Despite the fact that quite complex three-layered animals existed before the Cambrian, evolutionary development in the early Cambrian appears to be extremely rapid. Many attempts have been made to explain the reasons for this “explosive” development.

Environmental changes

Increase in oxygen concentration

The Earth's earliest atmosphere contained no free oxygen at all. The oxygen that modern animals breathe, both contained in the air and dissolved in water, is the product of billions of years of photosynthesis, mainly by microorganisms. About 2.5 billion years ago, the concentration of oxygen in the atmosphere increased dramatically. Until this time, all the oxygen produced by microorganisms was completely spent on the oxidation of elements with a high affinity for oxygen, such as iron. Until their complete binding occurred on land and in the upper layers of the ocean, only local “oxygen oases” existed in the atmosphere.

Lack of oxygen could prevent the development of large, complex organisms for a long time. The problem is that the amount of oxygen an animal can absorb from its environment is limited by its surface area. The amount of oxygen required for life is determined by the mass and volume of the organism, which, as the size increases, grows faster than the area. An increase in oxygen concentration in air and water could weaken or completely eliminate this limitation.

It should be noted that sufficient oxygen for the existence of large vendobionts was already present in the Ediacaran period. However, a further increase in oxygen concentration could provide organisms with additional energy to produce substances necessary for the development of fundamentally more complex body structures, including those used for predation and defense against it.

Snowball Earth

There is ample evidence that the Earth underwent a global glaciation in the late Neoproterozoic, during which much of it was covered in ice and surface temperatures were close to freezing even at the equator. Some researchers have indicated that this may be closely related to the Cambrian Explosion, since the earliest known fossils date from shortly after the end of the last complete glaciation.

However, it is quite difficult to indicate a cause-and-effect relationship between such catastrophes and the subsequent increase in the size and complexity of organisms. It is possible that low temperatures increased the concentration of oxygen in the ocean; its solubility in seawater almost doubles when the temperature drops from 30 °C to 0 °C.

Fluctuations in carbon isotopic composition

Sediments at the Ediacaran-Cambrian boundary show a very sharp decline, followed by unusually large fluctuations in the C/C isotope ratio throughout the Early Cambrian.

Many scientists have suggested that the initial decline is associated with a mass extinction just before the start of the Cambrian. It can also be assumed that the extinction itself was a consequence of the previous decay of methane clathrates. It is widely known that the emission of methane and the subsequent saturation of the atmosphere with carbon dioxide causes a global greenhouse effect, accompanied by various environmental disasters. A similar picture was observed in the Triassic, when life was restored after the Permian mass extinction.

However, it is quite difficult to explain how a mass extinction could cause a sharp increase in taxonomic and morphological diversity. Although mass extinctions, such as the Permian and Cretaceous-Paleogene, led to a subsequent increase in the number of individual species from insignificant to “dominant”, in both cases, ecological niches were replaced, albeit by other, but equally complex organisms. However, no abrupt growth of taxonomic or morphological diversity was observed in the new ecosystem.

A number of researchers have suggested that each short-term decline in the C/C fraction in the Early Cambrian represents a release of methane which, due to the small greenhouse effect and temperature rise it caused, led to an increase in morphological diversity. But this hypothesis does not explain the sharp increase in taxonomic diversity at the beginning of the Cambrian.

Explanations based on the development of organisms

Some theories are based on the idea that relatively small changes in the way animals develop from embryos to adults can lead to dramatic changes in body shape.

The emergence of a system of bilateral development

Regulatory Hox genes turn on and off “working” genes in various parts of the body, and thereby control the formation of the anatomical structure of the body. Very similar Hox genes are found in the genome of all animals, from cnidarians to humans. Moreover, mammals have 4 sets of Hox genes, while cnidarians make do with a single set.

