The 5 mass extinctions
Ordovician-Silurian extinction: ~ 440 million years ago
SPECIES MADE EXTINCT: 85%
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By the end of the Ordovician Period the seafloor was teeming
with shelled creatures such as trilobites. (Image credit: Alamy)
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The first mass extinction on
Earth
occurred in a period when organisms such as corals and shelled brachiopods
filled the world's shallow waters but hadn't yet ventured onto land.
Life itself
was beginning to spread and diversify, having first emerged around
3.7 billion years ago. But about 440 million years ago, a climatic shift caused sea temperatures
to change, and the majority of life in the ocean died.
At the end of the Ordovician period, a rapid onset of mass glaciation
covered the southern supercontinent,
Gondwana. Glaciation on this scale locked away high percentages of the world's
water and dramatically lowered global sea levels, which stripped away vital
habitats from many species, destroying food chains and decreasing
reproductive success, according to a study published in the journal
Oceanology.
It is not known exactly what triggered these events. One theory is
that the cooling process may have been caused by the formation of the
North American Appalachian Mountains, according to
Ohio State news. Large-scale erosion of these mountainous silicate rocks is associated
with the removal of the greenhouse gas carbon dioxide from the
atmosphere.
Not all scientists agree with this, however. Alternative theories suggest
that toxic metal may have dissolved into ocean waters during a period of
oxygen depletion, wiping out marine life, according to
National Geographic. Other scientists suggest that a gamma-ray burst from a supernova ripped
an enormous hole in the ozone layer, allowing deadly ultraviolet radiation
to kill life below, according to
APS News, and another theory suggests that volcanism was the cause, according to a
study published in the journal
Geology.
Late Devonian extinction: ~ 365 million years ago
SPECIES MADE EXTINCT: 75%
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Dunkleosteus were one of the giants of the sea before a mass
extinction killed them off. (Image credit: Alamy)
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Often referred to as the "age of fish," the
Devonian period
saw the rise and fall of many prehistoric marine species. Although by this
time animals had begun to evolve on land, the majority of life swam through
the oceans. That was until vascular plants, such as trees and flowers,
likely caused a second mass extinction, according to a 1995 study published
in the journal
GSA Today.
As plants evolved roots, they inadvertently transformed the land they lived
on, turning rock and rubble into soil, according to the
BBC. This nutrient-rich soil then ran into the world's oceans, causing algae
to bloom on an enormous scale. These blooms essentially created giant "dead
zones," which are areas where algae strips oxygen from the water,
suffocating marine life and wreaking havoc on marine food chains. Species
that were unable to adapt to the decreased oxygen levels and lack of food
died.
This theory, however, is debated, and some scientists believe that volcanic
eruptions were responsible for the decrease in oxygen levels in the ocean,
according to a study in the journal
Geology.
One sea monster that was wiped from the world's oceans was a 33-foot-long
(10 meters) armored fish called
Dunkleosteus. A fearsome predator, this giant fish had a helmet of bone plates that
covered its entire head and created a fang-like cusp on its jaw.
Permian-Triassic extinction: ~ 253 million years ago
SPECIES MADE EXTINCT: 96% MARINE LIFE; 70% TERRESTRIAL LIFE
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Some of the earliest land dinosaurs, such as dimetrodons,
were among the first to become extinct. (Image credit: Alamy)
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This extinction event, often referred to as the "
Great Dying," is the largest to ever hit Earth. It wiped out some 90% of all the
planet's species and decimated the reptiles, insects and amphibians that
roamed on land. What caused this catastrophic event was a period of rampant
volcanism,
Live Science
previously reported. At the end of the
Permian period, the part of the world we now call Siberia erupted in explosive volcanoes.
This released a large amount of carbon dioxide into the atmosphere, causing
a greenhouse effect that heated up the planet. As a result, weather patterns
shifted, sea levels rose and acid rain beat down on the land.
In the ocean, the increased levels of carbon dioxide dissolved into the
water, poisoning marine life and depriving them of oxygen-rich water,
according to the
Sam Noble Museum
in Oklahoma. At the time, the world consisted of one supercontinent called
Pangaea, which some scientists believe contributed to a lack of movement in the
world's oceans, creating a global pool of stagnant water that only
perpetuated carbon dioxide accumulation. Rising sea temperatures also
reduced oxygen levels in the water,
Live Science
previously reported.
Corals
were a group of marine life forms that were among the worst affected — it
took 14 million years for the ocean reefs to rebuild to their former glory.
Triassic-Jurassic extinction: ~ 201 million years ago
SPECIES MADE EXTINCT: 80%
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Tricinosuchus was one of the many species to go extinct at
the start of the Jurassic Period. (Image credit: Alamy )
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The
Triassic period
erupted in new and diverse life, and dinosaurs began to populate the
world. Unfortunately, numerous volcanoes also erupted at that time.
Although it remains unclear exactly why this fourth mass extinction
occurred, scientists think that massive volcanic activity occurred in an
area of the world now covered by the Atlantic Ocean, according to
MIT News. Similar to the Permian extinction, volcanoes released enormous amounts
of carbon dioxide, driving climate change and devastating life on Earth.
Global temperatures increased, ice melted, and sea levels rose and
acidified. As a result, many marine and land species became extinct; these
included large prehistoric crocodiles and some flying pterosaurs.
There are alternative theories explaining this mass extinction, which
suggest that rising carbon dioxide levels released trapped methane from
permafrost, which would have resulted in a similar series of events,
according to
Discover magazine.
