediacaran period, much of the acritarch fauna,
which had remained relatively unchanged for hundreds of millions of
years, became extinct, to be replaced with a range of new, larger
species, which would prove far more ephemeral. This radiation, the
first in the fossil record, is followed soon after by an array of
unfamiliar, large, fossils dubbed the Ediacara biota, which
flourished for 40 million years until the start of the Cambrian. Most
of this "Ediacara biota" were at least a few centimeters long,
significantly larger than any earlier fossils. The organisms form
three distinct assemblages, increasing in size and complexity as time
Many of these organisms were quite 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. _
Kimberella _, a triploblastic bilaterian, and possibly a
At least some may have been early forms of the phyla at the heart of
Cambrian explosion" debate, having been interpreted as early
Kimberella _), echinoderms (_
Arkarua _); and arthropods
Spriggina _, _
Parvancorina _). Still, debate exists about the
classification of these specimens, mainly because the diagnostic
features that allow taxonomists to classify more recent organisms,
such as similarities to living organisms, are generally absent in the
ediacarans. However, there seems little doubt that _Kimberella_ was
at least a triploblastic bilaterian animal. These organisms are
central to the debate about how abrupt the
Cambrian explosion was. If
some were early members of the animal phyla seen today, the
"explosion" looks a lot less sudden than if all these organisms
represent an unrelated "experiment", and were replaced by the animal
kingdom fairly soon thereafter (40M years is "soon" by evolutionary
and geological standards).
BECK SPRING DOLOMITE
Paul Knauth, a geologist at
Arizona State University , maintains that
photosynthesizing organisms such as algae, may have grown over a 750-
to 800-million-year-old formation in
Death Valley known as the Beck
Spring Dolomite. In the early 1990s, samples from this 1,000-foot
thick layer of dolomite revealed that the region housed flourishing
mats of photosynthesizing, unicellular life forms which antedated the
Microfossils have been unearthed from holes riddling the otherwise
barren surface of the dolomite. These geochemical and microfossil
findings support the idea that during the Precambrian period, complex
life evolved both in the oceans and on land. Knauth contends that
animals may well have had their origins in freshwater lakes and
streams, and not in the oceans.
Some 30 years later, a number of studies have documented an abundance
of geochemical and microfossil evidence showing that life covered the
continents as far back as 2.2 billion years ago. Many paleobiologists
now accept the idea that simple life forms existed on land during the
Precambrian, but are opposed to the more radical idea that
multicellular life thrived on land more than 600 million years ago.
EDIACARAN–EARLY CAMBRIAN SKELETONISATION
Ediacaran and lowest
Nemakit-Daldynian ) skeletal
fossils represent tubes and problematic sponge spicules. The oldest
sponge spicules are monaxon siliceous, aged around 580 million years
ago , known from the Doushantou Formation in
China and from deposits
of the same age in Mongolia, although the interpretation of these
fossils as spicules has been challenged. In the late Ediacaran-lowest
Cambrian, numerous tube dwellings of enigmatic organisms appeared. It
was organic-walled tubes (e.g. _
Saarina _) and chitinous tubes of the
sabelliditids (e.g. _Sokoloviina_, _Sabellidites_, _Paleolina_) that
prospered up to the beginning of the
Tommotian . The mineralized tubes
Cloudina _, _
Namacalathus _, _
Sinotubulites _, and a dozen more of
the other organisms from carbonate rocks formed near the end of the
Ediacaran period from 549 to 542 million years ago , as well as the
triradially symmetrical mineralized tubes of anabaritids (e.g.
Anabarites _, _Cambrotubulus_) from uppermost
Ediacaran and lower
Ediacaran mineralized tubes are often found in carbonates
of the stromatolite reefs and thrombolites , i.e. they could live in
an environment adverse to the majority of animals.
