When Earth Nearly Lost Everything: Top 5 Mass Extinctions

Life has existed on earth for as long as 4 billion years, and has spent that time evolving from simple, microscopic specs into all forms of life that we see on our planet today. However, it hasn’t always been smooth sailing. Nearly everyone is familiar with the extinction event that took the dinosaurs out of the picture around 65 million years ago, however, this is but one of the many times life on earth nearly ended, and it’s far from the worst one. Depending on how mass extinctions are classified, some researchers believe there to have been as many as 20 such events in our earth’s history, but today we’re going to cover what is agreed upon as The Big Five, that is, the five deadliest periods in our planet’s history that nearly brought complex life to its ultimate demise.

Ordovician-Silurian

At the end of the Ordovician Period, around 445 million years ago, animals were in some of their most primitive forms, and nearly all life existed in the oceans at this point, which makes sense considering that sea levels were so high that almost the entire continent of North America was underwater at times.

Going for a swim in this ancient ocean, you could expect to run into some strange creatures, such as the eel-like conodont, a trilobite, and maybe even some primitive fish. Life had diversified significantly throughout this period, forming the basis for many of the organisms that would later follow, but millions of years of evolutionary progress were suddenly reversed when two devastating events took a toll on the oceans, wiping out a staggering 85% of all species over the course of a million years or so.

There’s no evidence of a meteor or comet impact, or even intense volcanic activity, and so the cause of these two events is less clear than with others. One of the leading theories is that Gondwana, the super continent of the time, moved a bit too far toward the south pole, leading to rapid glacier formation and a subsequent dropping of sea levels. This drop also may have led to ocean anoxia, which is the name for what happens when the water in oceans is depleted of its dissolved oxygen, making it rather toxic for most life.

However, this isn’t the only possibility. Many researchers have suggested that this extinction event was actually initiated by a gamma ray burst. A gamma ray burst is an insanely powerful blast of gamma radiation that is released when certain sizes of stars explode in a supernova.

A typical GRB is so intense that it releases as much energy in a few seconds as our sun will in its entire 10-billion-year life span. They’re extremely rare, with only a few happening per galaxy per million years, but if one of these went off within a few thousand light years of the earth, it very well could have devastated the ozone layer and bathed life on earth in a serious amount of harmful rays.

It’s very possible that both explanations are true, and constitute the two separate events seen in the fossil record, but whatever the case, only 15% of species survived the ordeal, and it would take millions of years to recover.

Late Devonian

Fast forward about 70 million years or so, to the Late Devonian Period. During the Devonian, the very first animals had begun to live entirely on land, alongside primitive plants which quickly took over the landscape. There had been plants before, but it was during this time that they’d developed root systems, and huge forests were starting to pop up. Marine life had really taken off as well, with the seas now brimming with armored fish like the terrifying Dunkleosteus, one of the first apex predators in history weighing well over 600 kilograms.

But even this mighty hunter was nearing its end. Throughout the Late Devonian, a series of events rocked the earth’s biodiversity, and though there’s quite a bit of debate about how long it took for these events to unfold, with estimates varying from 500,000 years to over 20 million years, which, admittedly, is a pretty long stretch of time. There’s also confusion about what exactly happened, since the evidence is a bit lacking, and hypotheses vary.

One of the more interesting ideas is that since plants had begun to swell in size and population, it’s possible that this upset the balance of the atmosphere by absorbing too much carbon dioxide. Carbon dioxide is, of course, a greenhouse gas and its increasing presence in the atmosphere is largely correlated with a warming climate, but in this case the opposite would be true, and instead of global warming, these plants may have caused global cooling.

It’s also possible that a meteorite contributed to all of this, with the most likely candidate being the Siljan Ring in Sweden, the largest impact crater in Europe.

But this is mostly just educated speculation. What is known for sure is just how devastating this period was. Coral reefs were absolutely massacred, and would require 100 million years to recover, most of the armored fish went extinct, and so did the recently evolved ammonites. In total, an estimated 70% of all species went extinct.

