The "Cambrian Explosion" refers to the sudden appearance in the fossil record of complex animals with mineralized skeletal remains. It may represent the most important evolutionary event in the history of life on Earth. The beginning of the explosion is generally placed about million years ago, during the Cambrian Period at the start of the Palaeozoic Era the same time the Ediacarans disappear from the fossil record. While the explosion was rapid in geological terms, it took place over millions of years - the Burgess Shale, at million years old, records the tail end of the event.
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Before the Cambrian explosion, [b] most organisms were simple, composed of individual cells occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life began to resemble that of today. The seemingly rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the s,  and in his book On the Origin of Species , Charles Darwin discussed the then inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection.
Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks. The first discovered Cambrian fossils were trilobites , described by Edward Lhuyd , the curator of Oxford Museum , in In 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. 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, million years :  Rocks of that age at Warrawoona, Australia , were claimed to contain fossil stromatolites , stubby pillars formed by colonies of microorganisms. The intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B.
Whittington and colleagues, who, in the s, reanalysed many fossils from the Burgess Shale and concluded that several were as complex as, but different from, any living animals. Organisms such as the five-eyed Opabinia and spiny slug-like Wiwaxia were so different from anything else known that Whittington's team assumed they must represent different phyla, seemingly unrelated to anything known today.
Stephen Jay Gould 's popular 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 s 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 s, argue that complex animals similar to modern types evolved well before the start of the Cambrian. Radiometric dates for much of the Cambrian, obtained by analysis of radioactive elements contained within rocks, have only recently become available, and for only 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 becomes 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. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although plus phyla of living animals are known, two-thirds have never been found as fossils.
These allow paleontologists to examine the internal anatomy of animals, which in other sediments are only represented by shells, spines, claws, etc. More than 20, fossil specimens were collected, including many soft bodied animals such as jellyfish, sea anemones and worms, as well as sponges, arthropods and algae. In some specimens the internal body structures were sufficiently preserved that soft tissues, including muscles, gills, mouths, guts and eyes, can be seen.
The remains were dated to around Mya and around half of the species identified at the time of reporting were previously unknown. Trace fossils consist mainly of tracks and burrows, but also include coprolites fossil feces and marks left by feeding. Also, many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them.
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. 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. From the relationships, it may be possible to constrain the date that lineages first appeared.
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.
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. As chemical and genetic testing becomes more accurate, previously hypothesised phyla are often entirely reworked.
A phylum is not a fundamental division of nature, such as the difference 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 regarded them as also being a stem group relative to the arthropods. The term Triploblastic means consisting of three layers, which are formed in the embryo , quite early in the animal's development from 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. 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.
The term Coelomate means having a body cavity coelom containing the internal organs. Phylogenetic analysis has been used to support the view that during the Cambrian explosion, metazoans multi-celled animals evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates.
Changes in the abundance and diversity of some types of fossil have been interpreted as evidence for "attacks" by animals or other organisms. This decline has been attributed to disruption by grazing and burrowing animals.
Precambrian marine diversity was dominated by small fossils known as acritarchs. This term describes almost any small organic walled fossil — from the egg cases of small metazoans to resting cysts of many different kinds of green algae.
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. The layers of the Doushantuo formation from around 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. These fossils form the earliest hard-and-fast evidence of animals, as opposed to other predators. 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.
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.
This new habit changed the seafloor's geochemistry, and led to decreased oxygen in the ocean and increased CO2-levels in the seas and the atmosphere, resulting in global warming for tens of millions years, and could be responsible for mass extinctions. 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.
The organisms form three distinct assemblages, increasing in size and complexity as time progressed. At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, [ clarification needed ] having been interpreted as early molluscs Kimberella ,   echinoderms Arkarua ;  and arthropods Spriggina ,  Parvancorina ,  Yilingia.
Still, debate exists about the classification of these specimens, mainly because the diagnostic features that allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the ediacarans. Paul Knauth, a geologist at Arizona State University , maintains that photosynthesizing organisms such as algae may have grown over a to million-year-old formation in Death Valley known as the Beck Spring Dolomite.
In the early s, samples from this 1,foot thick layer of dolomite revealed that the region housed flourishing mats of photosynthesizing, unicellular life forms which antedated the Cambrian explosion.
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.
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 million years ago. The first Ediacaran and lowest Cambrian Nemakit-Daldynian skeletal fossils represent tubes and problematic sponge spicules. It was organic-walled tubes e.
Saarina and chitinous tubes of the sabelliditids e. Sokoloviina , Sabellidites , Paleolina   that prospered up to the beginning of the Tommotian. Anabarites , Cambrotubulus from uppermost Ediacaran and lower Cambrian. 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. Some Cloudina fossils show small holes in shells. The holes possibly are evidence of boring by predators sufficiently advanced to penetrate shells.
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. Some animals may already have had sclerites, thorns, and plates in the Ediacaran e. Kimberella had hard sclerites, probably of carbonate , but thin carbonate skeletons cannot be fossilized in siliciclastic deposits.
The Cambrian seas teemed with new types of animal, such as the predator Anomalocaris centre. A series of dark, craggy pinnacles rises 80 metres above the grassy plains of Namibia. The peaks call to mind something ancient — the burial mounds of past civilizations or the tips of vast pyramids buried by the ages. Earliest skeletal animals were reef builders.
Before the Cambrian explosion, [b] most organisms were simple, composed of individual cells occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life began to resemble that of today. The seemingly rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the s,  and in his book On the Origin of Species , Charles Darwin discussed the then inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks. The first discovered Cambrian fossils were trilobites , described by Edward Lhuyd , the curator of Oxford Museum , in In 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.