With 2020 turning out to be a very abysmal year, complete with natural disasters and pandemic, I’m positive that many of us, at some point in time, have heard, seen or shared jokes about how the ‘end’ is approaching. Though it doesn’t intimidate our dear Earth,as it already has some experience in witnessing the wiping out of most of the life forms, then, in habitation. Infact, this occurrence, known as mass extinction, has been a part of evolutionary history for several times. So, now the question arises; who, when, how and also, how many times? Let’s find out.

Introduction

Earth is believed to be approximately 4 billion years old, which gives ample time to geological and biological evolution. The compilation of all those changes and evolutions structure the present day Earth, aside the anthropogenic modifications. And some of the major events of the evolutionary history of Earth are mass extinction. That’s right. Even the destruction of species after species has contributed to the shaping of Earth’s present. 

The Catastrophes: Mass Extinction

Extinction is a natural phenomenon that takes place due to changes in the environment. Complete disappearance of a species is its extinction. A species is a group of organisms with similar biological, physiological and anatomical characteristics and which can interbreed and produce fertile offspring. The species which cannot adapt with the environmental changes, cannot evolve with subsequent reproduction or cannot reproduce fast enough to keep up with the decline of their population, perish and become extinct. 

A typical species becomes extinct within 10 million years of its first appearance. It is estimated that 99.9% of all species that ever habituated Earth are now extinct.

Mass extinction is referred to as the event which results in the simultaneous extinction of a large number of species present on Earth. These events resulted in severe reduction in the number of organisms in a short amount of time and also decreased the speciation. Hence, the rate of reproduction slowed down, and eventually led to the extinction. It is identified by sharp change in diversity and abundance of organisms.

Mass extinction has some special characteristics:.

• It took place over a short span of time and were quite intense
• It was a global phenomenon and included a large diversity of organisms
• Extinctions were believed to be more severe in case of marine organisms
• It is considered that plant species are more resistant to extinction than animals.

Mass extinction evidences can be found in fossils trapped in rocks and rock layers, which had been deposited on Earth’s surface for billions of years. On the basis of the incomplete fossil records, 5 major mass extinctions have been identified. In the descending order of age, they are:

1. Ordovician- Silurian Mass Extinction/Late Ordovician Mass Extinction (450-440 million years ago) at Ordovician-Silurian transition

In May 2020, studies suggested that due to global warming, caused mainly due to volcanism and anoxia, killed off 27% of all families, 57% of all genera and 60%-70% of all the species.49-60% of marine genera and nearly 85% of marine species were eliminated. There were two main causal events. In terms of percentage of genera becoming extinct, these are regarded as the second largest of the five major extinctions.

The LOME is considered to have occurred in two distinct pulses, one at the boundary of the between Katian and Hirnantian stages of the Late Ordovician Period. The second pulse occurred in the later half of the Hirnantian stage, and is associated with intense worldwide anoxia(oxygen depletion) and euxinia (toxic sulfide production), persisting through the Rhuddanian age of Silurian period.

The other plausible causes that were/are debated are glaciation, toxic metal poisoning, gamma-ray burst and weathering

2. Late Devonian Mass Extinction (375-364 million years ago) near the Devonian- Carboniferous transition

A prolong series of extinction resulted in elimination of 19%of all families, 50% of all genera and at least 70% of all species. It might have lasted for as long as 20 million years.The beginning and end of this period is marked by Kellwasser event and Hangenberg event respectively. According to Late Devonian sedimentary records, it was a period of several environmental changes, which might have directly affected biodiversity and caused extinction. Evidence of rate of carbon burial shooting up and widespread anoxia at ocean bottoms have been discovered. Possible triggers of these changes are:

• Asteroid impact 
• Supernovae event, which lead to drastic drop in atmospheric ozone ultimately resulting in genetic material damage due to UV rays.
• Plant evolution and expansion
• Weathering due to plant evolution, which caused drastic changes in nutrients cycles. That, in turn, led to deposition of surplus nutrients in water bodies, leading to eutrophication and subsequent anoxia.
• Reduction of atmospheric carbon dioxide, again due to evolution of plants
• Magmatism

3. Permian Triassic Mass Extinction/ Great Dying (252 million years ago) at Permian-Triassic transition

It is considered to be the most severe extinction in the evolutionary history of Earth. About 96% of all marine species and 70% of terrestrial vertebrates went extinct. In case of insects, 57% of all biological families and 83% of all genera faced extinction.

Evidence of about 1 to 3 pulses/phases of extinction are identified. The potential causes that had been identified include:

• Meteor impact event(s)
• Massive volcanic eruptions
• Climate changes due to large production of methane
• Sea level changes
• Increasing anoxia
• Increasing aridity
• Hydrogen sulfide emission

4. End Triassic Mass Extinction (201 million years ago) at Triassic-Jurassic transition

 In marine biodiversity, a whole class(conodonts) and 23-24% genera went extinct. The terrestrial organisms which suffered extinction include archosauromorphs (excluding crocodylomorphs, pterosaurs) and some dinosaurs went extinct. Groups like aetosaurs, phytosaurs etc., which were previously abundant, went extinct. The Jurassic age saw a diverse evolution of dinosaurs and  pterosaurs.

The possible causes of this mass extinction include:

• Gradual climate change, sea-level fluctuations or a pulse of oceanic acidification, decreasing diversity of land biomes
• Trend of increasing aridification
• Drop in sea level
• Extraterrestrial causes; includes impact from an asteroid or comet
• Volcanic eruptions

5. Cretaceous Tertiary Mass Extinction (65 million years ago) at Cretaceous-Paleogene transition 

This extinction is responsible for about 80% of all species of animals, including all the dinosaurs and many marine invertebrates. It also includes a range of fishes, mammals,pterosaurs and plesiosaurs, amongst other groups of animals. The main trigger for this extinction is considered to be asteroid impacts. This period recorded several large asteroid impacts.This hypothesis is further solidified by unusually high levels of iridium in the rocks of that period, which is usually not available on the surface of earth. 

As the after effect of this extinction,’impact winter’ was set into motion. Due to all the ash, dust and other matter that were dispersed in the atmosphere due to the asteroid impacts, blocked out sunlight for a long period of time.As a result, plants could not effectively perform photosynthesis, and keep up the oxygen level in the atmosphere. It also resulted in shortage of food for animals. The worst hit animals were the dinosaurs, due to their massive size.The smaller animals, on the other hand, strived with storage food and lesser oxygen, and thrived after the danger passed.Other calamities induced were tsunamis, earthquakes and increased volcanic activities.

Apart from these, scientists have identified several other minor extinctions, with the help of fossil and petrological records. 

Holocene Extinction: Living through History

At present, we are living through an extinction referred to as Holocene extinction or Anthropocene Extinction, which is considered to be the sixth major mass extinction. It is, needless to say, majorly a result of human activities. The current rate of extinction of species is 100 to 1000 times higher than the natural rate of extinction. The direct causes include hunting, deforestation, pollution, introduction of non native species in various diverse geographical regions etc.

If this doesn’t give mankind the wakeup call that it direly needs, I don’t know what would.

Websites referred:

https://en.m.wikipedia.org/wiki/Extinction_event

https://en.m.wikipedia.org/wiki/Holocene_extinction