Fun fact, because the oceans also went through a period of anoxia during the Late Devonian, it was rather hard for organic matter to decay, trapping lots of it in porous rock that would one day become an important source of oil in Canada and the United States.

Overall, the late Devonian was a disaster for life on earth, and though life on land was largely spared during this extinction, no one would be safe during the next.

The Great Dying

The Permian-Triassic extinction event was by far the deadliest in history, earning it the nickname “the Great Dying”. This event took place around 250 million years ago, and lasted for about 50,000 years. It was so deadly that even the mighty trilobites were wiped out, marking an end to their 270-million-year fossil record, having survived both previous mass extinctions.

As for what caused it, well, there’s a bit of a list. First and foremost were the long-term eruptions of a volcanic chain underneath modern-day Siberia, creating a huge area of volcanic rock that is still present to this day. These eruptions would have pumped high levels of sulfur dioxide and carbon dioxide into the atmosphere, which may have screwed with global temperatures and acidified the oceans. And these were no ordinary volcanos, in fact, this period of volcanic activity was one of the biggest in earth’s history, with so much lava being poured out that it covered an area of over 2 million square kilometers, which, for reference, is larger than the entire country of Mexico.

These eruptions also would have ignited massive hydrocarbon deposits underground, such as oil and coal, burning them and releasing even more greenhouse gases. At the same time, new microorganisms may have been producing high levels of methane, releasing it up from the sea floor, and the Araguainha impact crater has also been dated to have possibly occurred during this time.

Also, of importance here is that it was during this time, the Permian, that the supercontinent Pangea formed, which is believed by many to have helped initiate the devastating extinctions of marine ecosystems by severely disrupting oceanic circulation and weather patterns.

Finally, we have the argument from British astrophysicist John Gribbin, who points out that the timing of the Great Dying coincides with the last time the Solar System passed through a spiral arm of the Milky Way Galaxy. He states that this could have led to large amounts of interstellar dust clouding the solar system, effectively dimming the sun and causing an ice age, with similar theories being suggested in a few other papers.

Overall, the surface of the earth was just getting pummeled, and life would suffer the consequences.

On land, 70% of all species were eliminated, and in the sea, some researchers estimate that more than 95% of marine life went extinct. This period was also the only known extinction event for insects, meaning things were REALLY bad.

The result was a completely changed picture of life on earth. One of the few remaining species to survive was the Archosaurs, the ancestors of today’s crocodiles and, crucially, the dinosaurs.

Triassic-Jurassic

The Triassic period saw the earliest dinosaurs rise from the ashes of the Great Dying and start to make a name for themselves, though they had a lot of competition, such as Aetosaurs, which were large, omnivorous, armored reptiles, and capitosaurs, which were the largest amphibians to ever live.

But it was near the end of the Triassic that all this would change, with our fourth major mass extinction event. As far as paleontologists can tell, the Triassic-Jurassic extinction took place in less than 10,000 years, and it appears that 70% of all species died out.

The biggest culprit for this one seems to be the start of volcanic activity from the Central Atlantic Magmatic Province, or CAMP, which at this point you can probably guess, pumped a lot of carbon dioxide into the atmosphere and acidified the oceans, the classic recipe for mass extinction.

And it wouldn’t be a mass extinction without an accusation of a meteor impact, and there are actually a few candidates. The Manicouagan Reservoir in Quebec, for example, is a crater that’s been dated to the Late Triassic and is tied as the fourth largest on earth. There’s also a theory that this meteor may have broken up just before or during its entry into earth’s atmosphere, turning it into a multiple impact event.

Proponents of this theory point to the locations of several other craters, such as in Ukraine and North Dakota, that occurred at the same time and could have all originated from the same extraterrestrial object. If true, this would mean an impact event far more significant than previously thought, and potentially responsible for the extinction.

