The Cambrian Explosion , Cambrian Species Explosion or Cambrian Radiation (see Adaptive Radiation ) is the name given to the almost simultaneous first occurrence of representatives of almost all modern animal tribes in the geologically minute period of 5 to 10 million years at the beginning of the Cambrian , about 541 million years ago. The basic body plans of many multicellular animal phyla that have populated the earth since then have been handed down for the first time in the rocks of this epoch.
The first appearance of possible multicellular organisms is dated 2.1 billion years before our time ( Gabonionta ). Like all other fossils before the Mesoproterozoic , they are difficult to interpret cell colony-like formations with no reliable reference to later organisms. The first fossils that can be morphologically addressed as macroalgae with some conviction are the 1.2 billion year old Bangiomorpha (possibly a red alga ) and the approximately 800 million year old Proterocladus (probably a green alga from the Chlorophyta ). When exactly the first multicellular animals ( metazoa ) appeared is highly controversial. Comparing the genetic makeup of today's animal species using the methods of the molecular clock yields e.g. Sometimes very early dates for the last common ancestors of many animal tribes, without fossils from these epochs being found. One assumption is that the first multicellular cells were small and had neither a skeleton nor other hard parts, so that their fossil record would be unlikely.
Far fewer fossils are known from the geological ages before the Cambrian than from the times after, so that the entire period from the formation of the earth (roughly estimated approx. 4.5 billion years ago) to the Cambrian explosion 541 million years ago is called the Precambrian .
In the Ediacarium , the youngest formation in the Precambrian, fossils of larger and more complex animals appear for the first time ( Ediacara fauna ). The relationship of these forms to the later living animals is unclear. Many researchers interpret them as diploblasts; H. Living beings that, like today's jellyfish and rib jellyfish, only have two cotyledons . The physique of some forms is reminiscent of sea feathers . However, they have also been interpreted as gigantic single-cell organisms that formed their own, extinct realm alongside animals, fungi and plants, so-called vendobionts . Possibly they are also lichen-like fossil land plants. Since these shell and skeleton-free creatures left behind only indistinct and often difficult to interpret prints, a reliable assignment is hardly possible. The most developed and most interesting of the Ediacaric fossils in connection with the Cambrian explosion is the famous Kimberella , which is almost certainly a very primitive mollusc , that is, a "primal snail". Another fossil that can be considered a precursor to the Cambrian fauna is Spriggina , a segmented fossil that may represent an annelid worm. From the same period as the Ediacaran fauna, tiny, plastic microfossils (i.e. not only as imprints) preserved from phosphate rocks of the so-called Doushantuo formation from China, which are often interpreted as larvae or embryos of multicellular animals, although this interpretation is based on more recent studies has lost its probability. Some researchers interpret finds from this epoch ( Vernanimalcula ) as the remains of small worm-like animals that belong to the overstem of the Deuterostomia . However, this interpretation is contradicted by other scientists.
With the Cambrian Explosion, two new, basic body blueprint patterns appear. The fossils of the Cambrian are the first to document living beings with both a radially symmetrical structure and right-left symmetry and an additional longitudinal axis of the body running from top to bottom. These animals, known as " bilateria " ( bilateria ), presumably all descend from a common ancestor. This assumption is suggested by the discovery of the so-called Hox genes , which control the development of the basic body plans of all animals. The Hox genes of all Bilateria are homologous, i. H. their DNA sequences correspond so closely that one must assume that they originated from the same original gene. Amorphous multicellular cells only need one Hox gene for their body structure. Radially symmetrical organisms require two Hox genes as basic genetic makeup (realized in today's cnidarians). The descendants of the more complex creatures of the Cambrian who live today, however, have at least one Hox cluster made up of four Hox genes and a Para-Hox cluster with four Hox genes as well. For the ancestor of all Bilateria ("Urbilateria") a basic plan with seven Hox genes is reconstructed from the genes available today. There are eight Hox genes in today's protostomia and fourteen in the chordates . In the higher vertebrates there are 39 Hox genes in four clusters; here it is assumed that the original fourteen genes were created by doubling the entire genome twice (with subsequent loss of some genes). The increasing complexity of genetic blueprints is easy to explain, because in addition to the geometric body plan, the genes must provide a spatial and temporal coding for the specialization of different cells in different places.
Another important novelty in Cambrian fauna is the first appearance of animals that have hard shells or a skeleton. Even in animals without calcareous skeletal elements, stable body shells occur e.g. B. from chitin or cartilage substance, as z. B. in the Cambrian fossils from the so-called Burgess schist are handed down. In addition, ( apart from some representatives of the puzzling Ediacaran fauna ) animals with larger bodies have not appeared before.
Since with the Cambrian, for the first time, sediments with Ichnofossils , i. H. fossilized traces of animals appear, it can be assumed that the origin of animals that can crawl on the sea floor or burrow in it goes back to this era. In fact, the base of the Cambrian is not defined by a body fossil, but by the first complex trace fossil. The trace described as Treptichnus pedum probably goes back to a priapulid .
