There are obviously more than five mass extinctions on the earth. There should be more times, and some people estimate that it is as high as 20 times.
So how did the theory of five mass extinctions that we often hear come from? This statement comes from the research report published by Jack Sepkoski and David M. Raup in Science on March 1982. They counted the known events and identified five mass extinctions after the Cambrian explosion.
Ok, let's take a brief look at the geological evolution history of the earth.
Geological evolution history
According to the theory of five mass extinctions, the first mass extinction occurred at the end of Ordovician. From the above picture, it seems to have happened in the second half of the geological evolution history, but it is not like this. The extinction event at the end of Ordovician is only 450 million years ago, while the history of the earth is 4.6 billion years. The new discovery of 20 17 shows that the earliest organisms appeared 4.28-3.77 billion years ago. The word "five extinctions" should be accurately expressed as "five extinctions in Phanerozoic".
In fact, in the geological research after 1982, scientists found signs beyond fossil evidence, such as the change of isotope content in different strata, which confirmed the change of paleoclimate. In addition, the study of biogeographic sediments and some astronomical phenomena also makes us find that the origin of life is not simple.
Just as people still don't understand the cause of the Cambrian species explosion, people still don't understand the specific path of the origin of life. However, the activities of life have left a mark on the earth, which gives us a glimpse of the mystery.
In the literature in Note 2, British scientist Mattew S.Dodd and others found some microbial fossils in a geological structure named Nuvvuagittuq in Hudson Bay, Quebec, Canada, and they found some earliest evidence of life in these fossils.
Doubt the earliest life on earth
The geological structure of Nuvvuagittuq was formed around the spout of submarine hot springs in ancient times, which can eject iron and other minerals. Geologists believe that the age of Nuvvuagittuq is 3.77 billion years or 4.22 billion years, which means that they may have appeared as early as 340 million years after the formation of the earth. Dodd and his colleagues found filamentous bacteria in rocks, which contained iron compounds. The circular block connected with filamentous bacteria is very similar to the micro-anchor used by modern bacteria to attach to the rock surface. These rocks also contain all kinds of organic carbon that may be produced by bacteria. Modern bacteria living around hot spring exits, such as filamentous bodies, feed on iron compounds, which will cause tubular cavities in sediments. These features are also found in the above rocks. They think this is the oldest life on earth.
This is the earliest evidence of life published in authoritative scientific research journals, and some scholars (Francis Westel) are skeptical about it, because it is difficult for microorganisms to survive in fossils formed by volcanic eruption, and the microorganisms published in this paper seem to be a little too big.
However, there is a view recognized by academic circles that primitive life originated in the ocean.
Exploring the origin of life is a problem that integrates many disciplines, including geology, paleontology, earth science, astrobiology and earth science. The very hard data put forward by a discipline in the history of geological evolution are usually recognized by everyone and studied as a boundary.
The geology came out this time, which helped us to confirm the geological evolution history of oxygen.
In 2000, Farquhar of the University of Maryland published a study on sulfur isotopes in Science magazine [3], pointing out that the isotope ratio of sulfur changed about 2.45 billion years ago. He used a new isotope distillation method to determine the generation time of free oxygen on the earth.
His research found that rocks 2.45 billion years ago had a large number of reaction products containing S33 isotopes after the reaction. This mass-related fractionation (MIF) can only be realized when solar ultraviolet radiation is carried out in an oxygen-free atmosphere, and MIF sulfur disappears after that, so scientists believe that free oxygen began to appear in the earth's atmosphere in 2.45 billion years.
This helps us to realize the fact that at that moment of 2.45 billion years, the oxygen produced by photosynthetic organisms finally completely reacted with active chemicals in the ocean (such as iron and sulfur), so that it could escape into the atmosphere.
Before that, photosynthetic organisms did not occupy a dominant position, and the oxygen produced by photosynthetic organisms in the ocean was reacted by various active elements (mainly iron) in seawater, which was also manifested in sediments. The banded rust deposit (BIF) found in South Africa gives us evidence.
Rust deposits from 3 billion years ago, the red part in the picture.
