360 million years ago, an unknown event caused the rapid disappearance of much of Earth’s freshwater plants and species. Now, a team of researchers from the British University of Southampton has discovered that the “culprit” was the collapse of the ozone layer that protects our planet from harmful ultraviolet radiation. The finding, recently published in Science Advances, has profound implications for today’s world.
During the long history of life on Earth, there have been several large mass extinctions. Today we know that one, the one that killed the dinosaurs 66 million years ago, was caused by the impact of a meteorite. Others, like the one that took place in the Permian 252 million years ago, are due to massive massive volcanic eruptions on a continental scale.
And now, this study shows that a high level of ultraviolet radiation can also collapse forest systems and kill many aquatic species, such as fish and theropods (our distant ancestors), who lived in the Devonian period, 359 million years ago. . On this occasion, then, there were no meteorites or eruptions, but rather a large hole in the ozone layer caused by sudden global warming just after an intense ice age. Does it ring a bell?
In fact, it is inevitable to compare this situation with the current one, and the researchers suggest in their study that in a very short time, the Earth could reach comparable temperatures, possibly triggering a similar event.
For their research, scientists collected rock samples from the mountainous polar regions of eastern Greenland, which at that time formed the bed of an ancient lake in the arid interior of a continent that included present-day Europe and North America. The lake, in the southern hemisphere of the planet, could have been similar to the current Lake Chad, on the edge of the Sahara desert.
Rocks from the Andean mountains were also collected on Lake Titicaca, in Bolivia. Those samples come from the ancient southern continent of Gondwana, which in the Devonian was closer to the South Pole. In this way, the researchers obtained clues to what was happening at that time, both near the South Pole and at the equator.
Once in their laboratories, the rocks were dissolved in hydrofluoric acid, thereby releasing spores of microscopic plants such as ferns, preserved inside the rocks for hundreds of millions of years. Under the microscope, scientists discovered that many of those spores had strange spines on their surface, a way to respond to ultraviolet radiation that damaged their DNA. In addition, many spores had a kind of “wall” of dark pigmentation, which scientists consider as a kind of protective “tan” against the increase in UV rays.
The bottom line was that during that period of rapid global warming, the ozone layer collapsed for a time, exposing planet life to harmful levels of ultraviolet radiation and triggering a mass extinction event both on land and in shallow water. The deadly event took place 360 million years ago, on the border that separates the Devonian from the Carboniferous.
How ozone disappeared
After the ice sheets melted, the climate quickly warmed up, and this increase in temperatures pushed chemical elements capable of destroying ozone into the upper atmosphere, causing a hole that allowed large amounts of radiation to enter for several thousand years. ultraviolet.
John Marshall of the University of Southampton School of Ocean and Earth Sciences and director of the study explains that “the ozone shield disappeared for a time in this ancient period, coinciding with a brief and rapid warming of the Earth. Naturally, our ozone layer is always in a state of flux, constantly creating and losing itself, and we have shown that this also happened in the past and without the need for a catalyst, such as a volcanic eruption on a continental scale. ”
During extinction, some types of plants managed to survive, but their cycles were severely altered as the forest ecosystem collapsed. The then dominant group of armored fish became completely extinct and the surviving bony fish, such as sharks, thereafter became the dominant fish in our ecosystems.
Our ancestors, affected
The extinction, moreover, came at a key moment for the evolution of our own ancestors, the theropods. The first theropods, in fact, were fish that evolved to have limbs instead of fins, thanks to which they were able to take their first steps out of the water. They were creatures still eminently aquatic and whose limbs had a large number of fingers and toes. Extinction redirected their evolution, and those who survived were the best adapted to live out of the water. Creatures already totally terrestrial and with the number of fingers and toes reduced to five.
For Marshall, these findings have a series of surprising implications for current life on Earth: “Current estimates,” he says, “suggest that we will reach global temperatures similar to those of 360 million years ago, with the possibility that a similar collapse of the ozone layer could happen again, exposing the Earth’s surface and marine life to deadly radiation. That would take us from the current state of climate change to a true climate emergency. ” .