The sudden, parallel appearance of so many animals with very different body plans in a geologically short epoch has been an important problem for evolutionary research since it was discovered in the 19th century. Earlier attempts to explain it solely by the coincidence of fossil records , are no longer credible today because numerous new fossil sites have been discovered and evaluated worldwide. The Cambrian explosion is therefore now universally regarded as a real phenomenon. Although a much older age of many animal phyla still seems possible, corresponding representatives can at best have been millimeter-sized, soft-skinned creatures. Otherwise one should have discovered fossils of them, or at least of their traces, by now. Literally every eligible older fossil that has been appropriately interpreted has been bitterly debated among various researchers. Above all, it needs to be explained why life evidently originated comparatively quickly on the young earth, but then it took hundreds of millions of years for more complex multicellular organisms to appear. There is still no consensus within research about the trigger of the Cambrian explosion (or the geologically comparatively close foreplay of the Ediacaran fauna). The hypotheses discussed can be summarized in two groups:
Biological "arms race"
According to this hypothesis, it was the appearance of the first complex multicellular cells themselves that set the development in motion. The first multicellular cells would therefore have had very little adaptive advantages. However, once mobile animals, possibly with predatory nutrition, emerged in a slow development, they represented a profound selection factor. Shells and skeletons could then have emerged as a protective mechanism against predation. The emergence of animals with burrowing and burrowing ways of life was also a key event. It probably destroyed the stable microbial mats that previously covered the ocean floor and created completely new ecological conditions there.
Other hypotheses assume that the formation of higher multicellular cells would have been impossible at an earlier point in time because the living conditions in the oceans did not allow their earlier development. According to this, the Precambrian seas differed from today's seas in some key factor. The most common hypothesis assumes that the oxygen content in the sea only reached a level sufficient for higher life at this point in time. Newer hypotheses point to the possible importance of excessively high temperatures or salinity in the ocean, both factors that can also have a decisive influence on the oxygen content. Finally, consideration is also being given to the influence of the calcium content in seawater. According to this hypothesis, the shells and skeletons would initially have been something like waste products to excrete excess calcium.
Another fact known from geological research on rocks from the end of the Precambrian is that very strong ice ages occurred at that time. Many researchers interpret the findings to mean that almost the entire globe, including the seas, was covered with ice. The Sturtic glaciation and the subsequent Marino Ice Age are called “ snowball earth ” . It is believed that the break up of the supercontinent Rodinia released a lot of flood basalt , the weathering of which drew a great deal of carbon dioxide from the atmosphere. The resulting reduced greenhouse effect led to the Sturtische icing over.
In the latest Ediacarium, with Cloudina , Sinotubulites and a few other representatives, species with skeletal elements appear for the first time; in all cases it is an outer covering or tubular formation. The assignment of these fossil forms to extinct or recent animal phyla is uncertain and controversial, mostly they are interpreted as early cnidarians (or extinct representatives with a similar organization of the body, diploblasts with only two cotyledons ). In the earliest Cambrian (the stage of the Fortunian and the second stage in the Terreneuvian , formerly also called Tommotian), such skeletal-bearing forms became more frequent and diverse (after most Ediacaran fossils without hard parts had gradually disappeared in the late Ediacarian). In addition to tubular microfossils, there are tooth-like or hook-like formations ( protoconodonts ), today interpreted as the jaw apparatus of early arrowworms (Chaetognatha), formations and sclerites of lobopods interpreted as shells of early molluscs . For many of these forms, including Mobergella , which is also found quite frequently in Northern Europe , the taxonomic classification is still unclear. The isolated shells and sclerites, which mostly consist of calcium phosphate, are obtained by dissolving limestone in acid, leaving them behind as a residue. The fossil community is described as " Small Shelly Fauna ". While it was previously believed that the transition between the late Ediacaran fauna and the Anabarites trisulcatus - Protohertzina anabarica zone of the earliest Cambrian (as the first zone of the small shelly fauna) was relatively abrupt, possibly even from a mass extinction at the end of the Ediacarian accompanied, more recent finds from East Asia have shown a gradual transition spread over several million years.
The picture of an abrupt transition at the base of the Cambrian is thus significantly modified upon closer examination of the early stages of the Cambrian. Skeleton-bearing forms appear first in the sponges and cnidarians, from the Fortunium on in numerous Lophotrochozoa such as molluscs and brachiopods . In the third series of the Cambrian, the arthropods appear with the trilobites , and a little later with the first fossil echinoderms (Echinodermata) also the deuterostomia appear in the fossil record. This transition period reveals a transition period that is quite short in geological terms, but quite long for evolutionary processes, even if the exact time of occurrence of the animal phyla due to an assumed " ghost range " without fossil records can never be precisely specified.
The Cambrian Explosion was brought about by Stephen Jay Gould's book Chance Mensch , among others . Popularized the miracle of life as a game of nature (1989). He describes the animal families mentioned above, which are only documented in the Cambrian, as “unique”, “puzzling” or “amazing” in order to bring the subject closer to a broader public.
Journalists popularized the Cambrian Radiation, the most important fossil evidence of which comes from the Burgess Shale in North America, in the USA in the direction of uniqueness. The TIME magazine dedicated the Cambrian a cover story titled Evolution's Big Bang (issue of 4 December 1995), and comparing the appearance of many new species and strains in the Cambrian to the big bang of the universe. Although more and more forerunners of these species from much older formations have been discovered and the geological time scale has been refined by geologists for the Precambrian as well , the interpretation of these journalistic articles tended towards a one-off event in which many animal tribes were relatively short Time should have arisen.
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