Cambrian Explosion is calledPaleontologyAnd a big outstanding case in geology, which has been puzzling since Darwintheory of evolutionAnd other academic circles.From about 542 million years ago to 530 million years ago, it is geologically considered as the beginning time of Cambrian, and Cambrian strata suddenly appeared in many categories in more than 20 million yearsinvertebrateFossils, while in the earlier and older strata, no obvious ancestral fossils have been found for a long time, which is called "Cambrian explosion of life" by paleontologists, or "Cambrian explosion" for short.This is alsoPhanerozoicStart of.
Chinese name
Cambrian life explosion
Foreign name
Cambrian Explosion
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A major outstanding case in paleontology and geology
In a geological historical period called Cambrian, about 530 million years ago, various animals appeared on the earth in more than 20 million years, and they all started and appeared immediately.Arthropods, brachiopods, vermiform animals, sponges, chordates and a series of animals with the same morphology as modern animals have made a "collective appearance" on the earth, forming a prosperous scene in which many kinds of animals exist at the same time.
DarwinIn its《Origin of species》He mentioned this fact in his book and was greatly puzzled.He believes that this fact will be used as strong evidence against his theory of evolution.But he also explained that the ancestors of Cambrian animals must have come fromPrecambrianAnimals are produced through a long evolutionary process;Cambriananimal fossilThe "suddenness" and the lack of Precambrian animal fossils are due to incomplete geological records or the submergence of old strata in the ocean.
This is the "Cambrian explosion of life" which is still listed as one of the "top ten scientific problems" by the international academic community.According to the traditional and classical biological theory, namely Darwin's biological evolution, biological evolution has experienced a long evolutionary process from water to land, from simple to complex, and from low to high. This process is gradually realized through the slow rolling of the two wheels of natural selection and genetic variation,Chinese scientists have challenged this authoritative theory by studying ancient fossils.
The Chengjiang Biota, which is internationally known as "one of the most amazing discoveries in the 20th century", has opened a valuable scientific window for exploring the mystery of the "Cambrian explosion of life".On July 1, 1984, the "Chengjiang Biota" was first discovered in Chengjiang County, Yunnan ProvinceFossil groupThere are many types of animals, and the animal soft structure is very rare. For the first time, it vividly represents the magnificent landscape of ancient marine life and the original characteristics of living animals, and provides direct evidence for the study of the "Cambrian Explosion" with rich biological information.
The ancient fossil groups miraculously preserved the mineralized bones of organisms and a large number of soft tissue impressions, such as:epidermis, Sensorycilia, eyes, intestines, stomachDigestive gland, oral cavity, nerves, etc. Even some animals seem to have a full meal before dying, and the food filled in the digestive tract can still be identified.Chinese scientists are right“Chengjiang biota ”The continuous excavation, discovery and in-depth systematic research ofTrue arthropod、PincersAnd crustaceans, which confirmed that the modern phyla, subphyla and complex ecosystem originated in the early Cambrian, challenging the bottom-up inverted cone evolutiontheoretical model, providing fossil evidence for the top-down explosive theoretical model.There are more than 120 species of Chengjiang biota fossils that have been described, belonging to sponges, coelenterates, branchial drags, phyllopods, brachiopods, molluscs, arthropods, echinodermsChordateThere are more than ten animal phyla and some strange groups with unknown classification and location. In addition, there are many symbiotic algae.
By studying abundant fossil information, scientists have put forward various hypotheses to reveal the mystery of the Cambrian explosion.Researcher Chen Junyuan proposed the assumption that Cambrian biological mutation has extremely obvious spontaneous evolutionary behavior;Professor Shu Degan put forward the hypothesis of "Cambrian warm water and cold water paleobiogeographic divisions". But now there is no clear, conclusive and convincing explanation. Scientists are also continuing to devote themselves to field investigation and research, in order to fully solve the mystery of "Cambrian explosion" as soon as possible.
