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Cambrian explosion

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Cambrian explosion of species refers to the geologically sudden
   appearance in the fossil record of the ancestors of familiar 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 ("The fossil record had caused Darwin
   more grief than joy. Nothing distressed him more than the Cambrian
   explosion, the coincident appearance of almost all complex organic
   designs..." (Stephen Jay Gould, The Panda’s Thumb, 1980, pp. 238-239.),
   as have modern-day Creationists.

   Scientists have also long been puzzled by its abruptness, and the
   apparent lack of obvious predecessors to the Cambrian fauna. 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 William Buckland (1784-1856) realised that a
   dramatic step change in the fossil record occurred at the beginning 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 of Species to this problem. Further insights were provided by
   the remarkable amount of work on North American fauna by Walcott, who
   proposed that an interval of time, or the “Lipalian”, was not
   represented in the fossil record, or did not preserve fossils, and that
   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 Harry B. Whittington and colleagues on the
   redescription of the Burgess Shale (see below), together with Stephen
   Jay Gould’s popular account of this work, Wonderful Life, published in
   1989.

Dating the Cambrian

   One possible time-scale for the events around the Precambrian/Cambrian
   boundary. New data suggests that the Ediacaran may extend back to 635
   Ma at the end of the Marinoan Glaciations
   Enlarge
   One possible time-scale for the events around the Precambrian/Cambrian
   boundary. New data suggests that the Ediacaran may extend back to 635
   Ma at the end of the Marinoan Glaciations

   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. 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.

Significance of the data

Is the explosion real?

   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 Ma. 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, 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
   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 taphonomy of
   Doushantuo fossils, that the fossil is largely a diagenetic artefact.
   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, 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"). Steven 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.

   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 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, 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
   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 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.

What caused 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 Nursall in 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 either 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, 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”,
   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 centered 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 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 and 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 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, 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 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 in the history of Earth.
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