Cambrian explosion
The Cambrian explosion refers to the geologically sudden appearance in the fossil record of animals, starting about 542 million years ago (Mya). In addition, a similar pattern of diversification is seen in other organisms such as phytoplankton and the various colonial calcareous microfossils grouped together as calcimicrobes. The base of the Cambrian is also marked by strong geochemical perturbations, including excursions in carbon and sulfur isotopes.
Significance of the explosion
The Cambrian explosion has generated a great deal of interest and controversy among scientists and the public. Darwin saw it as one of the principal objections that could be lodged against his theory of evolution by natural selection, as have modern-day Creationists. Scientists have also long been puzzled by its abruptness, and the apparent lack of obvious predecessors to the Cambrian faunas. Three questions in particular are of importance currently: i) is the “explosion” real?; ii) what does it tell us about the origin and evolution of animals? and iii) what were its causes?
History
Geologists as long ago as Buckland (1784-1856) realised that a dramatic step change in the fossil record occurred at the base of what we now call the Cambrian. For Darwin, the apparent appearance in the fossil record of many animal groups with few or no antecedents caused great trouble – indeed he devoted a substantial chapter of the Origin to this problem. Further insights were provided by the remarkable amount of work on North American faunas by Walcott, who proposed that an interval of time, or the “Lipalian”, was not represented in the fossil record, and/or did not preserve fossils, and the ancestral forms to the Cambrian taxa evolved during this time. However, the intense modern interest in the subject was probably sparked by the work of Whittington and colleagues on the redescription of the Burgess Shale (see below), together with Gould’s popular account of this work, Wonderful Life, published in 1989.
Dating the Cambrian
The Cambrian explosion has proved to be difficult to study, partly because of the problems involved in matching up rocks of the same age across continents. In all of the below, it should be borne in mind that absolute radiometric dates for much of the Cambrian have only rather recently become available, and that especially for the Lower Cambrian, detailed biostratigraphic correlation across continents remains rather tenuous, particularly from the internationally-defined Precambrian/Cambrian boundary section in Newfoundland. Dating of important boundaries, and description of faunal successions should thus be regarded with some degree of caution until better data become available.
Data sources
The data from which the Cambrian explosion has been reconstructed consists largely of body and trace fossils, and geochemical isotopic analyses.
Trace fossils
Trace fossils, at a first approximation the traces made by organisms in the sediments they lived in or on, are of considerable importance in unraveling the Cambrian explosion.
In the latest Precambrian (from about 550 million years ago onwards) only simple two-dimensional trace fossils are found The organisms making the traces were clearly not exploiting deep sediments, but only the surface layers. The transition into the Cambrian is marked by the rapid diversification of many new types, including well-known vertical burrows such as Diplocraterion and Skolithos, and traces normally attributed to arthropods, such as Cruziana and Rusophycus. Indeed, some of these traces appear an appreciable period of time before the body fossils of the animals often thought to make them. These trace fossils show a clear “widening of the behavioural repertoire” (Conway Morris 1989) and are particularly significant because they represent a data source that is not directly connected to the presence of easily-fossilized hard parts. They show that at the very least, large benthic, bilaterally symmetrical organisms were rapidly diversifying during this time. While some cnidarians are effective burrowers, most of these trace fossils have been assigned to bilaterian animals, although exact assignment of trace fossils to their makers is, as usual, difficult beyond generalities.
Body fossils
The fossil record of the Cambrian is often divided into two categories, the “conventional” and “exceptional” record, although these two clearly grade into each other.
Conventional record
The conventional fossil record consists only of easily-preserved parts of organisms, above all their mineralized shells. Given that these are often found disarticulated, and that most living organisms do not have any hard parts at all, it is clear that reconstruction of ecosystems or any other advanced analysis of the Cambrian world is going to be very difficult based only on these data.
