Cambrian explosion

As Cambrian explosion, Cambrian species explosion or Cambrian radiation (see Adaptive Radiation) is called the almost simultaneous, first-time occurrence of representatives of almost all modern animal phyla in a geologically short period of 5 to 10 million years ago at the beginning of the Cambrian period before about 543 million years. The basic body plans of many multicellular animal phyla, which have since inhabit the Earth, are clearly over delivers for the first time in rocks of this era.

The first appearance of multicellular organisms is dated to 1.2 billion years before our time (first multicellular organisms: multicellular algae). Exactly when the first multicellular animals ( Metazoa ) occurred is still hotly disputed. From the comparison of the genome today's animal species using the methods of molecular clock is sometimes very early time points for the last common ancestor of many animal phyla, without fossils from these eras would have found result. Mostly, it is believed that the first multicellular organisms were very small and neither a skeleton nor other hard parts possessed, so that their stratigraphic range would be very unlikely.

From the geological eras before the Cambrian fossils are therefore much less known than from the times that followed, so that the entire period of the formation of the Earth is called the Precambrian ( approximately 4.56 billion years ago ) to the Cambrian explosion 542 million years ago. Today, you can this long period even without the aid of index fossils better break down (→ Geological time scale).

The Precambrian fauna

In the Ediacaran, the youngest formation in the Precambrian fossils first appear larger and more complex animals ( Ediacaran fauna). The relationship of these forms to the later live animals is unclear. Many researchers interpret them as Diploblasten, that is, organisms that possess only two germ layers as today's jellyfish and ctenophores. The conformation of some forms remembers about at sea pens. But they were already interpreted as a gigantic unicellular organisms that own, extinct kingdom next to animals, fungi and plants formed (so-called Vendobionten ). Possibly this is also to weave similar, fossil land plants. Since these shells and skeletal invertebrates living things have each left more or less vague and often difficult to interpret prints, a safe interpretation is hardly possible. The most developed and in connection with the Cambrian explosion most interesting among the ediacarischen fossils is the famous Kimberella, which is almost certainly a very primitive mollusks, ie a " Grand Snail " is. Another fossil that may be precursors of the Cambrian fauna, Spriggina, a segmented fossil that may represent an annelid worm. Probably about the same age as the Ediacaran fauna are tiny, plastic (that is, not only as imprints ) preserved microfossils from phosphate rocks of the so-called Doushantuo Formation of China, which are often interpreted as larvae or embryos of multicellular animals, although this interpretation lost according to recent studies of probability. Some researchers interpret findings from this era ( Vernanimalcula ) as remnants of small wormlike animals that belong to the tribe of About Deuterostomia. This interpretation is however disputed by other scientists.

Importance

With the Cambrian explosion, two new basic pattern for body plans appear. The fossils of the Cambrian creatures demonstrate for the first time with both a radially symmetric structure as well as with right-left symmetry and an additional running from top to bottom longitudinal axis of the body. These so-called " two-sided animals" ( bilaterian ) animals probably all descended from a common ancestor. This assumption is suggested by the discovery of the so-called Hox genes that control the basic body plans of all animals. The Hox genes of all bilateral animals are homologous, that is, their DNA sequences correspond to each other so much that you have to accept the emergence of the same original gene. Amorphous multicellular organisms need only one Hox gene for their body plan. Radial Symmetric living things require two Hox genes as genetic basic equipment ( at today cnidarians realized ). The living descendants of the more complex creatures of the Cambrian, however, have at least one Hox cluster of four Hox genes and to a Para- Hox cluster, also with four Hox genes. For the ancestors of all Bilateria ( " Urbilateria " ) is derived from the existing genes today reconstructs a basic plan with seven Hox genes. With today's Protostomia there are eight Hox genes in the chordate fourteen. In higher vertebrates, 39 Hox genes exist in four clusters; here one assumes a development of the original fourteen genes by two-time doubling of the entire genome ( with subsequent loss of some genes). The increasing complexity of genetic information easily explained, because in addition to the geometric body plan genes must specify a spatial and temporal coding for the specialization of different cells at different locations.

Another important new feature of the fauna of the Cambrian is the first appearance of animals that have hard shells or a skeleton. Also occur in animals without calcareous skeletal elements stable body wraps, for example, made ​​of chitin or cartilage on how they have survived in the Cambrian fossils from the Burgess Shale, for example, so-called. In addition, (apart from some representatives of the enigmatic Ediacaran fauna) animals did not occur with greater body before.