Although they are as hard to classify as most other Ediacaran
organisms, they are important in two other ways. First, they are the
earliest known calcifying organisms (organisms that built shells from
calcium carbonate ). Secondly, these tubes are a device to rise
over a substrate and competitors for effective feeding and, to a
lesser degree, they serve as armor for protection against predators
and adverse conditions of environment. Some _Cloudina_ fossils show
small holes in shells. The holes possibly are evidence of boring by
predators sufficiently advanced to penetrate shells. A possible
"evolutionary arms race " between predators and prey is one of the
hypotheses that attempt to explain the
In the lowest Cambrian, the stromatolites were decimated. This
allowed animals to begin colonization of warm-water pools with
carbonate sedimentation. At first, it was anabaritids and
Protohertzina _ (the fossilized grasping spines of chaetognaths )
fossils. Such mineral skeletons as shells, sclerites, thorns, and
plates appeared in uppermost
Nemakit-Daldynian ; they were the
earliest species of halkierids , gastropods , hyoliths and other rare
organisms. The beginning of the
Tommotian has historically been
understood to mark an explosive increase of the number and variety of
fossils of molluscs, hyoliths , and sponges , along with a rich
complex of skeletal elements of unknown animals, the first
archaeocyathids , brachiopods , tommotiids , and others. This
sudden increase is partially an artefact of missing strata at the
Tommotian type section, and most of this fauna in fact began to
diversify in a series of pulses through the
Nemakit-Daldynian and into
Some animals may already have had sclerites, thorns, and plates in
Ediacaran (e.g. _Kimberella_ had hard sclerites, probably of
carbonate), but thin carbonate skeletons cannot be fossilized in
siliciclastic deposits. Older (~750 Ma) fossils indicate that
mineralization long preceded the Cambrian, probably defending small
photosynthetic algae from single-celled eukaryotic predators.
Trace fossils (burrows, etc.) are a reliable indicator of what life
was around, and indicate a diversification of life around the start of
the Cambrian, with the freshwater realm colonized by animals almost as
quickly as the oceans.
SMALL SHELLY FAUNA
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
Tommotian ages; see timeline ). These are a very mixed collection
of fossils: spines, sclerites (armor plates), tubes, archeocyathids
(sponge-like animals), and small shells very like those of brachiopods
and snail-like molluscs – but all tiny, mostly 1 to 2 mm long.
While small, these fossils are far more common than complete fossils
of the organisms that produced them; crucially, they cover the window
from the start of the
Cambrian to the first lagerstätten: a period of
time otherwise lacking in fossils. Hence, they supplement the
conventional fossil record and allow the fossil ranges of many groups
to be extended.
EARLY CAMBRIAN TRILOBITES AND ECHINODERMS
A fossilized trilobite, an ancient type of arthropod : This
specimen, from the Burgess Shale, preserves "soft parts" – the
antennae and legs.
The earliest trilobite fossils are about 530 million years old, but
the class was already quite diverse and worldwide, suggesting they had
been around for quite some time. The fossil record of trilobites
began with the appearance of trilobites with mineral exoskeletons –
not from the time of their origin.
The earliest generally accepted echinoderm fossils appeared a little
bit later, in the Late
Atdabanian ; unlike modern echinoderms, these
Cambrian echinoderms were not all radially symmetrical.
These provide firm data points for the "end" of the explosion, or at
least indications that the crown groups of modern phyla were
BURGESS SHALE TYPE FAUNAS
Burgess shale type preservation
Burgess Shale and similar lagerstätten preserve the soft parts
of organisms, which provide a wealth of data to aid in the
classification of enigmatic fossils. It often preserved complete
specimens of organisms only otherwise known from dispersed parts, such
as loose scales or isolated mouthparts. Further, the majority of
organisms and taxa in these horizons are entirely soft-bodied, hence
absent from the rest of the fossil record. Since a large part of the
ecosystem is preserved, the ecology of the community can also be
tentatively reconstructed. However, the assemblages may represent a
"museum": a deep-water ecosystem that is evolutionarily "behind" the
rapidly diversifying fauna of shallower waters.
Because the lagerstätten provide a mode and quality of preservation
that is virtually absent outside of the Cambrian, many organisms
appear completely different from anything known from the conventional
fossil record. This led early workers in the field to attempt to
shoehorn the organisms into extant phyla; the shortcomings of this
approach led later workers to erect a multitude of new phyla to
accommodate all the oddballs. It has since been realised that most
oddballs diverged from lineages before they established the phyla
known today – slightly different designs, which were fated to perish
rather than flourish into phyla, as their cousin lineages did.
The preservational mode is rare in the preceding
but those assemblages known show no trace of animal life – perhaps
implying a genuine absence of macroscopic metazoans.