So we can realize that there should be a mass extinction at this time. Oxygen produced by photosynthetic organisms has caused great harm to the pre-existing benthic thermophilic bacteria, anaerobic bacteria, iron bacteria, sulfur bacteria and methanogenic bacteria. Photosynthetic organisms such as cyanobacteria release oxygen in the form of waste gas, which is fatal to bacteria that adapt to anaerobic environment. Oxygen is not the substance they need to survive, and even takes away the essential elements for their survival.
Because of this, the great oxidation event that began in 2.45 billion years is also called the oxygen catastrophe. Unfortunately, at present, there is little fossil evidence that can help us fully show the extinction process, and there is little fossil evidence that can prove the existence of cyanobacteria. However, almost all scientists believe that this catastrophe may lead to the extinction of more than 98% biological species.
In a research report published in Geology in 2006 [4], Adriana Dutkiewicz and others studied the oil-bearing fluid inclusions found in Elliot Lake, Canada, and found that there were cyanobacteria-like organisms before the great oxidation event, which provided geological evidence for our conjecture.
Ancient cyanobacteria
After cyanobacteria took over the ocean, the free oxygen in the atmosphere began to increase, which caused another harm, that is, the methane in the atmosphere would decrease, and the methane would be reacted into carbon dioxide and water by the oxygen produced by photosynthetic organisms. At the same time, there is also evidence that methanogens in the ocean lost to cyanobacteria in ecological competition. In the research report [5] published in the journal Nature in August 2009, Kurt O. Konhauser and others studied the banded iron ore deposit (BIF), and they found that the nickel content in the primitive ocean was 400 times that in the current water body. Microorganisms called methanogens like nickel-rich water, which will produce methane and release it into the atmosphere. Methane can prevent the accumulation of oxygen and keep the earth warm. Scientists also found that during the period from 2.7 billion to 2.4 billion years ago, the overall abundance of nickel in the ocean decreased by 50%. This corresponds to the great oxidation event. Lack of nickel may kill methanogenic bacteria and give photosynthetic organisms a chance to release oxygen. The contribution of methane to greenhouse gases is 23 times that of carbon dioxide, which may directly lead to the longest ice age in geological history-Huron Ice Age.
The Great Ice Age will inevitably lead to species extinction. We don't know the specific process of species extinction during this period, but the ice age has a great probability of causing large-scale species extinction. The cold living environment is very unfavorable to the development of living things. The ice age lasting about 300 million years (240-265.438 million years ago) will make the species on the earth unbearable, and many species will be difficult to become extinct. However, we haven't found any fossil evidence to prove this.
A research report published in August this year [6] found that the oxygen isotope content of sulfate ore produced during the Great Oxidation Event in Canada was very low, so they thought that the primary productivity of the earth decreased by 80% during the Great Oxidation Event. At the same time, geology believes that basalt weathering buried organic matter at this time, while silicate weathering consumed carbon dioxide to form calcium carbonate. All this led to the official appearance of the Huron Ice Age.
Huron ice age
We have reason to believe that most species will take refuge in submarine craters or places called submarine heat sources, which are also considered to be the places where primitive life on earth originated.
Sadly, the discovery of bacterial fossils or stromatolites in the above two periods was not extensive, and the existing fossils also had the problem of difficult dating. We can only infer the ecological environment at that time through limited evidence and logical reasoning. Part of the reason is that these individuals are too small and live in the ocean, leaving few opportunities for fossils.
Scientists believe that during the 300 million years of the Huron Ice Age, the greenhouse gases generated by volcanic eruptions finally retained enough solar radiation energy, and the earth began to thaw, entering what geologists call a boring billion-year stage (also known as the boring age of the earth), that is, the stage from1800 million years ago to 800 million years ago. At this stage, the earth's environment, biological evolution and lithosphere are extremely stable.
The oxygen content of the earth in the boring era has hardly changed, and it is also very low compared with today, which is about one third of the current 10- 100. However, the ozone layer was formed during this period, which protected the charged particles and ultraviolet rays of the solar wind, protected the healthy growth of organisms and ensured the stability of genetic material. The ozone layer paved the way for the CAMBRIAN species explosion.