Since the Cambrian also the first time to a greater extent with Ichnofossils sediments, ie fossilized traces of animals occur, it can be assumed that also the emergence of animals that crawl on the seabed or dig in it may date 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 track pedum described as Treptichnus probably goes back to a Priapuliden.

Interpretations

The sudden, parallel occurrence of so many animals with very different body plans in a geologically short period provides for evolutionary research, since it has been discovered in the 19th century, an important problem dar. Previous attempts to explain solely by the accident of fossil record, are no longer credible because now many fossil sites have been re- discovered and evaluated worldwide. The Cambrian Explosion is now therefore agreed to be a real phenomenon. Although a much higher age of many animal phyla still appears quite possible, appropriate representatives may be at best been millimeter-sized, weichhäutige creatures. Otherwise you would have by now fossils of them, or at least have discovered from their tracks. About literally every eligible older fossil, which was interpreted accordingly, is bitter dispute between different researchers. Is need to explain especially why obviously on the young Earth life emerged relatively quickly, but it has then taken hundreds of millions of years until the appearance of more complex metazoans. Agreement on the trigger of the Cambrian explosion (or geologically relatively short in front of it a prelude to the Ediacaran Fauna), there are not within the research to date. The discussed theories can be summarized into two groups:

• Biological " arms race": According to this hypothesis, it was the appearance of the first complex multicellular organisms themselves, which brought about the development going. The first multicellular organisms have therefore had very little adaptive advantage. Articles thereof but once moving animals, possibly caused by predatory diet in a slow development, they presented a profound selection factor dar. shells and skeletons could then be developed as a protective mechanism against predation. The emergence of animals with grave forming and burrowing lifestyle was also a key event. You probably destroyed the stable microbial mats, which previously covered the ocean floor, where he created entirely new ecological conditions.
• Abiotic conditions: Other hypotheses assume that the emergence of higher multicellular organisms would have been impossible at an earlier time because the conditions of life in the oceans their earlier development not allowed. Accordingly, the pre-Cambrian seas would have differed in some key aspect of the modern seas. The most common hypothesis assumes that the oxygen content is only at that time reached a sufficient level to higher life in the sea. More recent theories point to the potential importance to high temperatures or salinities in the ocean, both factors that may influence the oxygen content significantly. Finally, it is also thinking about an influence of calcium content in the sea water. - According to this theory, the shells and skeletons would initially have been such a thing as waste products to excrete excess calcium.

Another well-known from the geological exploration of rocks from the late Precambrian fact is that at that time appeared very strong ice ages. Many researchers interpret the findings in a manner that was as good as the entire globe, including the oceans, ice-covered. As a " Snowball Earth " the stubbornness tables icing and the subsequent ice age Marinoische be called. It is believed that the breakup of the supercontinent Rodinia put a lot of free flow basalt, whose weathering the atmosphere eluded much carbon dioxide. The resulting reduced greenhouse effect led to the stubbornness tables icing.

Development

In the following the Ediacaran lowest level of the Cambrian, in Tommotium, animals appear for the first time with shells, especially early molluscs. The Tommotium fauna is also called "small shelly fauna " because there have been handed down primarily small, only millimeter-sized fossils. A representative is in the attachments ( Glaukonitsandstein ) occurring genus Mobergella, which can be found in northern Germany. Mobergella is also one of the oldest body fossils of Scandinavia.

Then again follows the Cambrian explosion in the strict sense. Wide within 20 million years ago, arthropods massively and become the dominant group in the former fauna. Especially to mention are the trilobites, but also strange-looking shapes like Wiwaxia (probably a bizarre Annelide ) Xenusion and Hallucigenia ( probably both representatives of Lobopoden ) or predatory Anomalocaris ( likely to be among the arthropods ), and first forerunner of the vertebrates.

Popularization

The Cambrian explosion was Stephen Jay Gould's book by another random person. The miracle of life popularized as a game of Nature ( 1989). It refers to the above-mentioned detected only in the Cambrian animal families there as " unique," " enigmatic " or " amazing " to almost bring the issue to the general public.

Journalists popularized the Cambrian radiation, the most important fossil evidence from the Burgess Shale originate in North America, in the United States in the direction of uniqueness. The TIME magazine devoted to the Cambrian a cover story titled Evolution 's Big Bang ( edition of 4 December 1995), and comparing the appearance of many new species and strains in the Cambrian with the Big Bang of the universe. Although now more and more precursors of these species were discovered from much older formations, and thereby the division of geological time scale that could be refined for the Precambrian of geologists, the interpretation of these journalistic article went more in the direction of a one-time event in which many animal phyla short in relatively time to have incurred.