The Hox genes in different groups of animals are so similar that, for example, it is possible to transplant the human gene for “eye formation” into a Drosophila embryo, which will lead to the formation of an eye, but it will be a Drosophila eye, due to the activation of the corresponding “working” genes. This shows that the presence of a similar set of Hox genes does not at all mean that organisms are anatomically similar. Therefore, the emergence of such a system could lead to a sharp increase in diversity, both morphological and taxonomic.

Since the same Hox genes control the differentiation of all known bilaterian organisms, the evolutionary lines of the latter must have diverged before they began to form any specialized organs. Thus, the "last common ancestor" of all bilaterian organisms must have been small, anatomically simple, and most likely subject to complete decay without fossil preservation. This circumstance makes its detection extremely unlikely. However, a number of vendobionts may have had a bilateral body structure. Thus, such a development system could have arisen at least several tens of millions of years before the Cambrian explosion. In this case, some additional reasons are needed to explain it.

Small increases in genome complexity can have big consequences

In most organisms that reproduce sexually, the offspring receives approximately 50% of its genes from each parent. This means that even a small increase in genome complexity can give rise to many variations in body structure and shape. Much of biological complexity probably arises from the operation of relatively simple rules on large numbers of cells functioning as cellular automata.

Development track

Some scientists suggest that as organisms become more complex, evolutionary changes in the general structure of the body are superimposed by secondary changes towards better specialization of its established parts. This reduces the likelihood of new classes of organisms undergoing natural selection due to competition with “improved” ancestors. As a result, as the overall structure takes shape, a “development track” is formed, and the spatial structure of the body is “frozen.” Accordingly, the formation of new classes occurs “easier” in the early stages of the evolution of the main clades, and their further evolution occurs at lower taxonomic levels. Subsequently, the author of this idea pointed out that such “freezing” is not the main explanation for the Cambrian explosion.

The fossil evidence that could support this idea is mixed. It has been noted that variations in organisms of the same class are often greatest at the very first stages of development of the clade. For example, some Cambrian trilobites varied greatly in the number of thoracic segments, and subsequently this diversity decreased significantly. However, it was found that samples of Silurian trilobites have the same high variability in structure as Early Cambrian ones. The researchers hypothesized that the overall decline in diversity was due to environmental or functional limitations. For example, we might expect less variation in the number of segments after trilobites developed a convex body structure, which is an effective way of protecting it.

Ecological explanations

Such explanations focus on interactions between different types of organisms. Some of these hypotheses deal with changes in food chains; others consider an arms race between predators and prey that may have driven the evolution of hard body parts in the early Cambrian; Another number of hypotheses focus on more general mechanisms of coevolution.

"Arms race" between predators and prey

Predation, by definition, involves the death of the prey, due to which it becomes the strongest factor and accelerator of natural selection. The pressure on prey to adapt better should be greater than on predators because, unlike prey, they have a chance to try again.

However, there is evidence that predation was present long before the start of the Cambrian. Therefore, it is unlikely that it itself caused the Cambrian Explosion, although it had a strong influence on the anatomical forms of the organisms that arose.

The emergence of phytophages

Stanley suggested that the appearance of protozoa 700 million years ago, “gnawing” on microbial mats, greatly expanded food chains and should have led to an increase in the diversity of organisms. However, today it is known that “gnawing” arose more than 1 billion years ago, and the extinction of stromatolites began about 1.25 billion years ago long before the “explosion”.

Increase in size and diversity of plankton

Geochemical observations clearly show that the total mass of plankton became comparable to the present one already in the early Proterozoic. However, before the Cambrian, plankton did not make a significant contribution to the nutrition of deep-sea organisms because their bodies were too small to quickly sink to the seafloor. Microscopic plankton were eaten by other plankton or destroyed by chemical processes in the upper layers of the sea long before penetrating into the deep-sea layers, where they could become food for nekton and benthos.

As part of the early Cambrian fossils, mesozooplankton was discovered, which could filter out microscopic plankton. The new mesozooplankton may have served as a source of remains and also excreted excrement in the form of capsules large enough to sink quickly; these could have provided food for nekton and benthos, causing their size and diversity to increase. If organic particles reached the seabed, subsequent burial would increase the oxygen concentration in the water while simultaneously decreasing the concentration of free carbon. In other words, the emergence of mesozooplankton enriched the deep ocean with both food and oxygen, and thereby made possible the emergence and evolution of larger, more diverse inhabitants of the deep sea.