EARLY CAMBRIAN CRUSTACEANS
For more details on this topic, see
Crustaceans, one of the four great modern groups of arthropods, are
very rare throughout the Cambrian. Convincing crustaceans were once
thought to be common in Burgess Shale-type biotas, but none of these
individuals can be shown to fall into the crown group of "true
Cambrian record of crown-group crustaceans comes
from microfossils. The Swedish
Orsten horizons contain later Cambrian
crustaceans, but only organisms smaller than 2 mm are preserved. This
restricts the data set to juveniles and miniaturised adults.
A more informative data source is the organic microfossils of the
Mount Cap formation , Mackenzie Mountains, Canada. This late Early
Cambrian assemblage (510 to 515 million years ago ) consists of
microscopic fragments of arthropods' cuticle, which is left behind
when the rock is dissolved with hydrofluoric acid . The diversity of
this assemblage is similar to that of modern crustacean faunas. Most
interestingly, analysis of fragments of feeding machinery found in the
formation shows that it was adapted to feed in a very precise and
refined fashion. This contrasts with most other early Cambrian
arthropods, which fed messily by shovelling anything they could get
their feeding appendages on into their mouths. This sophisticated and
specialised feeding machinery belonged to a large (about 30 cm)
organism, and would have provided great potential for diversification;
specialised feeding apparatus allows a number of different approaches
to feeding and development, and creates a number of different
approaches to avoid being eaten.
EARLY ORDOVICIAN RADIATION
After an extinction at the Cambrian-
Ordovician boundary, another
radiation occurred, which established the taxa that would dominate the
During this radiation, the total number of orders doubled, and
families tripled, increasing marine diversity to levels typical of
the Palaeozoic, and disparity to levels approximately equivalent to
Different authors break the explosion down into stages in different
Ed Landing recognizes three stages: Stage 1, spanning the
Cambrian boundary, corresponds to a diversification of
biomineralizing animals and of deep and complex burrows; Stage 2
corresponds to the radiation of molluscs and stem-group Brachiopods
(hyoliths and tommotiids ), which apparently arose in intertidal
waters; and Stage 3 sees the
Atdabanian diversification of trilobites
in deeper waters, but little change in the intertidal realm.
Graham Budd synthesises various schemes to produce a compatible view
of the SSF record of the
Cambrian explosion, divided slightly
differently into four intervals: a "Tube world", lasting from 550 to
536 million years ago , spanning the E-€ boundary, dominated by
Namacalathus ans pseudoconodont-type element; a "Sclerite
world", seeing the rise of halkieriids, tommotiids, and hyoliths,
lasting to the end of the
Fortunian (c. 525 Ma); a brachiopod world,
perhaps corresponding to the as yet unratified
Cambrian Stage 2; and
Trilobite World, kicking off in Stage 3.
Complementary to the shelly fossil record, trace fossils can be
divided into five subdivisions: "Flat world" (late Ediacaran), with
traces restricted to the sediment surface; Protreozoic III (after
Jensen), with increasing complexity; _pedum_ world, initiated at the
base of the
Cambrian with the base of the _T.pedum_ zone (see
discussion at Cambrian#Dating the
Cambrian ); _Rusophycus_ world,
spanning 536 to 521 million years ago and thus corresponding exactly
to the periods of Sclerite World and
Brachiopod World under the SSF
paradigm; and _Cruziana_ world, with an obvious correspondence to
HOW REAL WAS THE EXPLOSION?
The fossil record as Darwin knew it seemed to suggest that the major
metazoan groups appeared in a few million years of the early to
mid-Cambrian, and even in the 1980s, this still appeared to be the
However, evidence of Precambrian Metazoa is gradually accumulating.
Ediacaran _Kimberella_ was a mollusc-like protostome (one of
the two main groups of coelomates ), the protostome and deuterostome
lineages must have split significantly before 550 million years ago
(deuterostomes are the other main group of coelomates). Even if it is
not a protostome, it is widely accepted as a bilaterian. Since
fossils of rather modern-looking cnidarians (jellyfish -like
organisms) have been found in the Doushantuo lagerstätte , the
cnidarian and bilaterian lineages must have diverged well over 580
million years ago .
Trace fossils and predatory borings in _Cloudina_ shells provide
further evidence of
Ediacaran animals. Some fossils from the
Doushantuo formation have been interpreted as embryos and one
Vernanimalcula _) as a bilaterian coelomate, although these
interpretations are not universally accepted. Earlier still,
predatory pressure has acted on stromatolites and acritarchs since
around 1,250 million years ago .
