The Planet’s 6 Great Mass Extinctions and Why We’re Experiencing One of Them Right Now

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In about 4.5 billion years of existence, planet Earth has gone through at least five major mass extinctions – and it is very likely that we are in the sixth phenomenon of the type precisely in the period in which we live.

These moments are characterized by an increase in the rate of living things that cease to exist.

To get an idea, about 98% of the organisms that inhabited the globe are no longer here. Scientists estimate that the “normal” average of extinctions is 0.1 to one species per 10,000 species every 100 years.

But in at least five or six episodes over the ages, that rate has accelerated beyond all measure: according to the Natural History Museum in London, UK, “a mass extinction event happens when species disappear much more faster than they are replaced”.

“This usually occurs if around 75% of the world’s species are lost in a ‘short’ period of geological time — less than 2.8 million years”, calculates the institution.

Paleontologist Mario Cozzuol, from the Institute of Biological Sciences at the Federal University of Minas Gerais (UFMG), mentions another fundamental characteristic of these events: they happened homogeneously in all parts of the world.

“We are talking about events on a global scale, with a great extent, in a relatively close geological time”, he adds.

But what mass extinction events are these? And how do scientists determine that they actually occurred?

1. Ordovician-Silurian, 440 million years ago

In a publication available online, the Museum of Life of the Oswaldo Cruz Foundation (Fiocruz) explains that, in that era, the planet was experiencing a moment of progress.

“The number of species, mainly of marine animals, was growing. It was at that time that the first land plants also appeared”, contextualizes the article.

But the bonanza ended with the disappearance of 85% of species, especially small invertebrate marine beings.

Among the possible causes for the crisis, scientists point to the movement of continents towards the south pole, drops in temperature, the formation of glaciers and the reduction of sea levels (on which much of life depended).

2. Devonian, 370-360 million years ago

About 75 million years later, Earth went through a new hecatomb, which wiped out between 70 and 80% of all species from the map.

At the time, the world “was populated by many primitive fish. The first terrestrial vertebrates with four limbs and insects also appeared. The plants, in turn, were getting taller”, describes the Museum of Life.

There is still no consensus on the reasons behind this mass extinction.

Evidence points to several changes in the environment, such as interspersed rises and falls in temperature, rising and falling sea levels, and a drop in the concentration of oxygen in the atmosphere.

Some also speculate about possible impacts from meteorites and comets.

3. Permian, 250 million years ago

Here is the worst event of all: it is estimated that more than 95% of beings became extinct in this period.

The American Museum of Natural History, in New York, explains that the phenomenon affected many vertebrates (beings that have a spine and a skull).

Known as “The Great Death”, this event is also related to changes in the environment.

It is possible that the movement of continents, volcanic eruptions, climate warming and increased acidity of the oceans represented the end of the line for many species that inhabited the planet.

“Some scientists point out that the Earth was hit by a large asteroid, which filled the air with dust particles, blocked sunlight and caused acid rain. Others think that a large volcanic explosion increased the amount of carbon dioxide (COtwo) and made the oceans toxic,” details the Natural History Museum in London.

4. Triassic, 200 million years ago

Studies estimate that three-quarters of the species disappeared at that time, marked by the development of pines and other plants from the gymnosperms, dinosaurs and first mammals group.

The main explanation for the phenomenon is the separation of Pangea – the supercontinent that brought together practically the entire land surface of the globe.

This colossal geological activity raised the amount of carbon dioxide (COtwo) in the atmosphere, made the oceans more acidic and triggered the eruption of several volcanoes.

With that, life ceased to be viable for many creatures.

The changes, however, represented an advantage for those who resisted, as was the case with some dinosaurs.

Reptile species that didn’t die out found fertile ground to thrive for the next few million years, according to the American Museum of Natural History.

5. Cretaceous, 65 million years ago

This is perhaps the most famous of all: it represented the extinction of most dinosaurs.

“Most of this group was wiped out. Only the lineage of birds was left”, summarizes the geneticist biologist Fabrício Rodrigues dos Santos, also a professor at UFMG.

It is estimated that about 80% of the species disappeared at that time.

“One of the most accepted arguments for the phenomenon is the fall of an asteroid, whose impact took on a global dimension”, adds the researcher.

Most likely, this asteroid hit the Yucatan Peninsula, territory that currently belongs to Mexico.

“And it is possible to map the impact waves until today, with evidence of this not only in the region, but in parts of Africa, the Americas and even Asia”, says Santos.

Of course, that asteroid alone didn’t wipe out all the dinosaurs overnight.

It is believed to have been the trigger for a series of changes in the environment—dust, diminished sunlight, plant death, oxygen depletion, acid rain, volcanic activity—that wiped out these reptiles little by little over a period of time. million years.