Finally, the emergence of phytophages among mesozooplankton could form an additional ecological niche for larger mesozooplankton predators, whose bodies, plunging into the sea, led to its further enrichment with food and oxygen. Perhaps the first predators among mesozooplankton were the larvae of benthic animals, whose further evolution was the result of a general increase in predation in the seas of the Ediacaran period.

Many empty niches

James Valentine has made the following assumptions in several papers: abrupt changes in body structure are “difficult”; changes are much more likely to survive if they face little competition for the ecological niche they target. The latter is necessary so that the new type of organism has enough time to adapt to its new role.

This circumstance should lead to the fact that the implementation of major evolutionary changes is much more likely in the initial stages of ecosystem formation, due to the fact that subsequent diversification fills almost all ecological niches. Subsequently, although new types of organisms continue to emerge, the lack of empty niches prevents them from spreading throughout the ecosystem.

Valentine's model does a good job of explaining the uniqueness of the Cambrian Explosion - why it only happened once and why its duration was limited.

The Cambrian explosion, which occurred about 540 million years ago, could not have been caused by an oxygen catastrophe. Chinese and Canadian scientists recently came to this conclusion.

The Cambrian explosion, or explosion of skeletal fauna, is the conventional name for a phenomenon that resulted in a sharp increase in the number of fossil remains of living beings in sediments corresponding to the beginning of the Cambrian geological period.

At that time, there was no significant increase in oxygen concentration in the air; it was only 10 percent. from the current level - they told the correspondent. Xinhua at the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences in Nanjing / Jiangsu Province, East China /.

For many years, the origin of organisms and the impact of environmental changes on them has been the subject of fundamental research.

According to the generally accepted point of view, 2.3 billion years ago an oxygen catastrophe first occurred on Earth, which resulted in the appearance of free oxygen in the atmosphere. According to researchers, at that time its concentration was only 0.1 - 0.01 percent. from the indicator of our days.

Then, 1 billion - 550 million years ago, a re-accumulation of oxygen in the air was recorded, then its content reached 10 percent. from the current level. In this regard, the first multicellular organisms appeared on Earth.

The dramatic increase in biodiversity during the Cambrian Explosion was previously thought by many scientists to be due to a global change in the composition of the Earth's atmosphere, thus bringing oxygen levels in the sea and air closer to current levels. The results of a study by a Chinese-Canadian team refuted this theory.

Doctor of Geological Sciences at the Institute, Xiang Lei, said that a team of Chinese and Canadian scientists studied the chemical composition of rock samples from the western part of Zhejiang province in 2014-2016, as a result of which they were able to refute the existing hypothesis.

All rocks used in the study were located at a depth of 400 - 760 m. Their age reached 510 - 550 million years. Scientists examined their content of organic carbon and elements such as iron, uranium, molybdenum, and also clarified the concentration of oxygen in the air and sea.

According to Dr. Xiang Lei, the oxygen content in the air only rose to today's levels 510 million years ago, having previously been at a consistently low level of about 10 percent. from the current level.

“The results of the study indicate the absence of large-scale oxidation of air and sea as a result of the Cambrian explosion. The hypothesis of a sharp increase in biodiversity due to an increase in oxygen content was not justified,” he was convinced.

What led to the Cambrian explosion? This question, as Xiang Lei noted, remains open for now.

The scientists' breakthrough research results were published in the British scientific journal Earth and Planetary Science Letters. The research work was supported by the Chinese Academy of Sciences and the National Natural Science Foundation of China.

For ease of study, the history of our planet and life on it was divided into periods of time, the boundaries of which are geological changes in the earth’s crust - processes of mountain building, rise and fall of land, changes in the outlines of continents, global climate changes.

