Here we go again: Earth’s major ‘mass extinctions’

Date:

Share post:

Most scientists agree that a “mass extinction” event is underway on Earth, with species disappearing hundreds of time quicker under the influence of human activity.

Here we go again: Earth's major 'mass extinctions'
Graphic on Earth’s ‘mass extinctions’ during the last 500 years
[Credit: Alain Bommenel/AFP]

But this is not the first: over the last half-billion years there have been five major wipeouts in which well over half of living creatures disappeared within a geological blink of the eye. All told, more than 90 percent of organisms that have ever strode, swam, soared or slithered on Earth are now gone.

Here are the biggest die-offs, each showing up in the fossil record at the boundary between two geological periods:




Ordovician extinction

When: about 445 million years ago

Species lost: 60-70 percent

Likely cause: Short but intense ice age

Most life at this time was in the oceans. It is thought that the rapid, planet-wide formation of glaciers froze much of the world’s water, causing sea levels to fall sharply. Marine organisms such as sponges and algae, along with primitive snails, clams, cephalopods and jawless fish called ostracoderms, all suffered as a consequence.

Devonian extinction

When: about 375-360 million years ago

Species lost: up to 75 percent

Likely cause: oxygen depletion in the ocean

Again, ocean organisms were hardest hit. Fluctuations in sea level, climate change, and asteroid strikes are all suspects. One theory holds that the massive expansion of plant life on land released compounds that caused oxygen depletion in shallow waters. Armoured, bottom-dwelling marine creatures called trilobites were among the many victims, though some species survived.




Permian extinction

When: about 252 million years ago

Species lost: 95 percent

Possible causes: asteroid impact, volcanic activity

The mother of all extinctions, the “Great Dying” devastated ocean and land life alike, and is the only event to have nearly wiped out insects as well. Some scientists say the die-off occurred over millions of years, while others argue it was highly concentrated in a 200,000-year period.

In the sea, trilobites that had survived the last two wipeouts finally succumbed, along with some sharks and bony fishes. On land, massive reptiles known as moschops met their demise. Asteroid impacts, methane release and sea level fluctuations have all been blamed.

Triassic extinction

When: about 200 million years ago

Species lost: 70-80 percent

Likely causes: multiple, still debated

The mysterious Triassic die-out eliminated a vast menagerie of large land animals, including most archosaurs, a diverse group that gave rise to dinosaurs, and whose living relatives today are birds and crocodiles. Most big amphibians were also eliminated.

One theory points to massive lava eruptions during the breakup of the super-continent Pangea, which might have released huge amounts of carbon dioxide, causing runaway global warming. Other scientists suspect asteroid strikes are to blame, but matching craters have yet to be found.




Cretaceous extinction

When: about 66 million years ago

Species lost: 75 percent

Likely cause: asteroid strike

An space rock impact is Suspect No. 1 for the extinction event that wiped out the world’s non-avian dinosaurs, from T-Rex to the three-horned Triceratops. A huge crater off Mexico’s Yucatan Peninsula supports the asteroid hypothesis.

But most mammals, turtles, crocodiles and frogs survived, along with birds as well as most sea life, including sharks, starfish and sea urchins. With dinosaurs out of the way, mammals flourished, eventually giving rise to the species — Homo sapiens — that has sparked the sixth mass extinction.

Source: AFP [April 25, 2019]

ADVERTISEMENT

spot_img

Related articles

Oxygen levels on early Earth rose, fell several times before great oxidation even

Earth's oxygen levels rose and fell more than once hundreds of millions of years before the planetwide success...

Deep waters spiral upward around Antarctica

Since Captain James Cook's discovery in the 1770s that water encompassed the Earth's southern latitudes, oceanographers have been...

150,000 Antarctica penguins die after iceberg grounding

Some 150,000 penguins died after a massive iceberg grounded near their colony in Antarctica, forcing them to make...

Antarctic ice reveals trapped secrets of climate change

They were found deep below Earth's surface, provide vital information about our climate's history and, for the first...

Study links dust to increased glacier melting

A University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science-led study shows a link between large...

Traces of life in the Earth’s deep mantle

The rapid development of fauna 540 million years ago has permanently changed the Earth -- deep into its...

Polar bear numbers to plunge a third as sea ice melts

Polar bear numbers could drop a third by mid-century, according to the first systematic assessment, released on Wednesday,...

An Ice Age savannah corridor let large mammals spread across Southeast Asia

New research from the University of Tubingen indicates that the Thai-Malay Peninsula—where parts of Malaysia, Myanmar and Thailand...