K-Pg extinction: ~ 66 million years ago
SPECIES MADE EXTINCT: 75%
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All non-avian dinosaurs were killed in the fifth mass
extinction. (Image credit: Getty Images)
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The most famous of all the mass extinction events is the
Cretaceous-Paleogene extinction — better known as the day the dinosaurs died. The
event is sometimes also known as the K-T extinction, and geologists call
it the“K-Pg extinction because the letter "C" is shorthand for a previous
geological period called the
Cambrian. The "K" is from the German word "Kreide," which means
"Cretaceous."
Crash-landing into what is today Yucatán, Mexico, an asteroid over 8 miles
(13 kilometers) wide plunged into Earth at around 45,000 mph (72,000
km/h). This punched a hole 110 miles (180 km) wide and 12 miles (19 km)
deep, called the Chicxulub crater. The impact would have scorched all the
land around it within 900 miles (1,450 km) and ended the 180 million-year
reign of the dinosaurs on Earth.
What followed the impact were months of blackened skies caused by debris
and dust being hurled into the atmosphere,
Live Science
previously reported. This prevented plants from absorbing sunlight, and
they died out en masse and broke down the dinosaurs' food chains. It also
caused global temperatures to plummet, plunging the world into an extended
cold winter. Scientists estimate that most extinctions on Earth at the
time would have occurred in just months after the impact. However, many
species that could fly, burrow or dive to the depths of the oceans
survived. For example, the only true descendants of the dinosaurs living
today are
modern-day birds
— more than 10,000 species are thought to have descended from impact
survivors.
Deep impact
HOW AN ASTEROID BROUGHT ABOUT THE END OF THE WORLD FOR THE
DINOSAURS
Death by volcano
HOW MASSIVE ERUPTIONS CAUSED THE BIGGEST MASS EXTINCTION ON EARTH
Turning back the clock
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Pandas became the poster species for extinction back in
1980s, when there were fewer than 1,114 individuals recorded
in China. (Image credit: Getty)
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Humans might be the driving force behind this accelerated extinction
event, but we are also the answer to stopping it. The world is awash with
scientists, conservationists and environmentalists working in the
laboratory, in conservation areas and in political battlegrounds to
protect endangered species.
From tackling global pollution emissions in the
2016 Paris Agreement to the U.K.'s
Global Resource Initiative that
combats deforestation, legislation will always be at the forefront of the
fight against mass extinction. In particular, one of the biggest direct
threats to endangered life is the illegal animal trade.
In the wake of the current pandemic, wildlife markets have been thrust
into the spotlight as not only being environmentally irresponsible, but
potentially dangerous to human health through zoonotic diseases — those
that jump from animals to humans — such as
COVID-19. These markets, trading
live exotic animals or products derived from them, are found throughout
the world. For example, bear farms in Asia cage 20,000 Asiatic black bears
for their bile, resulting in the decline of the wild population, according
to
Animals Asia.
Lawmakers are tackling these kinds of markets with growing success. In
Vietnam, for example, Prime Minister Nguyen Xuân Phúc signed a new
directive that bans wildlife imports and closes illegal wildlife markets,
according to a report in
The Guardian.
New eyes in the sky
One of the best ways to help prevent species from becoming extinct is to
monitor their populations and identify any
problems before it's too late to help. Currently camera traps and surveys
conducted on foot or from aircraft are the main method of data collection.
However, recent research has used a combination of satellite imagery and
artificial intelligence to observe animals from space. Using
high-resolution aerial photographs of Africa’s grasslands, researchers
created an algorithm to sweep over thousands of miles and count every
elephant pictured in the blink of an eye.
The technology, however, is still in its infancy and is limited to areas
where large animals, such as elephants, aren’t obscured by forest
habitats. "The main question here would be the size of the animals and
their habitats. We can’t see through clouds, obviously, but also we can’t
see through forested areas. Therefore we can’t look for animals that are
in dense forest," Olga Isupova, a computer scientist at the University of
Bath and creator of the elephant-tracking AI, told
How It Works magazine, a sister
publication to Live Science. "If they’re in open areas, then it’s just a
question of their size."
Nevertheless, it has huge potential to advance our ability to monitor
species. “We are currently looking at how we can improve the algorithm
itself to look specifically for smaller objects. We could also look for
those animals who appear in herds,” Isupova said. “For example, with
penguin colonies, the model can detect the whole colony and then have an
additional algorithm that approximates the count based on the size of the
colony. Also, you can look for the footprints of the animals. The animal
itself can be quite small, but if there are many of them and they leave
lots of footprints after them, we can also try to track that.”
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An aerial image of Addo Elephant National Park, showing the
algorithm counting elephants.
(Image credit: Maxar Technology)
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Another potential solution to combat extinction could be to
clone species. In February 2021,
scientists revealed they had successfully cloned a
black-footed ferret from an animal
that had died more than 30 years ago. Native to North America, these small
mammals were thought to be extinct until a small colony was found in the
early 1980s, which were entered into a breeding program and reintroduced
around the United States.
Due to inbreeding, the population of around 650 ferrets is at risk of
extinction once again. This inspired researchers to create a genetic copy
from the preserved cell of a wild female, named Willa, who died in the
1980s. The process of cloning was similar to that used to clone Dolly the
sheep back in the early 1990s. Scientists hope that after time spent in
captivity, cloned members of the species can successfully re-enter the
wild, offering a new conservation tool to protect endangered species.
It is legal, scientific and technological advances such as these that will
help to conserve Earth's wildlife and hopefully slow down the sixth mass
extinction.
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