The first organisms with hard parts in fact predate the Precambrian/Cambrian boundary. These include the “cone-in-cone” tube called Cloudina, and Namacalathus, which has a complex, stalked structure. These both appear to become extinct shortly before the base of the Cambrian boundary. The beginning of the Cambrian itself is marked chiefly by trace fossils, but during the poorly-defined Nemakit-Daldynian stage that follows, a variety of so-called “small skeletal fossils” or “SSF”s start to appear. Most of these are of uncertain affinities, and they represent a variety of tubes, caps, shells and sclerites. Among these may be represented early molluscs such as Latouchella, as well as a variety of sponge spicules. During the next stage, 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 most identifiable body fossils appear. These include the trilobites, echinoderms, and many more probable molluscan and brachiopod groups. Although as noted above, the dating and correlation of Cambrian strata is not particularly secure, this early Cambrian period before the Atdabanian may represent over twenty million years, and perhaps thirty million years after the appearance of the first widely recognised trace fossils.
Exceptional record
For reasons that are by no means clear (possible causes include the particular tectonic regime and the lack of large numbers of burrowing animals) the Cambrian is marked by a very high number of exceptionally preserved faunas, of which the most significant are the Lower Cambrian Chengjiang and Sirius Passet faunas, the Middle Cambrian Burgess Shale fauna, and the Upper Cambrian Orsten fauna. Exceptional faunas preserve a much wider range of tissue types than the conventional record, and thus many types of organisms are represented in the fossil record only by this sort of preservation. The exceptional faunas have therefore played a critical role in driving debates about the Cambrian explosion.
Of the major faunas, the Burgess Shale was discovered first, in the early years of the 20th Century by Walcott (1909). The Chengjiang fauna was actually discovered very shortly after this by Mansui in 1912, although it only became at all prominent in the 1980s after its “rediscovery”. 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 fauna is dominated by arthropods, with sponges and echinoderms making up less abundant components of the fauna. A significant number of taxa however, have consistently excited attention since their description, because these organisms do not fit readily into modern taxonomic categories. These include Opabinia, Anomalocaris, Amiskwia, Odontogriphus, Wiwaxia and Hallucigenia. In addition, most, or even all, of the agreed arthropods from the Burgess Shale, do not seem to fit into any modern arthropod class such as the insects, crustaceans and chelicerates. The information from the Burgess Shale has been supplemented greatly by the stream of fossils that have been described from the rather older Chengjiang fauna from China, and, to a lesser extent, from the perhaps even older Sirius Passet fauna from North Greenland, both of which seem to date from close to the Atdabanian/Botoman boundary, and thus within the Lower Cambrian.
The Chengjiang fauna has in particular yielded an enormous diversity, again dominated by arthropods, but including purported representatives of many other phyla including the vertebrates. In addition, it too has yielded some highly problematic forms such as Vetulicola and Yunnanozoan. Finally, the Sirius Passet fauna is again arthropod and sponge rich, and has yielded the problematic articulated Halkieria and the probably related taxa Kerygmachela and Pambdelurion among others.
Geochemistry
The Proterozoic/Phanerozoic transition is marked by fluctuations in at least three three major isotopic ratios: 87Sr / 86Sr, 34S / 32S and 13C / 12C. These fluctuations have been widely interpreted as being related to continental break-up, the end of the “snowball earth”, or a catastrophic drop in productivity caused by a mass extinction just before the beginning of the Cambrian. However, the variety of possible causes for these fluctuations means that currently, geochemistry is providing a very exciting new source of data that as yet has not been interpreted in a settled way.
Significance of the data
i) Is the explosion real?
The apparent suddenness of the Cambrian radiations has led many, including 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 of the origin of at least bilaterian animals place it 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 reasonable, or even obvious, 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 theories can be tested by phylogenetic reconstruction of the morphology of the most basal bilaterians. However, this has proven to be fraught with difficulty, although they seem to have at least possessed a through-gut and striated musculature – both perhaps indicative of at least not tiny size.
Proterozoic predecessors?