Scientists believe that there are green and purple photosynthetic bacteria in the ocean at this time. Scientists believe that the ocean at this time is purple [7]. Shil DasSarma, a microbial geneticist at the University of Maryland, believes that before chlorophyll appeared, there was a substance called retinaldehyde. The retina is easy to synthesize, which can absorb a lot of energy contained in green light waves, reflect red light and purple light, and make the ocean appear purple. Dassart Ma thinks that halophytes may be in an ecological dominant position at that time, absorbing a lot of green light waves with high energy, making chloroplast plants only use red and blue light waves with low energy density.
Purple ocean pictures on the internet.
At the same time, the famous geologist Donald? Canfield published a research report in Nature 1998, and put forward important viewpoints. The ancient ocean is very different from today's ocean. Today's ocean is rich in oxygen even in the depths. Ancient oceans were often stratified, with oxygen in the upper layer and extremely low oxygen in the lower layer. Canfield pointed out that when the deep sea becomes completely oxygen-free, sulfur bacteria will come out of the sediments and take over the seabed.
The metabolism of sulfur bacteria will produce waste hydrogen sulfide, making the deep sea fatal to oxygen-based organisms. The anoxic deep-sea layer is separated from the oxygen-filled upper seawater by the chemical thermocline, which rarely exceeds 200 meters from the sea surface. Canfield thinks that the ancient ocean has been in this state, and this theory is called canfield's ocean theory, also known as Euxinia. Today, this state can only be seen in the Black Sea.
Schematic diagram of canfield ocean
Then it seems that the living conditions of life on earth at that time were like this. Purple and green photosynthetic organisms grow slowly in the anoxic and sulfurized ocean, and even some bacteria can use solar energy to reduce hydrogen sulfide to sulfur as a photosynthetic mechanism to survive.
A billion years is a long time, but geologists have found some red algae fossils, which are considered as the earliest eukaryotes. The research report published in March, 20 17 [9] holds that fossil evidence shows that red algae appeared1600 million years ago.
Specimen overview B cell structure C intracellular structure
The discovery of red algae fossils can prove the ecological environment at that time and may open the way for the evolution of plants. Personally, at this time, some flagellates combined with various unicellular algae to form flagellate-like organisms, which opened the way for animal evolution. But there is still no fossil evidence to confirm this conjecture. At present, academic circles believe that the production of organelles should be the result of this way.
Fossil evidence also shows that plants landed about1300 million years ago, forming a combination of algae and cyanobacteria, namely lichen. This early lichen provided a preliminary environment for later plants to land. Higher plants may have landed 750-850 million years ago [10], which increased the free oxygen in the atmosphere.
A large number of plants landed, which made the total photosynthesis of the earth soar rapidly and greatly increased the oxygen content in the atmosphere. This may explain the CAMBRIAN species explosion that plagued Darwin.
At this stage of boring billion years, there is not much fossil evidence. We don't know how the ecosystem on the earth has changed during this period, but according to the distribution of life on the earth today, some bacteria should be extinct, and they occupy a small niche, such as halobacteria.
At this time, as analyzed above, life on the earth should be mainly bacteria, lichens, algae and other simple plants, and then it took a period of biological evolution until the Cambrian explosion 54 1 10,000 years ago, which produced almost all the animal "doors" today. The fossil evidence of the Cambrian explosion in the stratum is so obvious that many biologists, including Darwin, are puzzled.
This trend can be seen from the geological evolution history of earth's oxygen.
The red and green lines in the history of oxygen geological evolution are the upper and lower limits of the estimated value.
So the title mentioned five mass extinctions, when the earth entered Phanerozoic. The landing of plants makes the earth's environment more carrying capacity. At this time, biological evolution played a great role in the generation, forming a variety of large animals. The increase of species richness makes the fossil evidence colorful and diversified. It provides a good material basis for us to study paleontology.
Ok, now let's look at the five mass extinctions mentioned in note 1
Five species extinction events in Phanerozoic.
The severity of species extinction depends on the height of blue column.
Let's introduce it in chronological order.