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The origin of the Cambrian explosion of life -Ediacaran fauna
Trilobites
The Ediacaran fauna was first discovered by Sprigg in 1947 in the Pound sandstone in Ediacara area, south central Australia.At first, people could not determine the age of this fauna, but it was finally determined asPrecambrian, 670 million years old.The Ediacaran fauna consists of three phyla, 19 genera and 24 lower speciesinvertebrate。The three doors are:Coelenterata，AnnelidaandArthropoda。There are 7 genera and 9 species of jellyfish;Hydra has 3 genera and 3 species;There are 3 genera and 3 species of the order Hydrobranchiales (Corals);2 genera and 2 species of medusae;2 genera and 5 species of polychaete;Arthropods belong to 2 genera and 2 species.Save More AsImprinted fossilAlthough their morphology and structure are primitive, they are considered to be one of the most important discoveries of paleontology in the 20th century.This discovery made the scientific community abandon the long held belief thatCambrianThere can be no epigenesis beforeanimal fossilTraditional ideas.Metazoa refers to various multicellular animals relative to protozoa.
The Ediacaran fauna contains a variety of exoticanimal fossil: The body is huge and flat, mostly oval or strip shaped, with smooth organic plasma membrane. It is the oldest and most primitive fossil found so far, and also the most convincing biological evidence found in the Archean strata.According to Seilacher, the Aidicara fauna can be divided into three types: radial growth, bipolar growth and unipolar growth.Except for the radiating growth type, which may have a group related to coelenterates, the other two types have no parentage relationship with the phyla emerging after the Cambrian.
Although there are many controversies about the nature of Aidikara fauna, its strange morphology makes many scholars believe that Aidikara fauna is the first time after the emergence of metazoaAdaptive radiation, which is different from the form and structure changes adopted by most modern animals.It does not increase the complexity of the internal structure, but only changes the basic shape of the body, becoming very thin, in strips or pancakes, making all parts of the body fully close to the external surface, and breathing and nutrition intake without internal organs.For example, most modern large parasitic animals adopt the method of maintaining a round or spherical external shape while evolving complex internal organs to expand the corresponding surface area (such as lungs and digestive tract). From the fossils, it can be seen that these organisms have highly differentiated tissues and organs, indicating that they are not the most primitive type.They represent the first radiation evolution after the emergence of metazoans. Therefore, it can be considered that the Ediacaran fauna was formed in the atmosphere at the end of Proterozoicoxygen contentAn attempt by metazoans to occupy the shallow sea on a large scale under lower conditions failed and led to extinction.In the later evolution process, metazoans took the second way to complicate the internal organs and diversify the development of species, that is, the evolution of biological systems.[2]
The typical representative of Cambrian eruption is China, which is known as one of the most amazing scientific discoveries in the 20th centuryChengjiang Biota, YunnanIt is the oldest and best preserved shelled metazoan group found in the world.This fauna is a young paleontologist in ChinaHou XianguangIn Yunnan in 1984ChengjiangcountyMaotian MountainFirst discovered.This is a rich and perfectly preservedFossil groupIts members include jellyfish like organismsTrilobites, non trefoil arthropods with appendagesGolden armyworm、Vermiform animal、Spongiform motilityThingsInternal anal animal、Annelida, non twistedBrachiopodAnimalsSoft tongued snails、Open Bones, algae, and even lowerChordateOr hemichords (such as the famousYunnanozoon ）Etc.Because many animalssoft tissueWell preserved, early in the studyinvertebrateThe morphological structure, lifestyle, ecological environment, etc. provide excellent materials, and also become an important window for exploring the outbreak events of macrocrustaceans on the earth.
The discovery of the Chengjiang biota in Yunnan has made us understandPrecambrianThe evolution and development of life from late Cambrian to early Cambrian have been clearly understood.Its significance in biological evolution can be summarized as at least two points:
Cambrian life explosion
First of all, the discovery of this fauna has once again confirmed the existence of the "Big Bang of Life" and has become an important pillar of the "Cambrian Outbreak" theory.At the same time, it is also connectedPrecambrianAn important link in the evolution of life from late to early Cambrian.
There were two excitingPaleontologyDiscovery.One was discovered in North America in 1910, about 530 million years agoCambrianOf“Burgess Shale fauna ”The other was found in southern Australia in 1947 between 680 million and 600 million years ago“Ediacaran fauna”。YunnanChengjiang biota It has become an important link between the Burgess fauna and the Ediacara fauna. With the in-depth study of the Chengjiang biota, the evolutionary relationship among the three Ediacara Chengjiang Burgess fauna will become clearer.
