251 million years ago, before dinosaurs existed, the worst extinction ever recorded left the planet nearly bereft of plants and animals: More than three-fourths of all species perished, leaving a layer of fossils worldwide as a record. With evolution’s slate relatively clean, the door was wide open for new species to take over. Lizards leapt at the opportunity, evolving into dinosaurs within just a few million years.
But what triggered this earlier extinction? Researchers have speculated that it might have been an asteroid or comet impact, like the later event that did in the dinosaurs. Or, they have ventured, it could have just been heavy volcanic activity or extreme climate change. But until now, there has been no clear evidence.
Here’s why: Earth has changed drastically in die intervening 251 million years. Back then, all the continents were huddled together in one giant landmass, called Pangaea. If there were an impact, the resulting crater would long since have been, split apart or folded into the planet’s crust.
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And despite a thorough search, no increased levels of iridium have been associated with that time. So researchers have been looking for other evidence of an impact. And now they seem to have found it.
The new study, announced today, uncovered extraterrestrial gases trapped inside special molecules, known as Buck balls, in ancient soil layers. Scientists say the evidence points to a colossal whack from a comet or asteroid roughly 3.7 to 7.5 miles (6 to 12 kilometers) wide – about the same size as the one that ultimately destroyed most dinosaurs 186 million years later.
In the past 500 million years, there have been about five giant extinction events; researchers say the new finding means that at least two of them were caused by impacts. The others also may have been.
This suggests that the evolution of life on Earth is strongly coupled with our cosmic environment. The new work even has implications for the origin of intelligence on Earth, and possibly elsewhere. It hints at the possibility that Earth’s biosphere is regularly disrupted, every 100 million years or so, by giant impacts that would render human life impossible. Only a civilisation smart enough to spot the hazard and mitigate it can survive.
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Because Earth’s crust has stretched and folded so dramatically in the past 251 million years, the researchers say there is no way to pinpoint where the space rock hit. But they can guess at the wild events that followed. Worse, the combined effects of the object vaporising on impact, along with all die volcanism, poisoned what was left of the seas and choked the air with ash and deadly gases.
Sunlight may have disappeared for months. Or, Becker and her colleagues say, carbon dioxide may have trapped the Sun’s energy and sent temperatures soaring. Either way, it was ‘not a pretty picture for life, which is why it’s the greatest of all mass extinctions recorded on Earth. Some 90 percent of all sea life perished, along with 70 percent of land animals and most terrestrial plants.
The researchers say the volcanic activity was likely going on before the impact, but was then fuelled into frenzy. The one- two punch, it seems, may be what’s needed to precipitate the worst extinctions. The discovery, and the new technique used to make it, could lead scientists to find that some of the other 20 or so mass extinctions in the last billion years were also caused, or at least helped along, by cosmic collisions.