The longest chronological periods of Earth's history are called eras (they lasted hundreds of millions of years). Eras are in turn divided into periods.

Scientists obtain all the information about the Earth's past by studying the geological evidence of the world record. The interior of the planet consists of various layers of rock and sedimentary rocks, which were formed under the continuous influence of external conditions that determined the appearance of the Earth in the distant past. Geological formations have preserved and carried through millions of years information about living organisms that inhabited the oceans and land in different geological periods. Thanks to this, today we have the opportunity to imagine the appearance of the Earth in the distant past and trace the evolution of life over 3.5 billion years from the moment of its appearance.

By examining ancient rocks and fossils, scientists have discovered two unexplained phenomena in the Earth's geological and biological past. The first phenomenon is called the Universal Unconformity, and is the contact of rocks from different geological periods that do not follow one another. Such contact violates the logical sequence of the location of layers, according to the chronological periodicals of various historical stages in geology. It should be noted that illogical contact of rocks is found everywhere. This is explained by the mixing of the structures of the earth's crust as a result of tectonic activity and erosion processes. However, the Global Unconformity cannot be explained by this, since it is widespread and reflects the uncoordinated contact of rocks approximately 2.9 billion years old and young Cambrian sediments that formed approximately 500 million years ago.

The second phenomenon concerning the biological past of the Earth is called the “Cambrian Explosion”. Scientists-paleontologists have dubbed this term the sudden rapid increase in the species diversity of living organisms in the Cambrian period (the very beginning of the Paleozoic era). This occurred over a chronological period of 30 million years (approximately 542–510 million years ago). In such a short period of time by paleontological standards, the number of biological species has increased hundreds of times. Suddenly, a great variety of shelled organisms appeared, and the first chordates and protocrustaceans (called trilobites) arose.

The most famous and studied evidence of the existence of these two scientific phenomena is located in the United States. This is the Grand Canyon located on the Colorado Plateau, Arizona. A favorite place for tourists from all over the world. It was there that paleontologists found the largest number of fossilized life forms, so different from the soft-bodied organisms that lived in the Ediacaran period that preceded the Cambrian.

For a long time, scientists from all over the world have been looking for clues to the phenomena of the Cambrian period. Recently, a theory arose in scientific circles that explained the nature of the occurrence of the Universal Unconformity and the “Cambrian Explosion” and established the relationship between these two unique facts of planetary history.

About 600 million years ago, great changes began to occur in the bowels of the Earth, causing colossal shifts on the surface of the planet. There was a movement of lithospheric plates, which tore apart the once united continent - Gondwanaland, many volcanoes simultaneously erupted waves of lava. Widespread earthquakes generated huge tsunamis. The surface of the land was flooded several times by the waters of the world's oceans, which was the main reason for the formation of World Discord.

Younger and shallow sedimentary strata are subject to destruction by water and related factors several times slower than older and deeper rocks. It was during periods of continental flooding that erosion and degradation of sedimentary rocks occurred, and ancient rocks were exposed and subjected to rapid erosion. The products of rock destruction by the billions of tons dissolved in the waters of prehistoric oceans. The concentration of potassium, calcium, magnesium, iron, phosphate, and sulfate ions suddenly increased. The acid-base balance of the world's oceans has sharply shifted towards the alkaline side.

The main principle of life is that in order to exist, a living organism must constantly maintain a constant internal environment. The primitive soft-bodied descendants of modern organisms had to quickly evolve to withstand the sudden change in living conditions. The instantaneous leap in the evolution of ancient life was a forced response to a sudden increase in the concentration of various salts in sea water. The result of this evolutionary leap was mineralization mechanisms that directed the evolution of ancient animals in a different direction.

This theory is supported by the simultaneous appearance of mineral skeletons in unrelated organisms during the Cambrian period. The main three types of mineral salts determined the direction of the further evolution of life - calcium phosphate, the mineral basis of the chordate skeleton, calcium carbonate and silicon oxide, which are the material of the shells of the first shelled creatures. Calcium, silicon and phosphates are the main components of the Cambrian strata that formed the sites of the World Unconformity.