secondly,Chengjiang biota Found for“Discontinuous equilibrium”The theory provides a new factual basis forDarwinOftheory of evolutionImpact again.According to the theory of "discontinuous equilibrium", the evolution of living things is not like Darwin andNeo DarwinistIt is emphasized that it is a slow continuous gradual accumulation process, but a process of long-term stability (even constant) and short-term upheaval, leaving many gaps in the geological records.The discovery of Chengjiang biota shows that biological evolution is not always gradual, but a process of coexistence of gradual and leap forward.[2]
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Fossils of Cambrian life explosion
The Cambrian explosion has attracted numerous paleontologists andtheory of evolutionI went to look for evidence to explore its cause.The evidence of more than 100 years has produced two basic viewpoints to explain the Cambrian eruption.One point of view is that the Cambrian eruption is a fake, which isDarwinOr the view of the neo Darism.Because evolution is gradual, the so-called "explosion" only indicates that for the first timeBiofossilIt is found in the record thatPrecambrianThe living things that have been widely existing and developing, other living thingsFossil groupIt may be that the geological records are incomplete and "out of stock". The reason for this "out of stock" is that the Precambrian strata are experiencing heat and pressureFossilIt was destroyed.Due to the discovery of Precambrian fossilsSedimentary layerThere are a lot of bacteria andblue-green algaeSo simpleprokaryoteTherefore, this explanation is no longer convincing.Another view is that the Cambrian explosion represents a real event in the process of biological evolutionphysical environmentAnd the change of ecological environment.In 1965, two American physicists proposed that the Cambrian explosion was caused byEarth's atmosphereThe physical factor is the oxygen level.They believed that there was little or no free oxygen in the atmosphere of the early Earth. Oxygen wasPrecambrianalgaePlant photosynthesisAnd gradually accumulated.Metazoa need a large amount of oxygen. On the one hand, it is used for respiration, and on the other hand, it also usesozoneIt absorbs a large amount of harmful ultraviolet rays in the atmosphere to protect metazoans from harmful radiation damage.
Biologists discuss this problem from the ecological relationship of biology itself, because the geological evidence has denied this view of oxygen theory.About 1 billion to 2 billion years agoSedimentary layerIt contains a large number of severely oxidized rocks, which indicates that there were enough oxygen conditions for life explosion in this period.Therefore, biologists explore the cause of Cambrian outbreak from the emergence of two important events, namely, the emergence of sexual reproduction and the emergence of biological reapers.
Cambrian life explosion
According to the fossil data, eukaryotic algae had sexual reproduction about 900 million years ago. In fact, sexual reproduction appeared earlier.Sexual reproduction plays an extremely important role in the evolution of the entire biological world. Because sexual reproduction provides genetic variability, it may further increase biological diversity, which is one of the reasons for the Cambrian outbreak.The Bioreaper Hypothesis is AmericanecologistStanley proposed an ecological theory to explain the Cambrian outbreak, namely the harvesting principle.Stanley believes thatPrecambrianFor most of the 2.5 billion years, the ocean is a simplePrimary producerThe ecosystem formed by.The community in this system belongs to a single and unchanging community in ecology,Trophic levelIt is also simple and unique.Because physical space is characterized by a small number ofBiomeIt is tenaciously occupied, so the evolution of this community is very slow, and there has never been rich diversity.The key to the Cambrian outbreak is the emergence and evolution of herbivorous reapers, that is, eatingProkaryotic cellThe emergence and evolution of protozoa (cyanobacteria).The reaper isproducerGreater diversity creates space, and this increase in producer diversity leads to the evolution of more specific reapers.The nutrition pyramid has developed rapidly in two directions: the producers at the lower level have added many new species and enrichedSpecies diversityA new "reaper" is added at the top, which enriches the diversity of nutrition level.Thus, the biodiversity of the whole ecosystem is continuously enriched, and finallyCambrianThe emergence of a great explosion of life.
For“Harvesting theory”Scientists have not yet found direct evidence to prove its correctness. However, some indirect evidence supports this theory.One of the indirect evidence comes fromPrecambrianStromatoliteThese stromatolites composed of algae have preserved the most abundant producers in the Precambriancommunity。Stromatolites are only abundant in barren environments lacking metazoan harvesters, such as salt lakes with excessive salt.The abundance of algae in the Precambrian strata probably reflects the poverty of the harvesters at that time.In addition, the ecological field research also provides some indirect evidence. The research shows that putting predatory fish in an artificial pond will increase the diversity of plankton;Removing sea urchins from a diverse algal community will make an algae dominate the community and reduce its diversity.
As the second outstanding case in geological history, the Cambrian eruption has always been concerned by people.With the continuous discovery of fossils and the establishment of new theories, this mystery will finally be revealed to the world.
