Since the death of the dinosaurs, life on Earth has never known such deadly pressure. We have indeed entered into what scientists call the sixth great extinction. And humans may well be among the victims, according to one recent work. Such an extinction would mean the loss of a very large number of species, which would dig a huge hole in the planet's ecosystems, but would leave room for all kinds of strange and wonderful life forms that could evolve in the ecological niches left vacant.
Po find out how life rebounds after a mass extinction, let's look back. There have been five major mass extinctions in Earth's history, the sixth being the one I have offered with colleagues. Our hypothesis is based in particular on the comparison of the rates of change in the geological history of the five extinctions. And it seems to suggest that, this time, the warning signs are real.
So let us be pessimistic and assume that the apocalypse will take us. What will Earth look like after Armageddon?
The biggest crisis in history
251 million years ago, during the transition between the Permian and Triassic geological periods, the living world experienced the greatest crisis in its history: 90 % of the species disappeared so... Even the insects suffered huge losses, unique in their long history.
The cause of this mega-extinction is largely attributed to the effects of what experts call the "mega-extinction". Siberian traps "These are serial volcanic eruptions accompanied by huge lava flows and greenhouse gas emissions in what is now northern Russia. This has led to global warming, acidification of the oceans, acid rain, oxygen depletion of the oceans and contamination by toxic metals such as mercury. Imagine the bleakest climate predictions being made today, and sprinkle in a few more disasters.
The handful of species that survived the Permian-Triassic crisis gave life to all other subsequent creatures. Since then, there has been no such profound restructuring of ecosystems. Perhaps because the Darwinian rule of "survival of the fittest" has made descendants more robust to change.
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What did our planet look like in the time of the Lower Triassic ? On an Earth that had only one super-continent, the Pangaeait was hot - hot as hell ! - and apparently lifeless over vast stretches of land. In the tropics, the water temperature was as high as 45 degrees Celsius. In the vast Pangaea Desert, it was probably even hotter.
Because of this heat, there are no traces of land animals, marine reptiles and fish in the fossil record, except for the high latitudes, which are probably a little cooler. As a result, there are several "gaps" of several million years each for this geological period, like holes in the chronology.
Most of the coal in the Earth today comes from the processing of large quantities of ferns of the species Glossopterisa victim of the great extinction. An extinction that created a 12 million year gap in the fossil record. A series of "fungal tracks" on rocks with large numbers of spores is also a sign of the disaster: huge quantities of dead plant and animal matter would have been a source of abundant food for the fungi. Overall, the heat and soil destruction caused by acid rain (these gullied areas would have given off a vanilla aroma) will have made the planet uninhabitable during this period.
Without plants, there are no herbivores. Without herbivores, there are no carnivores. One of the few 'big' survivors on this desolate Earth was a lizard, Lystrosaurusa bizarre vegetarian reptile that, in the absence of predators and competitors, has diversified with some success during the Triassic.
The carnage was even worse in the oceans, where up to 96 % of species became extinct. The loss of all species of reef-building corals led to a 10 million year gap in the fossil record of the Lower Triassic. Imagine: a world without coral reefs, without all the diversity of living things they shelter.
But the Earth wasn't quite dead. Just as Lystrosaurus on Earth, there have been successes in the marine environment amidst all this desolation. Claraia For example, a bivalve species similar to the scallop survived the late Permian, then rapidly diversified to occupy the niches left vacant by the almost total annihilation of the brachiopods, inhabitants of the Permian sea floor. Claraia was robust and could withstand very low levels of oxygen - a handy feature when most life on the seabed was deprived of oxygen.
The doomed fate of the dinosaurs
Perhaps the most famous and spectacular extinction is the one that saw the death of (non-avian) dinosaurs about 66 million years ago at the limit of the Cretaceous and Tertiary periods. As important as the end of the very popular T. rexthe replacement of micro plankton at the other end of the food chain has put an end to the formation of the famous Cretaceous chalk cliffs that are so widespread throughout Europe (the name of this geological period comes from the German word 'Kreide', meaning chalk).
Whether due to a meteorite, or massive volcanic eruptions, or a bit of both, the extinction that killed the dinosaurs was more modest than that of the Permian-Triassic: only 75 % of global loss for the living and a faster recovery. Either the Earth itself recovered faster, or, after the "great slaughter" 185 million years earlier, life had become more able to adapt to, and evolve under stress.
Of course, we know that the dinosaurs didn't exactly disappear. Birds are their super-evolved representatives, descendants of the few surviving dinosaurs from Cretaceous-Tertiary events. No one can deny their evolutionary success over the 66 million years since the disappearance of the Chicken-like T-Rex.
Crocodiles and alligators, the birds' closest living relatives, are also prominent survivors. While it is clear that the birds' ability to fly to oases of calm and abundance allowed them to thrive in the midst of the upheavals of the time, it is less clear why the crocodiles survived. Some theories suggest that crocodiles were able to maintain and thrive because of their cold-blooded organisms (as opposed to the supposed dinosaur warm blood), their freshwater or brackish water habitats, and even their high IQ !
Beyond the death and destruction of extinctions, here is good news: life on Earth has always taken over even when it has been very severely affected. Without extinction, there is no evolution, the two are intrinsically linked.
The first dinosaurs evolved 20 million years after the Permian-Triassic losses. Their evolution was almost certainly driven by a cooling of the climate during what has been called theCarnian rainstorm (a period when it rained a lot), lush vegetation and whole sections of ecosystems to be colonized.
Dinosaurs lived for 165 million years before they died, but if they hadn't died out, humans probably wouldn't be here today to do damage.
If human beings are doomed, then we won't be around to see what will evolve to replace us. Rest assured that we geologists do not attach too much importance to our disappearance. For we know that the Earth is bigger than we are, and that it will bounce back.
David BondNERC Advanced Research Fellow and Lecturer in Geology, University of Hull
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