The mass extinction of biodiversity on Earth could end up being the seventh, not the sixth

Scientists call it the “Ediacaran fauna”, after the hills of South Australia where it has been discovered since 1946. It consists of animal fossils visible to the naked eye and 550 million years old, which form an exceptional biodiversity. You have to imagine the scene that has been “printed” in the sediments: strange “petalamides” with feather-shaped bodies rubbed on the shoulders with Kimberellasimilar to modern snails, when the ancestors of jellyfish (Cnidaria) were already floating.

But when we compare this rich Ediacaran fauna with the “fossil record” (all known fossils) dated only 10 million years later, the picture changes completely: 80% of the species are not found. . What could have happened in the meantime?

Humanity’s risk of extinction is underestimated, according to a study on global warming

Several hypotheses have been considered. For example, trilobites – marine arthropods with “armored” bodies – may have competed with other species to the point of extinction. Or, the conditions favorable to fossil formation—temperature and pressure that allowed shells and other dead animal remains to be preserved to this day—temporarily ceased to exist, which might have given the illusion that the fauna had disappeared.

The first mass extinction in Earth’s history?

Another possibility: a mass extinction would have enraged the entire planet. A hypothesis that has just received a serious “boost”, so to speak, with a new study published in the journal Proceedings of the National Academy of Sciences (11/2022). According to its authors, the very first mass extinction crisis on Earth took place 550 million years ago (Ma). Currently, the top five universally recognized biodiversity crises are:

  • The Ordovician-Silurian crisis (-440 Ma)
  • The Devonian Crisis (-365 Ma)
  • The Permian-Triassic Crisis or “Great Extinction” (-250 Ma)
  • The Trias-Jurassic Crisis (-210 Ma);
  • The Cretaceous crisis (-66 Ma) which notably saw the extinction of all dinosaurs (except the ancestors of birds).

The team, led by American Scott Evans, postdoctoral researcher at the Polytechnic Institute and State University of Virginia (Virginia Tech), compiled a database of fossils dating back 550 million years and already described in the scientific literature, before selecting each specimen based on various factors (location, size, power supply, etc.). In total, 70 types of animals have been identified, of which only 14 are still found some ten million years later.

In their analysis, the authors did not notice significant changes in the conditions needed to preserve the fossils, or even differences in feeding patterns between “extinct” and “surviving” genera, thus undermining the other two hypotheses mentioned above.

On the other hand, the researchers have highlighted a surprising trend. “We looked at the selectivity model: which (organisms) disappeared, which survived, and which subsequently thrived“explains Professor Shuhai Xiao, a geobiologist at Virginia Tech and co-author of the study, quoted by Live science. “It turns out that organisms that could not cope with low oxygen levels were selectively eliminated.

Surviving low oxygen levels

Therefore, all living things that survived all had one thing in common: their bodies were characterized by a high surface area to volume ratio, a characteristic known to help animals endure low oxygen conditions. . This observation, together with geochemical evidence for a decrease in oxygen 550 million years ago, suggests that the “Ediacaran” (geological period associated with the Ediacaran fauna) may have ended in a mass extinction caused by low oxygen availability in the Ocean.

Greenhouse gases raise fears of mass extinction in the oceans

However, the study does not conclude on the cause of this drop in oxygen levels. Lead author discusses with Live science the trace of volcanic eruptions, that of movements at the level of tectonic plates, or even of possible asteroid impacts. Without excluding a less “spectacular” explanation, such as a modification of the quantity of nutrients in the oceans – which would, all in all, be close to a phenomenon observed today.

Indeed, agriculture using synthetic fertilizers and wastewater discharge releases significant amounts of nutrients, particularly phosphorus and nitrogen, into marine and riverine ecosystems. These discharges cause algae blooms, which suffocate the environment by using up all available oxygen, creating what scientists call “dead zones”. “This study helps us understand (what could be) the long-term ecological and geological impacts of oxygen starvation episodes“, comments Professor Xiao.

Destruction of natural habitats and global warming

But pollution is not the only cause of the current crisis, defined by some researchers as the “sixth mass extinction” – and which therefore could now be called the “seventh mass extinction”, if the results of the American team had just been confirmed . Today, the main drivers of biodiversity decline are none other than the destruction of natural habitats (to the benefit of cities, livestock ranges and agricultural fields), followed closely by the overexploitation of natural resources and poaching.

Nearly 70% of wild animals have disappeared since 1970, according to the WWF

Global warming – caused by greenhouse gases emitted primarily by human use of fossil fuels – is set to become a major cause of plant and animal species extinction, according to WWF, whose Planet Vivante Index (IPV) has recorded a loss of 69% of populations (by number and not by number of species) of wild vertebrates – fish, birds, mammals, amphibians and reptiles – between 1970 and 2018 (WWF, 2022).

Read also:

Sixth Extinction: Is the Scale of the Crisis Underestimated?

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