Latest research
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Cambrian life explosion
fromNanjing Institute of Geology and Palaeontology, Chinese Academy of SciencesAn international team led by him published the latest results of the Cambrian explosion research in the internationally famous geoscience journal Geology.They are based on the strata andPaleontological fossilsThe data prove that typical Cambrian animal skeleton fossils have appeared in the late Precambrian, and mixed with typical weakly mineralized animal skeleton fossils in the late Precambrian, which indicates that there is a gradual transitional evolutionary relationship between Cambrian biota and late Precambrian Ediacaran biota.
This new research result breaks the existing understanding of the Cambrian explosion, which did not happen suddenly in the early Cambrian, but started in the late Precambrian.
Cambrian life explosion
It is known that life on the earth has an evolutionary history of about 3.8 billion years or even earlier, but why did we know that complex life (multicellular animals), similar to human beings, which can both move and prey, only began to burst out on the earth 540 million years ago ("Cambrian explosion")?This is from 1859DarwinismThe representative work Origin of Species has been a major scientific puzzle puzzling the scientific community since its publication.
In the past 30 years, scientists from all over the world, including China, have made a series of major scientific discoveries in the research field of animal origin and Cambrian explosion, making important contributions to solving this scientific problem.For example, the early Cambrian in Yunnan, ChinaChengjiang Fauna (520 million years ago), almost all known fossils of modern fauna including primitive vertebrates were found, and at the same time, many fossils with various skeletons and mineralized shells earlier than Chengjiang fauna were found around the worldanimal fossil。These early Cambrian fossil discoveries further confirmed the true existence of the Cambrian explosion of animals, and increased the suddenness of the Cambrian explosion.
Revealing the evolution history of animals before the Cambrian explosion is the fundamental way to solve the mystery of the Cambrian explosion.In recent decades, a large number of fossils with complex morphology and animal characteristics have also been found in the strata before Cambrian.For example, the typicalEdiacara biotaLantian biota in southern Anhui, Weng'an biota in Guizhou, etc.However, these fossils are closely related to the Cambriananimal fossilThe difference is very large. It is generally believed that there is no evolutionary relationship.
According to the discovered fossil data, the basic consensus of the academic community is that a globalMass extinctionThe event coincided with a huge negative carbon isotope anomaly event (BACE event) in the global ocean.Therefore, this mass extinction event is believed to be caused by abnormal changes in the earth's environment, and this understanding also supports the outburst of the Cambrian explosion, making the mystery of the Cambrian explosion more complicated and confusing.[3]
Cambrian life explosion
Different from modern oxygen rich oceans, the early oceans of the Earth were mainly iron rich.Reducing Fe2+in the ocean has become one of the main obstacles that must be overcome for the gradual accumulation of free oxygen in the early oceans of the Earth.A large number of samples with different facies and lithology near the Ediacaran Cambrian boundary in southern China have also been analyzed for iron composition.fromNanjing Institute of Geology and Palaeontology, Chinese Academy of SciencesA research team composed of Dr. Lei, Zhang Hua, Cao Changqun and collaborators from West Carolina University, University of Science and Technology of China and Nanjing University in the United States has conducted iron component research on the core data of a shale gas exploration coring well (Well Chunye 1) in Chun'an County, western Zhejiang Province in the early stage,The records of continuous vulcanization environment unique to the Lower Yangtze region during this period were obtained.[4]
In order to further explore this periodironThe source, migration mode, precipitation mechanism and other scientific problems of the iron isotope need to be traced by non-traditional isotope research methods such as iron isotope.For this reason, the research team conducted iron isotope research on 40 samples from Well Chunye 1, and combined with the ratios of different elements and other indicators, found that the scale of the iron rich ocean in the Ediacaran Cambrian transition period was very limited, which was roughly close to the modern ocean.The main source of ocean Fe2+supply is the iron transported by atmospheric dust and rivers, rather than the iron from hydrothermal sources.Pyrite mineralization, rather than oxidation, dominated the main precipitation of Fe2+in the iron rich ocean during this period.[4]
This research also strengthens the previous research team's conclusion that "in the main episode of the Cambrian explosion, the ocean in southern China did not undergo full basin oxidation, on the contrary, the anoxic water bodies in western Zhejiang and other places have expanded to a certain extent, and the rise of the ocean oxygen content to close to the modern value should at least occur after the main episode of the Cambrian explosion (the fourth stage of Cambrian)”Understanding.[4]