The devastating activation of a volcano in what we now know as India is an example of these developments.

And who survived the catastrophe? “Only very small animals, which needed few resources”, replies Santos.

“Although they appeared even before, the diversity of mammals was repressed because of the dinosaurs. They were restricted to certain environments”, says Cozzuol.

“With the disappearance of the dinosaurs, the mammals and birds that survived took advantage of this new ecological space to diversify. So, from then on, without the limitation that the dinosaurs placed, they were able to prosper”, explains the paleontologist.

6. ‘Anthropocene’, 2022

Some experts understand that the planet is currently going through the sixth mass extinction right now, in the moment we live.

And unlike the previous five episodes, the causes are not fortuitous environmental changes or the arrival of asteroids. This time, it’s humanity’s fault.

Remember that “normal” rate of species extinction? Because some calculations indicate that it is between a hundred and a thousand times faster since the appearance of hominids.

Research points out that human activity is behind this – and everything has only gotten worse in recent centuries.

“Since the Industrial Revolution, we’ve been increasing pressure on nature by using up resources, without thinking about how to get them back,” points out the Natural History Museum in London.

“For example, land-use change continues to destroy large portions of natural landscapes. Humans have already transformed more than 70% of land surfaces and use about three-quarters of freshwater resources”, continues the text.

Agricultural activity is one of the main sources of soil degradation, deforestation, pollution and loss of biodiversity.

And that, in turn, destroys the habitat of several species, which start to compete for the same resources, increasingly scarce.

“It’s not an exaggeration to think that several species that we don’t even know about are going extinct right now,” says Cozzuol.

In addition, the release of tons and tons of COtwo in the atmosphere through fossil fuels and other sources increases the average temperature of the planet and instigates droughts, floods and other catastrophes —which makes life unfeasible in many ways.

“If we compare it with the Cretaceous, the dinosaurs declined over the course of a million years. Now, we are seeing this happen on a much smaller time scale”, points out Santos.

“Even with so many changes, the planet will continue to exist. The biggest concern is precisely knowing what will happen to our own species”, he adds.

Cozzuol considers that, although the accelerated pace of current extinction is a consensus within the academic community, it is difficult to compare it to previous periods.

According to the researcher, the great difficulty is in the “temporal resolution”, or in the time scale between the past and the present.

“We had some extinctions on a global scale that took up to 5 million years. And the whole Pleistocene, which is the period in which humanity emerged and developed, comprises around 2 million years”, he informs.

“But in fact we are going through a time when the extinction rate is higher than the normal average. But it is difficult to compare this in terms of geological eras”, he concludes.

How do scientists know all this?

To collect this mountain of information, geologists, paleontologists and biologists use a variety of methods to analyze rock formations and fossils.

“We date these signs over the ages and compare qualitative aspects. From there, it is possible to see which species survived or became extinct”, explains Santos.

That is: if specialists saw fossils of a certain species and, after a certain period, this material disappears, this represents an indication that that living being ceased to exist on the planet.

“In this timeline, it is possible to see that a very large number of species did not cross over to the other side, that is, they did not manage to overcome some difficulty that appeared in the environment. And this suggests a mass extinction”, summarizes Cozzuol.

“And what determines the segmentation of geological time into periods are precisely the extinctions. All the great periods, such as the Paleozoic, Mesozoic and Cenozoic, were marked by these massive phenomena”, he adds.

An ancestral extinction?

In recent weeks, a team of American researchers has described the possible occurrence of a seventh mass extinction – or first, depending on the point of view.

According to the research, conducted at the University of California at Riverside and Virginia Tech, both in the United States, this phenomenon of species disappearance would have happened 550 million years ago, during the Ediacaran period.

It is estimated that 80% of living beings have disappeared from the map.

“The geological records indicate that the oceans lost a lot of oxygen during that period, and the few species that survived had organisms adapted to environments with low oxygenation”, says paleoecologist Chenyi Tu, one of the co-authors of the article, in a press release.

Among the ancient Ediacaran creatures that went extinct are the obamus coronatuswhich had a disk format, and the Attenborites janeaewhich resembles an egg —their names pay tribute to former US president Barack Obama and English naturalist David Attenborough, respectively.

Although the findings are interesting and curious, experts consulted by BBC News Brasil understand that this ancestral mass extinction still lacks more evidence to be considered on the same level as the other five.

“There is still no consensus on this Ediacaran extinction and we need more information before equating it with the others”, evaluates Santos.

In the biologist’s view, the great difficulty in analyzing such ancient eras lies in the difficulty in finding fossils and other geological records.

“We are talking about beings that do not fossilize, which makes it difficult to obtain an ‘extract’ from this period”, he concludes.

This text was originally published here.

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