Many scientists believe that Earth is at the beginning of a new "mass extinction", marked by the disappearance of species at an alarming rate, mainly because of human activity.
But this is not the first time: for 500 million years, the planet has experienced five previous episodes in which at least half of all living creatures have been eradicated in the blink of an eye to the geological history. .
In total, more than 90% of organizations that have ever walked, swam, stolen or crawled have disappeared.
Here is an overview of these five major mass extinctions, as scientists and diplomats from more than 130 countries gather until Saturday in Paris to adopt the first global assessment of ecosystems for nearly 15 years.
- Extinction of the Ordovician
When: about 445 million years ago
Disappearance of species: 60 to 70%
Probable cause: short but intense ice age
At that time, life was mainly in the oceans. Experts believe that the rapid formation of glaciers froze most of the world's water, causing a drop in sea level, and marine organisms like sponges and algae have paid the price. molluscs and primitive cephalopods and fish without mandibles called ostracodermos.
When: about 360 to 375 million years ago
Disappearance of species: up to 75%
Probable cause: depletion of oxygen in the oceans.
Again, marine organisms were most affected. It is believed that floating levels of the ocean, climate change or the impact of an asteroid are responsible for it. According to one theory, the proliferation of terrestrial plants led to anoxia (lack of oxygen) in surface water. The trilobites, arthropods of the ocean floor, paid the price.
When: about 252 million years ago
Disappearance of species: 95%
Probable causes: asteroid impacts, volcanic activity
Sometimes referred to as the "mother of all extinctions," this huge biological crisis devastated the oceans and lands. She is the only one who practically saw the disappearance of insects. Some scientists think this has happened over millions of years, others only over 200,000 years ago.
The trilobites that survived the first two extinctions eventually disappeared, as well as some sharks and bony fish. On land, moschops, herbivorous reptiles several meters in length also died.
When: about 200 million years ago
Disappearance of species: 70 to 80%
Probable causes: multiple, still under debate
The mysterious extinction of the Triassic has eliminated many large terrestrial species, most of them archosaurists, dinosaur ancestors and from which descend the birds and crocodiles of today. Most of the great amphibians also disappeared.
One theory evokes massive lava eruptions at the time of Pangea's disruption, the last supercontinent, eruptions accompanied by massive amounts of carbon dioxide causing unrestrained global warming. Other scientists suspect asteroids, but no craters have yet been identified.
When: about 66 million years ago
Disappearance of species: 75%
Probable cause: impact of an asteroid
The discovery of a huge crater in what is now the Mexican peninsula of Yucatan corroborates the hypothesis that the impact of an asteroid is responsible for this crisis, having seen the disappearance of non-avian dinosaurs such as T-Rex. and triceratops.
But most mammals, turtles, crocodiles, toads and birds survived, as did marine life, including sharks, starfish and sea urchins.
Without dinosaurs, mammals proliferated, leading to the birth of homo sapiens, the species responsible for the probable 6th extinction.
* Sources: National Geographic, Encyclopaedia Britannica, scientific studies.