The presence of Precambrian animals somewhat dampens the "bang" of
the explosion; not only was the appearance of animals gradual, but
their evolutionary radiation ("diversification") may also not have
been as rapid as once thought. Indeed, statistical analysis shows that
Cambrian explosion was no faster than any of the other radiations
in animals' history. However, it does seem that some innovations
linked to the explosion – such as resistant armour – only evolved
once in the animal lineage; this makes a lengthy Precambrian animal
lineage harder to defend. Further, the conventional view that all the
phyla arose in the
Cambrian is flawed; while the phyla may have
diversified in this time period, representatives of the crown groups
of many phyla do not appear until much later in the Phanerozoic.
Further, the mineralised phyla that form the basis of the fossil
record may not be representative of other phyla, since most
mineralised phyla originated in a benthic setting. The fossil record
is consistent with a
Cambrian explosion that was limited to the
benthos, with pelagic phyla evolving much later.
Ecological complexity among marine animals increased in the Cambrian,
as well later in the Ordovician. However, recent research has
overthrown the once-popular idea that disparity was exceptionally high
throughout the Cambrian, before subsequently decreasing. In fact,
disparity remains relatively low throughout the Cambrian, with modern
levels of disparity only attained after the early Ordovician
The diversity of many
Cambrian assemblages is similar to today's,
and at a high (class/phylum) level, diversity is thought by some to
have risen relatively smoothly through the Cambrian, stabilizing
somewhat in the Ordovician. This interpretation, however, glosses
over the astonishing and fundamental pattern of basal polytomy and
phylogenetic telescoping at or near the
Cambrian boundary, as seen in
most major animal lineages. Thus
Harry Blackmore Whittington 's
questions regarding the abrupt nature of the
remain, and have yet to be satisfactorily answered.
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. Possible explanations for this fall into three
broad categories: environmental, developmental, and ecological
changes. Any explanation must explain both the timing and magnitude of
CHANGES IN THE ENVIRONMENT
Earth\'s earliest atmosphere contained no free oxygen (O2); the
oxygen that animals breathe today, both in the air and dissolved in
water, is the product of billions of years of photosynthesis .
Cyanobacteria were the first organisms to evolve the ability to
photosynthesize, introducing a steady supply of oxygen into the
environment. Initially, oxygen levels did not increase substantially
in the atmosphere. The oxygen quickly reacted with iron and other
minerals in the surrounding rock and ocean water. Once a saturation
point was reached for the reactions in rock and water, oxygen was able
to exist as a gas in its diatomic form.
Oxygen levels in the
atmosphere increased substantially afterward. As a general trend, the
concentration of oxygen in the atmosphere has risen gradually over
about the last 2.5 billion years.
Oxygen levels seem to have a positive correlation with diversity in
eukaryotes well before the
Cambrian period. The last common ancestor
of all extant eukaryotes is thought to have lived around 1.8 billion
years ago. Around 800 million years ago, there was a notable increase
in the complexity and number of eukaryotes species in the fossil
record. Before the spike in diversity, eukaryotes are thought to have
lived in highly sulfuric environments. Sulfide interferes with
mitochondrial function in aerobic organisms, limiting the amount of
oxygen that could be used to drive metabolism. Oceanic sulfide levels
decreased around 800 million years ago, which supports the importance
of oxygen in eukaryotic diversity.
The shortage of oxygen might well have prevented the rise of large,
complex animals. The amount of oxygen an animal can absorb is largely
determined by the area of its oxygen-absorbing surfaces (lungs and
gills in the most complex animals; the skin in less complex ones);
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 increase the size to which an organism could grow
without its tissues becoming starved of oxygen. However, members of
Ediacara biota reached metres in length tens of millions of years
Cambrian explosion. Other metabolic functions may have
been inhibited by lack of oxygen, for example the construction of
tissue such as collagen , required for the construction of complex
structures, or to form molecules for the construction of a hard
exoskeleton. However, animals are not affected when similar
oceanographic conditions occur in the Phanerozoic; there is no
convincing correlation between oxygen levels and evolution, so oxygen
may have been no more a prerequisite to complex life than liquid water
or primary productivity.
The amount of ozone (O3) required to shield Earth from biologically
lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is
believed to have been in existence around the
Cambrian explosion. The
presence of the ozone layer may have enabled the development of
complex life and life on land, as opposed to life being restricted in
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. This may have caused a mass extinction,
creating a genetic bottleneck; the resulting diversification may have
given rise to the
Ediacara biota , which appears soon after the last
"Snowball Earth" episode. However, the snowball episodes occurred a
long time before the start of the Cambrian, and it is hard to see how
so much diversity could have been caused by even a series of
bottlenecks; the cold periods may even have _delayed_ the evolution
of large size organisms.
