User:Smith609/Cambrian explosion

, organisms were on the whole simple, comprised of individual cells occasionally organised into colonies; Over the subsequent 70-80 million years, the rate of evolution would accelerate by an order of magnitude, and the diversity of life would begin to resemble today's.

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

The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, can be distilled into three key points:


 * Was the “explosion” real?
 * What does it tell us about the origin and possible evolution of animals?
 * What were its causes?

A limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures left in Cambrian rocks, makes interpretation difficult; consequently, a wide variety of answers to these three questions have been touted. Further discoveries continue to bolster our knowledge, marking previously well-supported hypotheses as inaccurate, and allowing new ideas to crop up in their place.

History and significance of the concept
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. 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.

Victorian scientists grappled with the conundrum, and American palæontologist Charles Walcott proposed that an interval of time, the “Lipalian”, was either not represented in the fossil record, or did not preserve fossils — and that the ancestral forms to the Cambrian taxa evolved during this time.

The intense modern interest in the subject was probably sparked by the work of Harry B. Whittington and colleagues, whose redescription of the Burgess Shale (see below) from 1970 onwards, together with Stephen Jay Gould’s popular 1989 account of this work, Wonderful Life, bought the matter into the public eye, raising questions about what the explosion represented: whilst differing significantly in the detail, both proposed a sudden appearance of all animal types.

Today, creationists with a poor grasp of science maintain public interest in subject, highlighting the Victorians' concerns about its incompatibility with evolution. Advances in scientific techniques, along with new fossil finds, help refine scientific insights into the processes behind the explosion. Debate currently focuses on a number of points: firstly, whether the explosion of the fossil record represents an explosion of life's diversity; also, what the explosion tells us about animals' origins, and early evolution; and further, what — if anything — may have caused this geologically abrupt phenomenon.

Dating the Cambrian
The Cambrian explosion has proven difficult to study, partly because of the problems involved in matching up rocks of the same age across disparate continents. It should be borne in mind that 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 that, especially for the Lower Cambrian, detailed biostratigraphic correlation — which uses widespread but short-lived species to match rocks of the same age — remains rather tenuous, particularly around the internationally defined Precambrian/Cambrian boundary section. Dating of important boundaries, and description of faunal successions, should thus be regarded with some degree of caution until better data become available.

Trace fossils
Trace fossils — broadly speaking, the traces made by organisms in the sediments they lived in or on — are of considerable importance in unravelling the Cambrian explosion. Bona fide burrows first appear in the Precambrian, from about onwards; at first, only simple horizontal burrows occur. These marks were made by creatures moving across and below soft surfaces: the organisms making the traces were clearly not exploiting deep sediments, but only the topmost layers. As the Cambrian got underway, new forms of trace fossil appeared, including vertical burrows and traces normally attributed to arthropods. These represent a “widening of the behavioural repertoire”, both in terms of abundance and complexity.

Trace fossils are particularly significant because they represent a data source that is not directly connected to the presence of easily fossilized hard parts, which are of course rare during the Cambrian; indeed, many traces appear an appreciable period of time before the body fossils of the animals that are thought to make them. Whilst exact assignment of trace fossils to their makers is difficult, the trace fossil record seems to indicate that at the very least, large, bottom-dwelling, bilaterally symmetrical organisms were rapidly diversifying during the early Cambrian.

Body fossils
Body fossils are the physical remains of organisms, which, often after chemical alteration, are preserved in the rock record. The fossil record of the Cambrian is often divided into two categories, the “conventional” and “exceptional” record, which of course grade into one another.

The conventional record
The conventional fossil record consists only of readily-preserved parts of organisms, above all their mineralized shells. Since these fragments are usually found disarticulated, and the majority of organisms lack hard parts, reconstruction of ecosystems — or any other analysis of the Cambrian world — based only on these data is difficult.

The first organisms with hard parts in fact pre-date the Precambrian/Cambrian boundary, and the complex stalked structure called Namacalathus. and appear to have become extinct shortly before the base of the Cambrian. The beginning of the Cambrian itself is marked chiefly by the appearance of new trace fossils, but a variety of small skeletal fossils, the small shelly fauna, gradually appear over the next few million years. This fauna incorporates a variety of tubes, caps, shells, and sclerites, mostly of uncertain affinity - perhaps including early molluscs such as Latouchella, and a variety of sponge spicules. During the second stage of the Cambrian, the Tommotian, a much greater variety of small shelly fossils start to appear, including the first probable brachiopods. However, it is not until the next stage, the Atdabanian, that a significant proportion of the body fossil record can be readily attributed to modern groups. Groups represented include the trilobites, echinoderms, and many more with probable molluscan and brachiopod affinities. Although the dating and correlation of Cambrian strata, as noted above, is not particularly secure, this pre-Atdabanian early Cambrian period may represent a period of time spanning over 20, and perhaps as many as 30, million years from the appearance of widely recognised trace fossils.