1, Ordovician-Silurian (O-S) extinction event
It occurred in the late Ordovician or the transitional period between Ordovician and Silurian, about 450-440 million years ago. Extinction is global, with 49-60% of marine genera and nearly 85% of marine species extinct. At this time, there are a large number of animals such as poda, brachiopod, exodonta, cephalopod, trilobite, graptolite, filter-feeding plankton and so on.
There are several possible reasons.
1. The Earth Ice Age-Early Paleozoic Great Ice Age, also known as the Andean-Saharan Ice Age.
This is the most widely accepted statement at present [1 1]. 420 million years ago, a huge plate called Gondwana (Southern Continent) moved to the South Pole. The ice sheet formed, then the seawater condensed, and the seawater was released after the earth entered the interglacial period. The rise and fall of sea level changed the climate and living environment, and many species became extinct. Related strata were found in the Late Ordovician strata in North Africa, which came from the Antarctic at that time. These strata recorded five glacial pulses at the same time, which is strong geological evidence.
The southern continent is connected with the South Pole.
B gamma ray bursts (GRB) destroyed the ozone layer of the earth.
In 2005, scientists from NASA and the University of Kansas published a study in the International Journal of Astrobiology [12], suggesting that it may be caused by a gamma-ray burst released by an extreme supernova, which lasted for ten seconds, seriously destroying the ozone layer, making ultraviolet rays from the sun reach the earth, leading to the death of a large number of creatures on the ground and near the ocean, thus destroying the food chain. At the same time, this process can cool the earth and form unfavorable climatic environment such as glaciers.
Because of my educational background, I think this idea is very big, and the authors obviously think that their data is not very hard, which also shows that this phenomenon has contributed at least part of the destructive power.
GRB
Volcanic activity and weathering hinder the global carbon cycle.
As mentioned above, geologists believe that volcanic activity will consume carbon dioxide in the atmosphere, which is related to the fact that rock weathering will bury some organic matter and prevent it from re-entering the global carbon cycle. Simply put, it is the formation of fossil fuels.
Carbon dioxide is a greenhouse gas, and their reduction helps to form an ice age and make species extinct.
D, metal poisoning
After the global carbon cycle was blocked, the oxygen production capacity of photosynthetic organisms decreased, and the metal elements in seabed sediments escaped, making most of the marine organisms extinct.
2. Late Devonian extinction event
It happened 376-360 million years ago and can be divided into Kelwasser incident and Hangenburg incident. This long-term extinction event puzzled scientists. However, the sedimentary records show that the environment changed obviously in the late Devonian, and there is evidence that the bottom water of the ocean is generally anoxic. The carbon burial rate has soared, and benthos have been destroyed, especially in tropical areas and coral reef communities. This directly affects the survival of living things and leads to extinction. The reasons for these changes are still controversial.
The possible reasons are as follows
First, foreign objects (comets or asteroids) hit the earth.
This statement was put forward by 1969, and I personally think it was influenced by the cold war environment at that time. So the research direction is skewed. Unlike when the dinosaurs died out, they found the exact crater to prove it. This time, they didn't find the exact crater.
Suspected Alamo hit Nevada
B. Plant evolution influenced the global carbon cycle and triggered the Late Paleozoic Great Ice Age, also known as the Karoo Ice Age.
In Devonian, the vascular bundle structure evolved after plants landed, which made the height of plants grow from 30 cm to 30 m. Higher plants represented deeper and larger roots, which further accelerated soil weathering, made nutrients in the soil enter the ocean, resulting in marine eutrophication, and then algal blooms broke out, leading to the decrease of marine species, global hypoxia, temperature drop and further deterioration of the environment, endangering terrestrial life.
At the same time, the rapid growth of land plants led to the rapid decline of carbon dioxide content in the atmosphere at that time, and the rapid growth of plants also made some plants buried underground as fossil fuels (oil), and there was no way to re-enter the carbon cycle. The content of carbon dioxide in the atmosphere has been reduced from 15 times to 3 times today. Evidence such as glacial deposits in northern Brazil (near Devonian Antarctic) shows that extensive glacial activities occurred at the end of Devonian. This glacial activity triggered a serious species extinction event.