Increase In The Calcium
Concentration Of The
Newer research suggests that volcanically active midocean ridges
caused a massive and sudden surge of the calcium concentration in the
oceans, making it possible for marine organisms to build skeletons and
hard body parts. Alternatively a high influx of ions could have been
provided by the widespread erosion that produced Powell's Great
Evolutionary developmental biology
A range of theories are based on the concept that minor modifications
to animals\' development as they grow from embryo to adult may have
been able to cause very large changes in the final adult form. The Hox
genes , for example, control which organs individual regions of an
embryo will develop into. For instance, if a certain _Hox_ gene is
expressed, a region will develop into a limb; if a different Hox gene
is expressed in that region (a minor change), it could develop into an
eye instead (a phenotypically major change).
Such a system allows a large range of disparity to appear from a
limited set of genes, but such theories linking this with the
explosion struggle to explain why the origin of such a development
system should by itself lead to increased diversity or disparity.
Evidence of Precambrian metazoans combines with molecular data to
show that much of the genetic architecture that could feasibly have
played a role in the explosion was already well established by the
This apparent paradox is addressed in a theory that focuses on the
physics of development. It is proposed that the emergence of simple
multicellular forms provided a changed context and spatial scale in
which novel physical processes and effects were mobilized by the
products of genes that had previously evolved to serve unicellular
functions. Morphological complexity (layers, segments, lumens,
appendages) arose, in this view, by self-organization .
Horizontal gene transfer has also been identified as a possible
factor in the rapid acquisition of the biochemical capability of
biomineralization among organisms during this period, based on
evidence that the gene for a critical protein in the process was
originally transferred from a bacterium into sponges.
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, and others focus on the
more general mechanisms of coevolution . Such theories are well suited
to explaining why there was a rapid increase in both disparity and
diversity, but they must explain why the "explosion" happened when it
Ediacaran Mass Extinction
Main article: End-
Evidence for such an extinction includes the disappearance from the
fossil record of the
Ediacara biota and shelly fossils such as
_Cloudina_, and the accompanying perturbation in the δ13C record.
Mass extinctions are often followed by adaptive radiations as
existing clades expand to occupy the ecospace emptied by the
extinction. However, once the dust had settled, overall disparity and
diversity returned to the pre-extinction level in each of the
Evolution Of Eyes
Evolution of the eye
Andrew Parker has proposed that predator-prey relationships changed
dramatically after eyesight evolved. Prior to that time, hunting and
evading were both close-range affairs – smell, vibration, and touch
were the only senses used. When predators could see their prey from a
distance, new defensive strategies were needed. Armor, spines, and
similar defenses may also have evolved in response to vision. He
further observed that, where animals lose vision in unlighted
environments such as caves, diversity of animal forms tends to
decrease. Nevertheless, many scientists doubt that vision could have
caused the explosion. Eyes may well have evolved long before the start
of the Cambrian. It is also difficult to understand why the evolution
of eyesight would have caused an explosion, since other senses, such
as smell and pressure detection, can detect things at a greater
distance in the sea than sight can; but the appearance of these other
senses apparently did not cause an evolutionary explosion.
Arms Races Between Predators And Prey
The ability to avoid or recover from predation often makes the
difference between life and death, and is therefore one of the
strongest components of natural selection . The pressure to adapt is
stronger on the prey than on the predator: if the predator fails to
win a contest, it loses a meal; if the prey is the loser, it loses its
But, there is evidence that predation was rife long before the start
of the Cambrian, for example in the increasingly spiny forms of
acritarchs, the holes drilled in _Cloudina_ shells, and traces of
burrowing to avoid predators. Hence, it is unlikely that the
_appearance_ of predation was the trigger for the Cambrian
"explosion", although it may well have exhibited a strong influence on
the body forms that the "explosion" produced. However, the intensity
of predation does appear to have increased dramatically during the
Cambrian as new predatory "tactics" (such as shell-crushing) emerged.