Instances of exceptional preservation
For reasons that are by no means clear — perhaps the particular tectonic regime, or the low abundance of burrowing animals — the Cambrian is marked by an unusually high number of exceptionally preserved faunas, of which the most significant are the Lower Cambrian Maotianshan shale faunas of Chengjiang (Yunnan, China) and Sirius Passet (Greenland), the Middle Cambrian Burgess Shale (British Columbia, Canada) fauna, and the Upper Cambrian Orsten (Sweden) fauna. Exceptional faunas preserve a much wider range of tissue types than the conventional record, and thus many types of organisms are only represented in the fossil record by this sort of preservation. The exceptional faunas have therefore played a critical role in driving debates about the Cambrian explosion.

Whilst they have been known since the early 20th century, exceptional faunas rose to prominence in the 1970s and 1980s after they were “rediscovered”. The Burgess Shale has, in particular, yielded many of the most famous fossils ever discovered, and forms the subject of Gould’s Wonderful Life. The exceptional record displays a fauna dominated by arthropods, with less abundant sponges and echinoderms; in the case of the Chengjiang, purported representatives of many other phyla, even including vertebrates, are present. A smaller but significant number of taxa, including the famous Opabinia, Anomalocaris, Yunnanozoon, Halkieria, Odontogriphus, Wiwaxia and Hallucigenia, have consistently excited attention since their description, because these organisms do not fit readily into modern taxonomic categories. In addition, most, or even all, of the agreed arthropods from the exceptional faunas do not seem to fit into any modern arthropod class such as the insects, crustaceans or chelicerates. The information from the Burgess Shale is supplemented greatly by the stream of fossils described from the rather older Chengjiang fauna from China, and, to a lesser extent, from the potentially older still Sirius Passet fauna from North Greenland, both of which seem to date from close to the Atdabanian/Botoman boundary, and thus well within the Lower Cambrian.

Molecular phylogenetics
The field of molecular phylogenetics attempts to reconstruct the relationships between organisms by comparing details of their biochemistry, such as their DNA. It provides an alternative line of evidence into the Cambrian, although the need for calibration against the fossil record means it is not entirely independent. Further, since the "clocks" measure molecular evolution, a period of rapid evolution is indistinguishable from a longer period of slow change, adding a huge margin of error to Cambrian considerations. Whilst the rapidly developing science must be treated with a degree of caution, it has yielded some useful results: for example, evidence that the three major animal groups diverged some time before the Cambrian, then independently underwent a rapid Cambrian diversification — although the implications of this apparent finding are a matter for discussion.

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. However, the wide variety of possible causes for these fluctuations means that geochemistry is currently an exciting new source of data, which is still to be interpreted in a settled way.

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Significance of the data

 * 1) INCLUDECambrian_explosion

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

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

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

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

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

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

The sum of the evidence, then, suggests that neither large bilateral animals (which would probably have been capable of leaving a body or trace fossil record) nor tiny ones (which would perhaps be expected to be found in the Doushantuo Formation) existed before close to the end of the Proterozoic. While this viewpoint is by no means generally accepted, it is also somewhat supported by revised molecular clock estimates, which tend to converge towards a much later bilaterian divergence date, and close to that suggested by the fossil record.

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

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

Appearance of phyla
While the general rapidity of the Cambrian explosion thus seems to remain a reality, attempts have been made to downplay the “amount” of evolution that was required to generate the taxa actually seen in the Cambrian. In particular, the distinction between “crown” and “stem” groups has been applied to claim that many or even most lower-middle Cambrian taxa fall outside the crown groups of the modern phyla. This in some cases somewhat legalistic argument allows the origins of many of the phyla as we see them today to be pushed up into the succeeding Ordovician Period, or even later. Thus, the view that all modern phyla essentially suddenly appear at the base of the Cambrian has come under assault. One aspect of this reassessment is that many or most of the problematic Cambrian fossils have begun to be seen in the light of a stem-group placement to modern phyla or groups of phyla. Rather than being seen as one-off oddities, they can in this view be seen as representing the progressive adaptive stages of the assembly of modern-day body plans, albeit ones with their own particular adaptations. An analogy can be drawn with the origin of the tetrapods or mammals, which have also been sequentially mapped out in the fossil record. Of course, many problematica remain, but in at least some of these cases, such as Odontogriphus, not enough has been known until recently about their morphology in order to come to a reasonable conclusion.

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

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

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

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

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

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

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

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

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