The newly emerged young life forms had an advantage over the primitive soft-bodied ones, lacking solid organs. The new organisms had teeth for attack and defense, shells for defense, chords and hard skeletons that allowed them to move through the water with purpose and at higher speeds. The sudden acquisition of mineralization mechanisms allowed young creatures to multiply in unprecedented numbers and displace older life forms. The mass of the very first creatures with mineral organs was the basis for the formation of geological strata of the Cambrian period, which lay on ancient layers of rock.

Life forms with mineral skeletons began to form in the Precambrian, but it was the geological anomalies that formed the World Unconformity that accelerated this process many times over and gave it an explosive character. The trigger for the processes that created the appearance of the bulk of modern animal species was the rapid mineralization of the water of the world's oceans. Geological processes determined biological evolution for millions of years to come.

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Meaning) the appearance in Early Cambrian (about 540 million years on the evolutionary scale) fossil deposits of representatives of many divisions of the animal kingdom, against the background of the absence of their fossils or fossils of their putative ancestors in Precambrian deposits.

The theory of evolution has no reliable explanation for this “phenomenon.” The theory of creationism sees in this phenomenon further evidence of the failure of the theory of evolution and evidence in favor of Creation.

History of discovery

The discoverer of the Cambrian Explosion was the Englishman Robert Murchison, an aristocrat by birth who, under the influence of his ambitious wife, decided to take up science. While studying the fossils of multicellular ancient eras and the rock layers in which they were located, he encountered a clearly defined boundary, below which there were traces of only the simplest single-celled organisms - bacteria and algae. And the layer above (in the “Cambrian” deposits) contains a rich variety of biological forms. Murchison was a practicing Christian and shared Linnaeus' belief that “there are exactly as many species as the Creator originally created them”. In the revealed phenomenon he saw evidence of the action of God's hand. In the 1830s, Murchison published the results of his research.

Cambrian Explosion and Darwinism

Latest discoveries

For the past hundred years, it has been believed that vertebrates appeared later in the history of life. However, in 1999, fish fossils were discovered in Cambrian rocks in China. which confirms that fish appear suddenly in the fossil record along with all other types of animals. The Cambrian explosion became even louder. For these fully formed fish to appear in the Cambrian, the putative vertebrate ancestor must be pushed back millions of years into the Precambrian period, where there are no transitional forms for them or for all major types of organisms.

In 2006, in China, geologists discovered a fossilized embryo of an animal that, according to evolutionists, is 600 million years old. If evolution did occur, then modern embryos, after hundreds of millions of years of evolution, would be very different from those found in China. However, the embryos found in China are completely identical to the embryos of modern animals.

In 2008, fossil jellyfish were discovered in Cambrian strata that are almost identical to modern living species. Some had muscles, a number of stinging cells, complex sex organs and behaviors (including mate recognition and courtship), and complex eyes. According to evolution, while these jellyfish remained the same jellyfish to this day, during this time (about 500 million years) almost all of nature should have evolved - birds, pine trees, crocodiles, kangaroos, elephants, dogs, flowers, tomatoes, whales, geckos, etc.

See also

Links

The birth of complexity [Evolutionary biology today: unexpected discoveries and new questions] Markov Alexander Vladimirovich

Cambrian explosion

Cambrian explosion

At the very beginning of the Cambrian period, approximately 542 million years ago, many groups of animals began to acquire hard, mineralized skeletons almost simultaneously. Since just such skeletons are usually preserved in the fossil state, and the soft parts disappear without a trace, this event in the fossil record looks like a sudden, “explosive” appearance of many groups of animals (mollusks, arthropods, sponges, archaeocyaths, brachiopods, which were later joined by echinoderms, corals, bryozoans and others). Hence the generally accepted name for this event is the “Cambrian explosion.”

