Six (Mass) Extinctions in 440 Million Years
All things must pass. But the idea that a species could go extinct
is a relatively new one, first proposed by anatomist Georges Cuvier in a
presentation in Paris in 1796 in a lecture on the extinction of the
mastodon, then thought by some to still be roaming the ill-explored
western reaches of North America.
Cuvier’s suggestion that life on Earth was not static, and that
species could disappear, was groundbreaking. Studying the collections of
the National Museum of Natural History in Paris and records from other
collections around the world, he soon identified several species whose
like we would never see again, including the mosasaur, the cave bear,
and the Irish elk.
Buoyed by the research of scientists like Charles Lyell and Charles
Darwin, the idea that species developed gradually, over time, gained
acceptance in the scientific community. For generations, it was dogma
that extinctions happened slowly, too. The idea that species could be
wiped out in a fell swoop, even one with catastrophic consequences,
wasn’t given much credence.
That changed in the late 1980s and early 90s, when the Alvarez
hypothesis, which stated that a huge comet or asteroid impact was
responsible for the sudden disappearance of non-avian dinosaurs and many
other forms of life 66 million years ago. Proposed by physicist Luis
Alvarez and his son, geologist Walter Alvarez, the hypothesis took time
to gain acceptance, but buoyed by evidence like the crater pictured
below, an impact is now the most widely accepted explanation for the
Cretaceous-Paleogene (K-Pg) extinction.
That acceptance also opened the door for further study of geological
and fossil records, which led researchers to a surprising conclusion:
While the K-Pg extinction event was a very bad day for life on Earth, it
was by no means the only one on record. Researchers now think that the
K-Pg was just the latest of five major extinction events—and that we’re
currently in the middle of a sixth mass extinction, one caused not by a
volcano or asteroid impact, but by humans.
Each event had a different impetus. Some took place over the span of
millions of years while others were extremely sudden. What they have in
common, though, is that they reshaped the face of life on Earth by
wiping out a significant portion of it.
About 445 Million Years Ago: Ordovician Extinction
The earliest known mass extinction, the Ordovician Extinction, took
place at a time when most of the life on Earth lived in its seas. Its
major casualties were marine invertebrates including brachiopods,
trilobites, bivalves and corals; many species from each of these groups
went extinct during this time. The cause of this extinction? It’s
thought that the main catalyst was the movement of the supercontinent
Gondwana into Earth’s southern hemisphere, which caused sea levels to
rise and fall repeatedly over a period of millions of years, eliminating
habitats and species. The onset of a late Ordovician ice age and
changes in water chemistry may also have been factors in this
extinction.
About 370 Million Years Ago: Late Devonian Extinction
Towards the end of the Devonian period around 370 million years ago,
a pair of major events known as the Kellwasser Event and the Hangenberg
Event combined to cause an enormous loss in biodiversity.
Given that it took place over a huge span of time—estimates range
from 500,000 to 25 million years—it isn’t possible to point to a single
cause for the Devonian extinction, though some suggest that the amazing
spread of plant life on land during this time may have changed the
environment in ways that made life harder, and eventually impossible,
for the species that died out.
The brunt of this extinction was borne by marine invertebrates. As
in the Ordovician Extinction, many species of corals, trilobites, and
brachiopods vanished. Corals in particular were so hard hit that they
were nearly wiped out, and didn’t recover until the Mesozoic Era, nearly
120 million years later. Not all vertebrate species were spared,
however; the early bony fishes known as placoderms met their end in this
extinction.
252 Million Years Ago: Permian-Triassic Extinction
The Permian-Triassic extinction killed off so much of life on Earth
that it is also known as the Great Dying. Marine invertebrates were
particularly hard hit by this extinction, especially trilobites, which
were finally killed off entirely. But you don’t get a nickname like the
Great Dying for playing favorites; almost no form of life was spared by
this extinction, which caused the disappearance of more than 95 percent
of marine species and upward of 70 percent of land-dwelling vertebrates.
So many species were wiped out by this mass extinction it took more
than 10 million years to recover from the huge blow to global
biodiversity. This extinction is thought to be the result of a gradual
change in climate, followed by a sudden catastrophe. Causes including
volcanic eruptions, asteroid impacts, and a sudden release of greenhouse
gasses from the seafloor have been proposed, but the mechanism behind
the Great Dying remains a mystery.
201 Million Years Ago: Triassic-Jurassic Extinction
This extinction occurred just a few millennia before the breakup of
the supercontinent of Pangaea. While its causes are not definitively
understood—researchers have suggested climate change, an asteroid
impact, or a spate of enormous volcanic eruptions as possible
culprits—its effects are indisputable.
More than a third of marine species vanished, as did most large
amphibians of the time, as well as many species related to crocodiles
and dinosaurs.
66 Million Years Ago: Cretaceous-Paleogene Extinction
The most recent mass extinction event is also likely the best understood of the Big Five.
In addition to its most famous victims, the non-avian dinosaurs, the
K-Pg event caused the extinction of pterosaurs and extinguished many
species of early mammals and a host of amphibians, birds, reptiles, and
insects. Life in the seas was also badly disrupted, with damage to the
oceans causing the extinction of marine reptiles like mosasaurs and
plesiosaurs, as well as of ammonites, then one of the most diverse families of animals on the planet.
In all, scientists estimate that 75 percent of species living at the time of the K-Pg extinction were wiped out.
Now: The Holocene Extinction
The Holocene Extinction hasn’t been defined by a dramatic event like
a meteor impact. Instead, it is made up of the nearly constant string
of extinctions that have shaped the last 10,000 years or so as a single
species—modern humans—came to dominate the Earth. Some have even
suggested that the Holocene Extinction would be more aptly named the
Anthropocene Extinction, after the role humans have played in this
ongoing loss of biodiversity around the world.
“Many of the past mass extinction events are mysterious in some ways
because we really don’t know the cause,” says Michael Novacek, the
Museum’s provost of science and a curator in the Division of Paleontology.
"But we have a good idea of what the cause of the current changes are,
this century and the centuries before: it’s human activity.”
Humans have contributed to factors like climate change and the
introduction of invasive species, which are leading to even more
extinctions as animal habitats disappear or are disrupted by new
species. “Some biologists think that the current rate of species loss
is probably a thousand times what the normal rate is,” says Novacek.
Many of the species going extinct are doing so before they are even
identified. In light of this, researching new species for a fuller
understanding of the world’s biodiversity grows ever more urgent for
institutions like the American Museum of Natural History. These records,
Novacek says, are vital to our knowledge of the world around us.
“The collections in the Museum here and other museums are really a
record of life," Dr. Novacek says. “They’re very important for not only
telling us what went extinct, but what survived.”
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