The hunt for Precambrian metazoans has obviously 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. In addition, in the Ediacaran Period immediately preceding the Cambrian, apart from the trace fossils and tubes previously mentioned, the record contains 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 from 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 most well known are those from the phosphorites. The fossils described from here include algae, giant acritarchs, and, spectacularly, phosphatised embryos that may represent non-bilaterian animals such as sponge or cnidarian grade organisms. Other embryos assigned to the Bilateria have also been described, along with a purported adult bilaterian. 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. As a result, the current most widely accepted view is that there are no convincing bilaterians until at least the close of the Proterozoic. Clearly, further research is required to clarify the many problematic aspects of Doushantuo diversity.
Early trace fossils?
Finally, the various trace fossils reported from the Proterozoic should be mentioned. While most of these have been accepted to be pseudofossils, a few have been recently reported, including one from approx 1 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. Thus, it is fair to say that no convincing trace fossils before the end of the Ediacaran are currently accepted.
The sum of the evidence, then, suggests that neither large bilaterians (which would probably have been capable of leaving a body or trace fossil record) or tiny bilaterians (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.
ii) Evolutionary significance
The rapidity of the Cambrian explosion, the lack of precursors in the fossil record, 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 “Evo-Devo”. The promulgation by Wonderful Life of the view that the Cambrian represented an unprecedented riot of disparity, of which only a shadow 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.
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, both the older Chengjiang and Sirius Passet faunas represent a period of time perhaps more than 10 million years before. 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, and clearly much more work is needed on these taxa.
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 Cambrian “Problematica” 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 highly problematic fossils 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 for the Cambrian explosion
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 “disparity” or morphospace occupancy 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.
iii) What caused the Cambrian Explosion?
The ultimate cause of why the Cambrian explosion happened when it did – in the Cambrian, rather than the Jurassic or Archaean, say,has also been of intense interest. Proposed explanations revolve around three major themes: i) extrinsic forcing events such as environmental change; ii) intrinsic mechanisms such as the acquisition of complex genetic architectures; 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 Nursall in the 1950’s, is that animals did not evolve before the beginning of the Phanerozoic because of low atmospheric oxygen. Low oxygen levels could prevent animals from evolving either, in the extreme case, by preventing the synthesis of collagen, present in metazoans, and now also known in other eukaryotes, and which requires at least 1% of present atmospheric levels (the “Towe limit”). However, more likely would be a physiological constraint. Animals living in low oxygen environments today tend to have low diversity, have 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 explanation, and a current popular one, is “Snowball Earth” that ties in 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 genes (most famously 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 entangle 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
Given the difficulties presented by the above two sorts of explanation, and without necessarily abandoning their explanatory interest, several recent examinations of the Cambrian explosion have suggested that ecological diversification is the primary motor for the Cambrian explosion: indeed, it could even be claimed that the Cambrian explosion is simply 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.
Why did the Cambrian Explosion take place when it did?
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 intrinsic or extrinsic trigger. A popular example of the latter would be a meteoritic impact (the Australian Acraman impact 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 up, and indeed 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, and asking why it did not take place earlier is akin to asking why WWI, or the invention of the atom bomb, did not take place in the 14th century. Butterfield, for example, has argued that the presence of animals, with their vigorous ability to move and predate on other organisms, would have speeded 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 hetereotrophs 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 of in the history of Earth.
References
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{{cite journal}}: CS1 maint: multiple names: authors list (link) - Parker, A. (2004). In the Blink of an Eye, Free Press, ISBN 0-7432-5733-2.
- Wang, D. Y.-C., S. Kumar and S. B. Hedges (1999). "Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi". Proceedings of the Royal Society of London, Series B, Biological Sciences. 266 (1415): 163–71. doi:10.1098/rspb.1999.0617.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Xiao, S., Y. Zhang, and A. Knoll (1998). "Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite". Nature. 391: 553–58. doi:10.1038/35318.
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