All that paleontology that we have talked about so far - Precambrian paleontology, that is, studying the Archean and Proterozoic eons with all the biomarkers, silicified cyanobacteria, acritarchs, gorodiscians and soft-bodied Vendian animals - began to develop intensively only relatively recently. Until this moment, the Precambrian strata seemed to scientists to be practically dead, containing almost no traces of life. The “Cambrian Explosion” seemed to be the sudden appearance of a wide variety of organisms, seemingly out of nowhere. Therefore, the Precambrian was called the cryptozoic - the time of “hidden life”, and the last stage of the development of the biosphere, which began with the Cambrian and included the Paleozoic, Mesozoic and Cenozoic eras, is called the Phanerozoic (the time of “explicit life”).

Darwin considered the Cambrian explosion to be one of the facts that did not fit into his theory of gradual evolutionary changes. It later turned out that the “explosion” was not actually that explosive. As we now know, the ancestors of many Cambrian groups lived before, but they were mostly skeletonless, soft-bodied. That is why paleontologists for a long time could not discover their remains in Precambrian rocks.

The mystery of the Cambrian explosion, however, remains, only now we are not talking about the sudden emergence, as if “out of nothing,” of many types of animals, but about the more or less simultaneous appearance of a mineral skeleton in them. This could be due to changes in environmental conditions. For example, such an effect could be caused by a sharp decrease in the acidity of water, as a result of which calcium carbonate (CaCO 3) - the most common skeletal-forming material in animals - became less soluble in sea water and easier to precipitate. A number of other explanations have also been proposed. Good popular stories about the Cambrian explosion and the theories proposed to explain it can be found in the books by A. Yu. Rozanov “What Happened 600 Million Years Ago” (1986). This is a real “living fossil”: the genus Astrosclera has existed for more than 200 million years (since the end of the Triassic period), and in the structure of its carbonate skeleton this sponge is very close to the forms that flourished in the Paleozoic (the so-called stromatoporates).

The skeleton of the astrosclera consists of small spherical elements that gradually grow and merge with each other. Scientists isolated the organic fraction from the sponge skeleton, and from it all the proteins. The three predominant proteins were found to be carbonic anhydrases. The researchers determined their amino acid sequence, and then used this sequence to “fish out” the three corresponding genes from the genome. This made it possible, by comparing the nucleotide sequences of the carbonic anhydrase genes of primitive sponges and higher animals, whose genomes have already been read, to reconstruct the evolution of these proteins in animals.

Scientists have concluded that all the many and varied carbonic anhydrases in animals come from a single ancestral protein, which was present in the last common ancestor of all animals. In different evolutionary lines, the gene of this original carbonic anhydrase was repeatedly subjected to independent duplications (doublings). This is how various new variants of carbonic anhydrases arose. The "last common ancestor of all animals" undoubtedly lived long before the Cambrian skeletal revolution. It turns out that animals were initially well prepared (preadapted) for the development of a mineral skeleton - from the very beginning they had enzymes that could sharply accelerate the formation of calcium carbonate. These enzymes were apparently used by Precambrian soft-bodied animals for other purposes - as already mentioned, carbonic anhydrases have enough work in the animal body even without skeleton formation. When environmental conditions began to favor biomineralization, different groups of animals, without any agreement, “attracted” part of their carbonic anhydrases to perform a new function.

It turned out that these primitive multicellular organisms already have a significant part of the complex of so-called postsynaptic proteins, which in more highly organized animals function in nerve cells and are involved in “signal reception.” Sponges, however, do not have nerve cells. Why do they need these squirrels? Apparently, they are involved in the exchange of signals between sponge cells. An animal may not have a nervous system, but if its cells do not “communicate” with each other at all, it will no longer be an animal, but a cluster of single-celled organisms. Later, when animals developed a nervous system, these “communication” proteins were useful for forming a system for exchanging signals between nerve cells. This example, like many others, shows that most evolutionary innovations do not arise out of nowhere, but are assembled from “available material,” and often very small genetic changes are sufficient to radically change the function of a protein or protein complex.