Global carbon cycle
The marine life affected by the late Devonian extinction event includes brachiopods, trilobites, ammonites, conodonts, jawless superfamily and all shield fish. However, land plants and freshwater creatures are relatively less affected.
3. Permian-Triassic extinction events
It happened between Permian and Triassic, about 250 million years ago. In terms of extinct species, at that time, 70% of terrestrial vertebrates and up to 96% of marine life disappeared, and this extinction event also caused the only extinction of insects. It will take millions of years for the ecosystem to fully recover, which is longer than other mass extinctions. It is the most serious of the five extinction events, and it is also known as the most serious extinction event so far-if proportional, I think the extinction ratio caused by the great oxidation event should be higher than this one.
There are many scholars who have studied this extinction event, so there are many speculations.
1. Planets or meteorites hit the Earth.
I have to vomit, this theory is a panacea, never absent. This time they didn't even find a crater worthy of serious doubt. First, the shaking Shi Ying layer in Antarctica was used as evidence. Later, the microstructure proved to be the product of volcanic activity. Later, they discovered several craters with unknown geological age, which once again aroused serious doubts. Later, they decided to use the power of art, so we saw a photo of a meteorite hitting the ocean. In this way, fans of this theory can not only explain why the crater could not be found (the traces left by meteorites entering the sea were worn away by later earth activities), but also explain that meteorites did cause mass extinction.
Meteorites hit the ocean.
This idea only found some exact geological evidence in the extinction of dinosaurs, which is very unreliable in this incident.
B, volcanic eruption leads to sunshade, and after destroying terrestrial ecosystem, it leads to acid rain and global warming, leading to species extinction.
Siberian basalt
Evidence from basalts in Siberia and Emei Mountain in Sichuan, China shows that the most serious volcanic eruption occurred at the end of Permian, especially in Siberia, which contains 20% pyroclastic rocks. Dust clouds and acidic aerosols formed by volcanic eruption blocked sunlight, leading to the collapse of terrestrial ecosystems, and falling acid rain flowed into the ocean, leading to the collapse of marine ecosystems.
The carbon dioxide produced by this process also makes global warming, further destroying the living environment of living things.
This statement has also been questioned, and people suspect that the power of this volcanic eruption is not enough to change the global ecology.
C. The formation of Pangaea led to the collapse of the offshore ecosystem.
Pangu continent was formed at the end of Permian.
Part of the East Asian plate did not converge with Pangea until the end of Permian. The formation of Pan-continent made most shallow waters in the world disappear, and shallow waters are the zone with the most life in the ocean. This can explain why marine life will be greatly reduced, while terrestrial life is unlikely to be affected, which contradicts reality.
It is believed that this certain geographical phenomenon should not cause such a serious extinction event.
D, gasification of combustible ice
Combustible ice is mainly methane hydrate. Scientists found that the ratio of carbon 13/ carbon 12 fluctuated in the stratum at that time. At the same time, there is evidence that [13] global temperature has increased by about 6 C near the equator, and even more at higher latitudes.
The gases that cause the greenhouse effect are mainly methane and carbon dioxide. Global warming has destroyed the earth's ecosystem and caused tragic species extinction.
This theory can explain global warming, but it is not easy to make methane in the atmosphere disappear quickly.
E, marine anoxic vulcanization
This is a bit like canfield's ocean theory. Geological evidence shows that in the late Permian, there was lack of oxygen in the ocean and sulfide escaped from the seabed. Hydrogen sulfide produced in seawater is discharged into the atmosphere, which harms the global biological system and the ozone layer. Ultraviolet rays further harm the global life, which leads to species extinction. A large number of evidence of the existence of green sulfur bacteria was found in shallow water strata in the late Permian, which can support this conjecture.
The advantage of this conjecture is that it can explain the mass extinction of plants, which will increase the content of methane, otherwise plants should flourish in an environment with high carbon dioxide content. Spore fossils at the end of Permian further support this theory. Most of them are abnormal in shape and may have been irradiated by ultraviolet rays.
