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Science. A new study explores the possibility of human extinction. If previous mass extinctions came from external events, this time we’re the responsible party.

The mass extinction is coming

Every year, on average, two species of vertebrates become extinct. In the last century, then, about 200 species of vertebrates have disappeared from the face of the Earth. Two million years ago, it would have taken “not 100 years on average, but 10,000 years” to achieve the same result, according to a new study by Gerardo Ceballos (Autonomous University of Mexico City), Paul Ehrlich and Rodolfo Dirzo (Stanford University, California) published in the journal Proceedings of the National Academy of Sciences.

Together with Nature and Science, PNAS is one of the most serious and prestigious scientific journals, so the data must be taken seriously. According to the three scientists, and many of their colleagues, we are in a “mass extinction” event: a true ecological revolution that, within a few thousand years, could cause the disappearance of most animals, plants and micro-organisms living on Earth. And it is our fault.

The three scholars believe that the data on extinctions might even lead one to underestimate the ongoing phenomenon, because two cases a year may seem negligible. But if one focuses on other aspects, the picture is even more alarming.

Ceballos and his colleagues turned to the International Union for Conservation of Nature, a consortium of governments and NGOs. The group provides information on the geographical distribution of species. For example, a map of areas of Sub-Saharan Africa indicates there are approximately 35,000 lions left. The researchers compared these data with other historical sources, according to which in the 19th century, lions were present not only throughout Africa, but also in the Balkans, in the Arabian Peninsula and even in India.

They repeated the analysis on other 176 species of mammals and observed the same phenomenon: Species that occupied one or more continents today are restricted to small areas. Almost half of the registered species have lost more than 80 percent of their habitat in 100 years. Africa and Asia, and in particular their tropical regions, are the regions where the species were reduced the most.

Scientists associate the geographic extent of a species with the number of genetically homogeneous populations into which the species is divided. Therefore, even in species that are not at immediate risk of extinction, we have already witnessed a dramatic reduction of population size — that is, their biodiversity and adaptability. Therefore, the loss of population is a direct precursor of extinction.

All signs indicate that the decimation is not limited just to mammals. “We describe it by the name of ‘biological annihilation’ to emphasize the scope of the ongoing extinction,” say the authors. “It is the prelude to the disappearance of many species and the decline of the natural systems that made civilization possible.”

The warning of the three biologists is not isolated. The accelerated pace of extinctions has been known for at least 30 years. Back in the ‘80s, the biologist Edmund O. Wilson estimated that the extinction rate increased by about 10,000 times since man started running around the globe, as he wrote in the magazine Scientific American. In recent years, new data have refined the estimates without changing the substance.

Manmade global warming is just one of the phenomena that is radically changing global habitats, and it has affected quite a small historical period — the last two centuries when we learned to burn hydrocarbons on a large scale. But the early damage began with agriculture, about 10,000 to 12,000 years ago. Organized hunting, pollution and ocean acidification have done the rest. Assuming there is still time to reverse course, it is not measured by the span of centuries but of decades. Otherwise, mass extinction is inevitable.

This would not be the first mass extinction. According to paleontologists, based on the fossil records, over the past 500 million years there have been five other large-scale extinctions (the “big five”), as well as other smaller incidents.

Evolution, in fact, has not been a gradual process, as initially believed by Darwin himself. Rather, it alternated periods of relative quiet, where species and environment found a stable equilibrium and extinctions were rare, and very turbulent times.

One of the most recent estimates of the “calm” rhythm of extinctions, which allows us to evaluate how huge our impact has been, was indicated by paleontologist Jurriaan de Vos. Before us, each species could reasonably remain on Earth a few million years. Biologists call it “background rate of extinction.” Today, according to De Vos’ calculations, species go extinct a thousand times faster, an obvious sign that the balance was broken.

It is a recurring phenomenon. A system as complex as the Earth can spontaneously generate abrupt changes (for example, in temperature or in the abundance of certain chemical elements) that force the entire ecosystem to renew itself almost completely.

Two hundred fifty-two million years ago, due to the most severe mass extinction, 96 percent of marine species and 70 percent of terrestrial vertebrate species disappeared, and so far, only more or less plausible hypotheses about the causes have been determined. Another event was clearer. It happened 66 million years ago when a 20 kilometer wide meteorite struck Yucatan, causing the extinction of the dinosaurs and 75 percent of all other animal species.

However, other mass extinctions, smaller than the big five, were explained with the spread of new animal species (like ours) that were able to undermine the habitat of all those pre-existing species. It happened about 540 million years ago, when the first animals broke the balance in which the so-called “Ediacaran fauna” flourished, consisting of peaceful and immobile multicellular organisms that had dominated the entire planet simply by feeding on oxygen.

The current “sixth extinction” would be the first caused by a single species. We humans could be among the likely victims of the extinction. For the rest of the ecosystem, that would be very good news.

Scientists are already wondering about what forms of life will survive the next extinctions. It is little more than a game, because the dynamics of ecosystems are essentially unpredictable. But there are some fixed milestones. Tardigrades will not go away easily; these are very common small organisms (which can also be found in common puddles) but have incredible abilities. They can live up to 30 years without water and food, they can survive temperatures up to 300 degrees Fahrenheit and as low as near-absolute zero, and the vacuum of space.

Yet in the latest issue of the journal Scientific Reports (Nature group), physicists David Sloan, Rafael Batista and Abraham Loeb, of Harvard and Oxford, have examined the possibility that a cosmic catastrophe will cause the extinction of tardigrades in the next 10 billion years, after which the sun will go out. According to information available to astronomers, between now and then there will not be any asteroid big enough or supernova explosions or radiation close enough to destroy the solar system. Tardigrades will enjoy the last sunset.

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