One of the fascinating things in the history of our planet is that it's gone through a series of supernatural and it is very likely that you already know, and that every so often, on the earth, "something" happens and everything changes. That "something" can be a slow process, and in some cases almost imperceptible, or all the opposite, something violent and catastrophic.
Whatever it is has consequences that are inexorable: Death looms over the entire planet.
With these words may seem the statement for a mediocre horror film, however, it does not stop being true.
In fact, on our planet it has happened several times, although it is very difficult to find its evidences.
They are called "Great Mass Extinctions" and five of them have already occurred.
A mass extinction is a type of terminal extinction in which 10% or more of the species disappear without offspring over a year, or 50% or more of the species in a period between one and three million and half of years.
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You can say with good judgment that the numbers seem a bit arbitrary, but it happens that (like everything that human beings measure) certain limits are needed to be able to call things in one way or another. It is clear that nature does not care at all about our definitions (for that of the "Sorites paradox") and simply carries out its task without worrying about the extravagant name that we put to it.
In any case, we can identify five major "events" throughout the history of our planet where some of those limits were overcome ... very widely. One must be very careful with the mentions of "estimated duration" and "probable causes" because the scientific progress has varied remarkably with the passage of time, the two responses based on new data and permanent investigations.
It is estimated that it happened approximately 440 million years ago. They were actually two consecutive events (in relative terms, of course), which are considered as a single extinction, where 85% of the living species of the planet perished.
Image courtesy of Charles R. Knight
At that time, life only inhabited the seas, and the event produced the near disappearance of brachiopods, bryozoans, trilobites, conodonts and graptolites.
The causes? There are two possible theories:
The first one points to cataclysmic events, causing a gigantic glaciation on the habitats of these coastal species.
The second postulates that the explosion of a nearby supernova swept the Earth of gamma rays for a few moments, enough to kill all or most of the ozone layer, killing or contributing to kill the living beings that inhabited shallow waters .
However, the death of the dominant predators, was an opportunity for the first animals endowed with a primitive spine and an internal skeleton, giving an opportunity to the first fish.
Image courtesy of Ghedoghedo
It is estimated that it occurred some 360 to 400 million years ago, it also includes two events called the Kellwasser event and the Hangenberg event, and especially affected marine fauna in the tropics. Approximately 83% of the species became extinct, affecting especially the coral species during the first event, and the vertebrates (both marine and terrestrial) in the second.
It was a very long process, estimated at several million years. Although the causes of this process are very little known, it is suggested that it could occur due to a severe global cooling, together with conditions of anoxia (reduction of oxygen in the seas) associated with excessive volcanism, among other possible causes.
This event occurred about 250 million years ago, it was the largest known extinction. Approximately 96% of marine species and 70% of terrestrial vertebrate species disappeared, with such catastrophic consequences that life took a long time to recover.
With an estimated duration of one million years, or less, it was a rapid event in geological terms and extremely deadly.
Image Courtesy of Fredrik
Again, the causes are uncertain and theories abound in this regard. Some are based on records that indicate severe volcanism in present-day Siberia, causing a cascading effect of extreme global warming. Others point to a meteoric impact that has been found in Antarctica footprints, with a diameter of 500 km and 1.6 km depth.
Whatever the cause, it was by far an event that could annihilate life on earth, perhaps permanently.
This event happened approximately 210 million years ago, and produced the disappearance of 76% of the species, including the archosaurs, a large part of the therapsids and the last great amphibians.
The most plausible hypothesis considers that the event could be produced by massive volcanic eruptions in what is now the Atlantic during the break of Pangea and the formation of the current continents.
The release of so many ecological niches allowed the dinosaurs to assume the dominant role during the subsequent Jurassic period.
Image courtesy by Arthur Weasley, Dmitry Bogdanov, Smokeybjb
Image courtesy of ABelov2014
It is by far the best known, it occurred 65 million years ago, and although its duration is not known with certainty, it has been established that at least 75% of the species were annihilated.
Although several possible hypotheses were investigated as the cause of the event, the most accepted one is now based on an investigation carried out in 1980 by a group of researchers led by the physicist Luis Álvarez (Nobel Prize), who found an iridium concentration hundreds of times higher than normal in samples from the intermediate layers between the Cretaceous and Tertiary periods of 65 million years ago, worldwide. This finding is called the K / T limit.
The presence of iridium raised the hypothesis that the extinction of dinosaurs and many other life forms would have been caused by the impact of a large meteorite on the surface of the Earth 65 million years ago.
In 1990, there were signs in Haiti of a tsunami of great proportions that dragged iridium residues. That guided the search in the Caribbean of some crater that could justify the event, being one compatible with it in Chicxulub, in the Yucatan Peninsula.
Image courtesy of Donald E. Davis
While there are factors that do not fit perfectly into this theory, it is by far the most accepted until now.
The event caused the ecological collapse of most dinosaurs, flying reptiles (pterosaurs), most aquatic reptiles (plesiosaurs, pliosaurs and ichthyosaurs) and ammonites.
As a result of this, the mammals, who by then were no more evolved than a shrew, achieved an unparalleled opportunity for evolution, of which we are heirs.
Only one thing is certain. It has already happened five times, and it will happen again.
We may still be here when it arrives, maybe not, but life on earth will sooner or later suffer a new crisis, and maybe it will evolve again.
it can be a gigantic meteorite, an unpredictable tectonic-volcanic event, a nearby supernova or a sudden glaciation, you just know something will happen.
However, think for a moment ... what would happen if that extinction was already in progress?
Maybe we are already living an "event of mass extinction" and we do not realize it.
Let's see some numbers:
In the last 500 years more than 300 species of vertebrates have become extinct.
More than 170 species of mammals have lost between 30% and 80% of their habitat.
This happened in only 500 years ... imagine now what can happen in a few thousand years.
We live in a geological age that is already called Anthropocene, and this would be a "defaunación del anthropocene".
According to the biologist Rodolfo Dirzo of Stanford University:
"It is a cryptic phenomenon that, unlike logging, is difficult to detect and quantify. However, the profound consequences that arise from the absence or reduction of animals will soon make the problem less and less cryptic "
"The massive decline in families and animal species is mainly due to the loss and fragmentation of their habitat, hunting, trade, overexploitation of the territory and the appearance of invasive species"
Image courtesy of Pixabay
At this point, an awkward truth begins to be glimpsed: perhaps we ourselves have initiated the process.
To reverse this situation, protection areas should be increased and the populations of animals and plants in the regions dominated by man should be preserved. It would also be necessary to reduce the growth of the human population, reduce consumption and bet on the use of green technologies.
It does not seem easy, right? It definitely is not, and maybe even with our best efforts, we are not doing enough.
As you can now imagine, the situation can be much more serious than it seems, and we are collaborating with the problem: If a sixth extinction arrives, we may be part of the causes of the "event".
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