Ever since life first appeared on Earth in the form of Prokaryotes about 4.28 billion years ago, more than five billion different life forms have thrived on our planet. Life originated from the hydrothermal vents in the depths of early oceans and gradually occupied every ecosystem on Earth from soil to rock to hot springs by multiplying and diversifying into different species. As the evolutionary life cycle continued, older species vanished from sight and passed into oblivion while new ones came to the fore to maintain a balance. However, there have been geological instances in history when the loss of species outstripped the emergence of new species and disrupted the evolutionary life balance in catastrophic events known as mass extinctions.
What is Mass Extinction?
Mass extinction is an event marked by a sharp decrease in the diversity and abundance of life on Earth. It occurs when the current extinction rate exceeds the standard extinction rate and results in the loss of about three-quarters of all existing life forms spread across Earth’s biosphere.
In the last 540 million years, since the Cambrian Explosion, when life first diversified into a wide variety of forms, there have been five major extinction events that surpassed the standard extinction rates and can be characterized as mass extinctions.
Today Earth is facing a biodiversity crisis due to the exploitation of the planet by its residents. To ascertain whether this crisis might be a part of the sixth mass extinction, we have to look back in time at the Big Five and understand what caused these cataclysmic geological events and led to the disappearance of more than 99 percent of all life forms that ever existed on Earth.
The Big Five— Finding the Key Driver of Mass Extinctions
The five mass extinctions stated below each lasted between fifty thousand years and 2.76 million years. Although paleontologists cannot discern a pattern from their timings, they have found that all these major extinction events happened after every 100 million years on average following the Caribbean Explosion.
1. Ordovician-Silurian Extinction
The first mass extinction transpired about 440-450 million years ago during the epilogue of the Ordovician period. It wiped out over 70 percent of all species, making it the second-deadliest of the five mass extinctions in Earth’s geological history.
The end-Ordovician mass extinction was widely considered to be consisting of two pulses associated with a planetary-scale period of glaciation followed by a rapid warming period. However, recent studies suggest that this mass extinction was a single-pulse event that was an emanation of global warming due to volcanism and anoxia.
2. Late Devonian Extinction
The second mass extinction happened near the Devonian–Carboniferous transition around 383-359 million years ago. It lasted roughly twenty million years, extirpating about 75 percent of all species on Earth, the majority of which were bottom-dwelling invertebrates.
The demise of the Devonian period was a time marked by dramatic changes in Earth’s climate. It saw rapidly alternating conditions of global warming and cooling, accompanied by plummeting sea levels. At the same time, plants began to take over dry land, which resulted in a drop in the global carbon dioxide concentration, which, in turn, led to periods of anoxia.
3. Permian-Triassic Extinction
The third and the most pernicious of the mass extinctions struck at the end of the Permian period around 250 million years ago. It eliminated more than ninety-six percent of all marine species and about seventy percent of all land species existing at that time, thus earning the title ‘The Great Dying’. Out of all the mass extinctions, it is the only one that annihilated such a large number of insect species.
This calamitous extinction event had long-lasting repercussions on the entire evolutionary cycle. It took marine ecosystems between four to eight million years to recover from the devastation of Permian mass extinction, while the vertebrates recovered in no less than thirty million years.
Most paleontologists suggest that a colossal volcanic eruption in what is today Siberia was the biggest trigger for this catastrophic mass extinction. The massive volcanic activity released over fourteen trillion tons of carbon dioxide into the air, resulting in global warming. The magma from the volcano infiltrated coal basins, releasing more greenhouse gases and increasing ocean toxicity.
Other suggested causes from scientists include an asteroid impact that permeated the air with pulverized particles. Such a heavily polluted atmosphere would have blocked the sunlight and resulted in acid rains. The toxic air could also have caused breathing problems in many land species.
4. Triassic-Jurassic Extinction
The Great Dying left a blot on the evolutionary life cycle that wiped off after a long time; however, it also opened up the planet for new forms of life and blossomed diversity once the species recovered. But after only fifty million years, once again, life suffered a deadly blow that annihilated up to seventy-five percent of all land and marine species. According to scientists, the emission of high amounts of greenhouse gases from the modern-day Atlantic Ocean warmed the Earth and raised carbon dioxide concentration in the atmosphere, triggering this mass extinction.
5. Cretaceous-Paleogene Extinction
The most recent and most popularly known mass extinction event occurred during the Cretaceous period around sixty-six million years ago, wiping out three-quarters of the plant and animal species. Around seventy-five percent of all life forms on Earth at that time went extinct in the aftermath of this cataclysm.
The Cretaceous-Paleogene extinction had a significant impact on human evolution as it marked the demise of non-avian dinosaurs. On a random day about sixty-six million years ago, an asteroid hit the waters of modern-day Yucatán in Mexico. This extraterrestrial impact started a chain of events that resulted in the collapse of ecosystems supporting non-avian dinosaurs. Some scientists also argue that the volcanic eruption in Deccan Flats of modern-day India could have aggravated the grave climate conditions caused by the extraterrestrial impact and dealt the final blow leading to the mass extinction. The obliteration of these super predators presented a unique opportunity for mammals to diversify and occupy new habitats, paving the way for human evolution.
The Ongoing Biodiversity Crisis: Are We Headed Towards a Sixth Mass Extinction?
By studying the Big Five, we can deduce that most major extinction events were triggered by the same driving force: abnormal changes in Earth’s carbon cycle. Monstrous igneous and volcanic eruptions that ejected massive amounts of greenhouse gases or catastrophic extraterrestrial impacts that flung enormous volumes of debris into the atmosphere all led to the same end-result: global warming.
We are currently treading on a somewhat similar path and getting in the throes of another mass extinction. However, the difference is that our biodiversity crisis is not driven by some igneous activity or asteroid impact but by our own actions. Human activities like deforestation, destruction of habitats, overfishing, pollution, and human-caused global warming are putting up to a million different species of plants and animals at risk of extinction.
The current extinction rate is about ten to a hundred times faster than any in the past mass extinctions while at least a thousand times higher than the background extinction rate. About 99.9 percent of all species that ever existed on Earth have already perished. There are about 8.7 million species on the planet right now, out of which a million are expected to go extinct in a few decades. If the rate of extinction continues to accelerate this way and the species threatened with extinction perish in the imminent future, we could be in the clutches of the sixth mass extinction in less than 540 years! Could we put a halt to this impending catastrophic event? According to scientists, the extinction storm is already well under its way, and stopping it is now scientifically impossible!