Genetic mutations found in plant spore fossils, caused by exposure to ultraviolet rays, suggest the collapse of the ozone layer at the time.
Researchers at the University of Southampton, UK, found that the mass extinction that occurred 360 million years ago, at the end of the Devonian period, was caused by a disruption in the ozone layer. The study was shared on Wednesday (27) in the journal Science Advances.
The team collected rock samples during expeditions to polar and mountainous regions in eastern Greenland, which, millions of years ago, formed a huge lake when Europe and North America were still close. This lake was located in the southern hemisphere of the Earth and would have been similar to Lake Chad, located on the edge of the Sahara Desert.
Other samples were collected from the Andes Mountains, above Lake Titicaca, in Bolivia, which, also millions of years ago, was closer to the south pole. According to experts, analyzing rocks from different places made it possible to compare the extinction event near the pole with the one near the Equator.
The scientists dissolved the samples in hydrofluoric acid, releasing microscopic spores from plants that remained preserved for millions of years. In the examination, the researchers found that many of these spores – plant reproduction units – had peculiar spines on their surface, in addition to darker cell walls.
These changes are a response to DNA damage caused by ultraviolet (UV) radiation, which is why scientists believe the Earth’s ozone layer was damaged at that time.
The team believes that 360 million years ago there was rapid global warming whose cause is still unknown. This warmer climate resulted in the melting of the polar ice caps, which in turn led to the release of harmful chemicals that opened a hole in the ozone layer. “Our ozone shield vanished for a short time in this ancient period, coinciding with a brief and quick warming of the Earth,” said John Marshall, the research leader, in a statement.
According to the scientists, during the extinction, the plants survived selectively, but were severely damaged when the forest ecosystem collapsed. The dominant group of fish from that period was extinct – and those that survived (sharks and bony fish) remain dominant in our ecosystems today.
These extinctions occurred at a fundamental moment for the evolution of our own ancestors, the tetrapods, explain the researchers. These first animals were fish that evolved to have limbs instead of fins, but still lived mainly in the water. Extinction redefined the direction of its evolution: the survivors were mostly terrestrial and had the number of fingers and toes reduced to five.
Professor Marshall believes his team’s findings have implications for life on Earth today, as it highlights the importance of stopping climate change. “Current estimates suggest we will reach similar global temperatures to those of 360 million years ago,” said the researcher. “[This increases] the possibility that a similar collapse of the ozone layer could occur again, exposing surface and shallow sea life to deadly radiation. This would move us from the current state of climate change, to a climate emergency.”