Not too long ago I rewrote my will, bringing it up to date. There’s nothing like tackling a project like that to remind me of my mortality. But imagine not just your own individual death, but the finality of the death of all members of your species – that’s the idea behind what geologists and paleontologists investigate when they muse on extinctions and what can cause them.
You’ve likely heard of the mass extinction that removed the non-avian (non-bird) dinosaurs from the face of the Earth some 65 million years ago. There have been other periods, too, of enormous “die offs” in Earth history. And even apart from times of mass extinction, some species are always going belly-up. In short, most species that have ever lived are now extinct. As I like to say, extinction isn’t rare, but as common as dirt.
The largest extinction the planet has seen wasn’t the one that wiped out the dinosaurs. It came many millions of years before, between what geologists call the Paleozoic and Mesozoic Eras. But the extinction that marked the end of the dinosaurs is famous because the public is understandably interested in everything connected to the dinosaurs’ reign on the planet.
If we look at the fossils that are just a bit older than the time of the dinosaurs’ extinction and compare them to the fossils that are just a bit younger, we can see just how different life on Earth became. Organisms in the oceans were particularly hard hit during the great transition, as were plants on land. Interestingly, mammals were comparatively unaffected (go team!).
The first part of an important theory for what happened when the dinosaurs disappeared was put forward in 1980 by a team of researchers from Cal Berkeley. The idea is one you may be familiar with, namely that a large meteorite slammed into the Earth. Later research work put the location of the impact in the Yucatan peninsula of Mexico – the place became known to as the “Crater of Doom.” The meteorite that hit the Yucatan, as the theory goes, carried with it an unusual metal, one that can be found in a narrow layer of sediment that occurs at or near the “boundary” between the dinosaurs’ era and our own.
Soot and ash in Earth’s atmosphere, according to the theory, was so strongly increased by the impact event that not much sunlight reached the surface of the planet. Plants died as a result. After them, of course, many animals dependent on the plants gave up the ghost as well.
But not everyone has been satisfied with the meteorite theory of extinction. There has always been some evidence of massive ecological upheavals before the special layer that contains the unusual metal linked to a meteorite impact. Enter Gerta Keller, a professor at Princeton who has gone her own way on the matter of what led to the great die-off.
Keller agrees with others in the field that there was a giant impact in the Yucatan. But she argues that mass extinction events occurred before that time. The cause? Massive volcanic eruptions in what’s now India.
India is home to major lava flows known as the “Deccan traps.” They are just a bit older than the Yucatan impact event. Well, that’s “just” in geologic time – likely 150,000 – 300,000 years.
Keller has taken samples at 150 different places around the globe from the layers just around the time of the mass extinction. She says her observations indicate the mass extinction was well underway before the impact of the meteorite. And, in addition, she says there were really multiple impacts, not just one.
There’s a lot yet to be unraveled about mass extinctions. Some hypotheses that have been put forward don’t yield testable ideas – and so we can’t prove or disprove them. In short, it can be tough to know what to make of some of the data and observations we have.
But extinctions fascinate us, which is why geologists and paleontologists have investigated them for generations. What causes the catastrophic changes in species, however, is often hard to pin down. As some have said, only a time-travel machine would let us observe what really caused the extinctions of species. But nothing will stop the good efforts of many scientists to try to understand mass extinctions better.