This rise of predation during the
Cambrian was confirmed by the
temporal pattern of the median predator ratio at the scale of genus,
in fossil communities covering the
but this pattern is not correlated to diversification rate. This lack
of correlation between predator ratio and diversification over the
Ordovician suggests that predators did not trigger the
large evolutionary radiation of animals during this interval. Thus the
role of predators as triggerers of diversification may have been
limited to the very beginning of the "
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. Before the start of the Cambrian, their corpses and
droppings were too small to fall quickly towards the seabed, since
their drag was about the same as their weight. This meant they were
destroyed by scavengers or by chemical processes before they reached
the sea floor.
Mesozooplankton are plankton of a larger size. Early Cambrian
specimens filtered microscopic plankton from the seawater. These
larger organisms would have produced droppings and corpses that were
large enough to fall fairly quickly. This provided a new supply of
energy and nutrients to the mid-levels and bottoms of the seas, which
opened up a huge range of new possible ways of life. If any of these
remains sank uneaten to the sea floor they could be buried; this would
have taken some carbon out of circulation , resulting in an increase
in the concentration of breathable oxygen in the seas (carbon readily
combines with oxygen).
The initial herbivorous mesozooplankton were probably larvae of
benthic (seafloor) animals. A larval stage was probably an
evolutionary innovation driven by the increasing level of predation at
the seafloor during the
Metazoans have an amazing ability to increase diversity through
coevolution . This means that an organism's traits can lead to traits
evolving in other organisms; a number of responses are possible, and a
different species can potentially emerge from each one. As a simple
example, the evolution of predation may have caused one organism to
develop a defence, while another developed motion to flee. This would
cause the predator lineage to split into two species: one that was
good at chasing prey, and another that was good at breaking through
defences. Actual coevolution is somewhat more subtle, but, in this
fashion, great diversity can arise: three quarters of living species
are animals, and most of the rest have formed by coevolution with
Evolving organisms inevitably change the environment they evolve in.
Devonian colonization of land had planet-wide consequences for
sediment cycling and ocean nutrients, and was likely linked to the
Devonian mass extinction . A similar process may have occurred on
smaller scales in the oceans, with, for example, the sponges filtering
particles from the water and depositing them in the mud in a more
digestible form; or burrowing organisms making previously unavailable
resources available for other organisms.
The explosion may not have been a significant evolutionary event. It
may represent a threshold being crossed: for example a threshold in
genetic complexity that allowed a vast range of morphological forms to
be employed. This genetic threshold may have a correlation to the
amount of oxygen available to organisms. Using oxygen for metabolism
produces much more energy than anaerobic processes. Organisms that use
more oxygen have the opportunity to produce more complex proteins,
providing a template for further evolution. These proteins translate
into larger, more complex structures that allow organisms better to
adapt to their environments. With the help of oxygen, genes that code
for these proteins could contribute to the expression of complex
traits more efficiently. Access to a wider range of structures and
functions would allow organisms to evolve in different directions,
increasing the number of niches that could be inhabited. Furthermore,
organisms had the opportunity to become more specialized in their own
UNIQUENESS OF THE EXPLOSION
Cambrian explosion" can be viewed as two waves of metazoan
expansion into empty niches: first, a coevolutionary rise in diversity
as animals explored niches on the
Ediacaran sea floor, followed by a
second expansion in the early
Cambrian as they became established in
the water column. The rate of diversification seen in the Cambrian
phase of the explosion is unparalleled among marine animals: it
affected all metazoan clades of which
Cambrian fossils have been
found. Later radiations , such as those of fish in the
Devonian periods, involved fewer taxa , mainly with very similar body
plans. Although the recovery from the Permian-Triassic extinction
started with about as few animal species as the
the recovery produced far fewer significantly new types of animals.
Whatever triggered the early
Cambrian diversification opened up an
exceptionally wide range of previously unavailable ecological niches .
When these were all occupied, limited space existed for such
wide-ranging diversifications to occur again, because strong
competition existed in all niches and incumbents usually had the
advantage. If a wide range of empty niches had continued, clades would
be able to continue diversifying and become disparate enough for us to
recognise them as different phyla ; when niches are filled, lineages
will continue to resemble one another long after they diverge, as
limited opportunity exists for them to change their life-styles and
There were two similar explosions in the evolution of land plants :
after a cryptic history beginning about 450 million years ago , land
plants underwent a uniquely rapid adaptive radiation during the
Devonian period, about 400 million years ago . Furthermore,
Angiosperms (flowering plants ) originated and rapidly diversified
during the Cretaceous period.
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