Regardless of its cause, one interesting things about this extinction event is that there seems to have been a relatively rapid recovery from it, and this is probably because there were many surviving species who quickly filled in ecological niches. The best example of this is the dinosaurs, who were among the only remaining reptiles to survive, allowing them to quickly rise to dominance on every continent.

K-T

But all good things must come to an end. Although the dinosaurs reigned supreme for more than 140 million years, their existence would also come to an abrupt halt around 66 million years ago, at the Cretaceous-Tertiary extinction event, or K-T for short.

In contrast to the other extinction events, scientists are fairly confident about the cause of K-T. The idea that a massive asteroid had struck the earth at the end of the Cretaceous Period was first suggested in 1980 by Luis Alvarez, who, by the way, was a recipient of the Nobel Prize in Physics because he was apparently just good at everything. Luis and his son Walter put the idea forward based on the fact that the rock layer associated with the end of the Cretaceous has a higher-than-normal concentration of the element iridium, which is found in abundance in asteroids.

Their idea was received with skepticism, but soon gained popular support with the discovery of the Chicxulub crater in the Yucatan Peninsula, perfectly coinciding with the extinction. And this asteroid was certainly big enough to cause a global catastrophe, with high-end estimates placing its diameter at 15 kilometers, or 9 miles, which, for reference, means that if the asteroid were sitting at sea level, it would be taller than Mt. Everest.

When this enormous chunk of rock smashed into the surface of the earth at a speed of 20 kilometers per second, or 12 miles per second, it exploded with a force 5-10 BILLION times stronger than the bomb dropped on Hiroshima, and the end of the dinosaurs was rapidly approaching.

The initial explosion, the ash it launched into the sky, and the shockwaves it sent in every direction plunged the earth into irreversible climate change and induced intense volcanic activity. The oceans acidified, dooming all but the most resilient marine species, and the sun was nowhere to be seen, dooming plant life. With no plants to eat, the herbivores began to die off, and with no herbivores to eat, the carnivores began to die off. Soon, 75% of all life on earth had vanished.

However, it was from this moment onward that a new dominant life form emerged, mammals. Mammals had been on the edge of survival and quite small for millions of years, but with the dinosaurs no longer in the scene, they could now take over. Just a few million years after the K-T extinction, mammals had diversified, becoming whales, dolphins, and primates, paving the way for the rise of humankind, the most dominant species the planet has ever seen.

Today

That concludes the big five extinction events, but there is a sixth worth mentioning, and it’s the one happening around us today.

The domination humans have gained over our planet’s resources has been paramount to our technological and societal success, but much of it has come at the cost of the earth’s biodiversity. Many people are familiar with the extinction of the dodo bird, or the Tasmanian tiger, and that many species of whale or rhinoceros are endangered, but this really doesn’t do it justice.

Since 1900, extinction rates have occurred at 1000 times the rate they are expected to in a normal ecosystem. In 2018, the global biodiversity assessment estimated that of the 8 million species on earth, a staggering 1 million of them are endangered due to human activity such as deforestation, over-fishing, and pesticide use.

But there’s one thing that sets this emerging extinction event apart from the rest, and it’s that humans are aware of what is happening. Unlike the archosaurs of the Triassic or the armored fish of the Devonian, we know what is causing this loss of life, how it affects us, and how we can ultimately put a stop to it.

Key Takeaways

• Earth has experienced five major mass extinctions, each wiping out a significant portion of life.
• The Permian-Triassic extinction, or ‘The Great Dying,’ was the most severe, eliminating 95% of marine life.
• The Cretaceous-Tertiary extinction, caused by an asteroid impact, led to the demise of the dinosaurs.
• Current human activities are causing a sixth mass extinction, with 1 million species endangered.
• Unlike past extinctions, humans are aware of the current biodiversity crisis and its causes.

Sources

• Original Side Projects video: When Earth Nearly Lost Everything: Top 5 Mass Extinctions

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