Cambrian explosion
2008/9 Schools Wikipedia Selection. Related subjects: Geology and geophysics
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. Over the following 70 or 80 million years the rate of evolution accelerated by an order of magnitude,
The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the mid 19th century, and Charles Darwin saw it as one of the main objections that could be made against his theory of evolution by natural selection.
The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid evolution; and what it would imply about the origin and 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.
fossils
- (last æon of the Precambrian)
Palæozoic- (first æon of the Phanerozoic)
- (first æon of the Phanerozoic)
around the Cambrian/Precambrian boundary.
Axis scale: millions of years ago.
History and significance
Geologists as long ago as Buckland (1784–1856) realised that a dramatic step-change in the fossil record occurred around the base of what we now call the Cambrian. Charles Darwin considered this sudden appearance of many animal groups with few or no antecedents to be the greatest single objection to his theory of evolution: indeed, he devoted a substantial chapter of The Origin of Species to this problem.
American palæontologist Charles Walcott 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.
More recently it was discovered that the history of life on earth goes back at least : rocks of that age at Warrawoona in Australia contain fossils of stromatolites, stubby pillars that are formed by colonies of micro-organisms. Fossils ( Grypania) of more complex eukaryotic cells, from which all animals, plants and fungi are built, have been found in rocks from , in China and Montana. Rocks dating from contain fossils of the Ediacara biota, organisms so large that they must have been multi-celled, but very unlike any modern organism. Cloud argued in 1948 that there was a period of "eruptive" evolution in the Early Cambrian, but as recently as the 1970s there was no sign of how the relatively modern-looking organisms of the Middle and Late Cambrian arose.
The intense modern interest in this "Cambrian explosion" 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 as 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 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, only distantly related to anything known today. 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. This view was influenced by 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.
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. There has also been intense debate whether there was a genuine "explosion" of modern forms in the Cambrian and, to the extent that there was, how it happened and why it happened then.
Types of evidence
Deducing the events of half a billion years ago is tricky, and evidence comes from biological and chemical signatures in rocks.
Dating the Cambrian
Accurate absolute radiometric dates for much of the Cambrian, obtained by detailed analysis of radioactive elements contained within rocks, have only rather recently become available, and for only a few regions.
Relative dating (A was before B) is often 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.
Body fossils
Fossils of organisms' bodies are usually the most informative type of evidence. Fossilisation 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 further back in time. Despite this, they are often adequate to illustrate the broader patterns of life's history. There are also biases 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 there are 30-plus phyla of living animals, two-thirds have never been found as fossils.
The Cambrian fossil record includes an unusually high number of lagerstätten, which preserve soft tissues. These allow palæontologists 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 Cambrian Maotianshan shale beds of Chengjiang ( Yunnan, China) and Sirius Passet (Greenland); the middle Cambrian Burgess Shale ( British Columbia, Canada); and the late Cambrian Orsten (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.
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 is not limited to animals with easily-fossilized hard parts, and which 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. 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
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.
Phylogenetic techniques
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. Characters which 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 fallible, as some features (e.g. 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 years ago.
It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last common acestor 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.
Explanation of a few scientific terms
A 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 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.
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.
Groups which cannot easily be placed in an existing phylum are considered to be stem groups. A stem group annelid, for instance, is any group which was closely related to, but not within, the crown group "annelida". The stem group split off from the lineage that would lead to annelids, and eventually became extinct.
(ectoderm)
(mesoderm)
organ
(peritoneum)
(endoderm)
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.).
The bilaterians are animals which are approximately symmetrical (by reflection) at some point in their life history. 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 ( starfish, sea urchins, sea cucumbers, etc.) look radially symmetrical (like wheels) rather than bilaterian, but their larvae exhibit bilateral symmetry and some of the earliest echinoderms may have been bilaterally symmetrical. Porifera and Cnidaria are radially symmetrical, non-bilaterian and non-triploblastic..
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 – the non-coelomate 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).
Precambrian life
Our 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 beforehand?
Evidence of animals around
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 , but their abundance and diversity declined steeply after about . 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. After appearing around , acritarchs underwent a boom around , 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 anti-predator defenses. A consideration of taxon longevity appears to support an increase in predation pressure around this time, However, in general, the rate of evolution in the Precambrian was very slow, with many cyanobacterial species persisting unchanged for billions of years.
If these predatory organisms really were metazoans, this means that Cambrian animals didn't appear "from no-where" at the base of the Cambrian - predecessors had existed for hundreds of millions of years.
Fossils of the Doushantuo formation
The Doushantuo formation harbours microscopic fossils which may represent early bilaterians. Some have been described as animal embryos and eggs, although some of these 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.
Burrows
The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the Ediacaran period, about . 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 ) 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 there 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.
Ediacaran organisms
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 progresses.
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.
At least some may have been early forms of the phyla at the heart of the "Cambrian explosion" debate, having been interpreted as early molluscs ( Kimberella), echinoderms ( Arkarua); and arthropods ( Spriggina, Parvancorina). There is still debate about the classification of these specimens, mainly because the diagnostic features which 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 to-day, 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 (40M years is "soon" by evolutio