4. Triassic-Jurassic extinction event (TR–J)
It happened about 200 million years ago, and the impact of this extinction event spread all over the land and sea. Among marine life, 20% families have disappeared, including the famous conodonts, many large crocodiles, most theropods, and many large amphibians. The Triassic-Jurassic extinction event made at least 50% species disappear at that time. This extinction event caused a niche vacancy on the land, making dinosaurs the dominant land animals in Jurassic. The extinction event occurred before the division of Pangea and lasted less than 10 thousand years. This extinction event caused obvious differences between Triassic dinosaurs and Jurassic dinosaurs.
The possible causes of this extinction event are as follows
A, aliens, asteroids or comets hit the earth.
Traditionally, no corresponding crater was found this time, but this idea is inextricably linked with species extinction. Every time a species goes extinct, someone insists that the earth has been hit. Several suspicious craters are either too small or too old.
B, the volcano continues to erupt, causing extreme climate.
The approximate location of the magma area in the middle of the Atlantic Ocean
Kemp is the largest igneous province in the world, with an area of about 1 1000000 km. The eruption of Mount Kemp occurred about 2,065,438 million years ago and lasted about 600,000 years. Volcanic eruptions release carbon dioxide or sulfur dioxide and aerosols, which will lead to strong global warming (from the former) or refrigeration (from the latter).
This may lead to drastic changes in the earth's environment and mass extinction of species.
C. climate change caused by natural evolution process
The geological structure of Europe seems to indicate that the sea level declined in the late Triassic and rose in the early Jurassic. Although sea level decline is sometimes regarded as the chief culprit of marine extinction, the evidence is not conclusive, because many sea level declines in geological history have nothing to do with the increase of extinction. However, there is still some evidence that marine life is affected by secondary processes related to sea level decline, such as decreased oxygenation (due to poor circulation) or increased acidification. These processes do not seem to spread all over the world, but they can explain the local extinction of marine animals in Europe. This is not enough to explain the global extinction phenomenon.
Later research pointed out that by the end of Triassic, the trend of drought had obviously intensified. Although high latitudes like Greenland and Australia have actually become humid, geological evidence shows that the climate change in most parts of the world is more intense. Evidence includes the increase of carbonate and evaporite deposits (most abundant in dry climate) and the decrease of coal deposits (mainly formed in wet environment). In addition, the climate may become more seasonal, and long-term drought is interrupted by severe monsoon.
But none of these can perfectly explain species extinction.
5. Cretaceous-Paleogene extinction event (K-Pg)
It happened 66 million years ago, also known as Cretaceous-Tertiary extinction event (K-T extinction for short), commonly known as dinosaur extinction. In this extinction, almost all large terrestrial vertebrates were extinct, only birds and some creatures that could hide in water or caves survived. Therefore, the terrestrial niche is almost empty, providing conditions for mammals to occupy the earth.
Although I have been talking about planetary extinction, it is generally believed that the cause of this extinction is that asteroids hit the earth, and scientists found a lot of iridium at the boundary of strata.
Located near Interstate 25 in Colorado, USA. The red arrow is the Cretaceous-Paleogene boundary (famous for its rich iridium)
Scientists artificially created a planetary fragment with a diameter of 10 km, which hit the Mexican Yucatan Peninsula 65 million years ago and formed the chicxulub crater. Meteorite impact formed dust and sulfuric acid aerosol covering the sky. This led to the obstruction of photosynthesis of terrestrial organisms, and aerosol landed in the ocean in the form of acid rain, which led to serious ocean acidification.
Artist's description of the collision scene of planets.
However, there are still many theoretical challenges to this conjecture, such as the sea level decline exposing the continental shelf and the partial extinction of marine life. On land, dinosaurs became extinct because of the lack of food caused by the evolution of angiosperms.
This requires further research by scientists.
There is no particularly hard data in the current research to convince everyone. In modern science, our understanding of ecology, a macro discipline, is not thorough, and even such a simple problem as global warming is highly controversial. If ecology develops further, I think we can discuss mass extinction from a more systematic level. I only mentioned the well-known extinction event in academic circles at present, and did not discuss it in depth. But I think the evolution of species before Phanerozoic is also interesting. There have been many extinctions.