- [Brian Huber] Hi,
and welcome to the David H. Koch Hall of Fossils
at the Smithsonian National Museum of Natural History.
My name is Brian Huber
and I am a curator here at the museum.
Life on Planet Earth is diverse and ever-changing.
Part of that constant change
includes the loss of some species due to extinction.
Sometimes extinctions are caused by abrupt climatic
or environmental shifts,
and these can kick off a cascade of extinctions
that devastates the global ecosystem, called the biosphere.
These are known as mass extinction events.
Past mass extinctions demonstrate
the fragility of ecosystems,
but they also highlight the resilience
of life on the planet.
In this exhibit, we explore two mass extinction events,
what caused them, and the effects they had on our planet.
The largest mass extinction event in Earth's history
occurred 252 million years ago
and is known as the End-Permian Extinction.
Leading up to that time,
a series of volcanic eruptions lasting for a million years
occurred in what is now Siberia.
These eruptions ignited underground coal deposits,
which burned and released carbon dioxide and methane,
causing the Earth's temperature to rise.
The trapped heat and carbon dioxide created acid rain,
which acidified the ocean,
and, with the addition of toxic chemicals
released by the volcanoes,
led to cascading extinctions in the ocean.
This reef model shows the striking change
in marine ecosystems
before and after these volcanic eruptions.
Before, the ocean teemed with coral reefs
and complex ecosystems
that co-evolved for millions of years.
After, environmental catastrophes caused
over 80% of ocean organisms to become extinct.
And the variation in morphology among
what little marine life survived on the ocean floor
was much reduced.
It would take millions of years
for the pre-extinction diversity to recover.
The most recent mass extinction event
is also the most famous.
It ended the age of the dinosaurs.
The End-Cretaceous mass extinction event,
66 million years ago, was caused by a massive asteroid
impacting the Earth.
The asteroid was almost six miles wide
and crashed into a shallow ocean
that covered what is now the Yucatan Peninsula, Mexico.
Over the next few days,
massive tsunamis and shock waves from the impact
rippled across the planet.
In some places, frictional heat and radiation energy
generated from the asteroid traveling
at more than 25,000 miles per hour ignited wildfires,
causing smoke plumes to permeate the atmosphere.
While this is not the first asteroid
to crash into our planet, this was the first
that has been linked to a global mass extinction.
Scientists think the reason this particular impact
caused such devastation include the fact
that the asteroid's landing place was shallow
and the ocean floor contained a highly sulfurous rock
that vaporized upon impact
and combined with water to fall as acid rain.
The other major factor is that the ejecta cloud
that blocked sunlight from reaching Earth's surface
after the impact reduced the ability of plants
to photosynthesize, causing a collapse of food webs
on land and throughout the oceans.
The End-Permian and End-Cretaceous events
are not the only mass extinctions in Earth's history.
Four additional mass extinctions are identified
on the timeline on the north wall of our exhibit.
But additional extinctions in geologic history
may also qualify as mass extinction events.
The criteria for what distinguishes background
or minor extinctions from mass extinction events
is not cut and dry.
And as with all paleontological science,
it depends on what evidence has been preserved
and how scientists can interpret it today.
For example, we know what happened
with the End-Cretaceous mass extinction
because the fossil record is clear.
Dinosaur fossils can be found below the layer of rock
and volcanic ash that marks the asteroid impact event,
but they're absent from the layers above.
But what would a mass extinction event look like
when all life was microscopic?
Such an event would have had the same devastating effects
on the biosphere,
but we cannot observe evidence left behind in the rocks
that we find today.
Regardless of the exact number of mass extinction events
we currently identify,
it is clear that the environmental effects
of mass extinctions may last for millions of years
and cause permanent changes to global ecosystems.
There's evidence that we are at the onset
of another mass extinction event right now.
This one has nothing to do with asteroids
or volcanic activity.
It is caused by human actions.
Increased human populations, overhunting, and global warming
caused by the burning of fossil fuels
strain ecosystems to the breaking points.
The species loss and environmental changes happening
in what has recently been labeled
as Earth's most recent mass extinction,
puts global ecosystems and all of us at risk.
We have time now to act and prevent the destruction
from reaching critical levels,
but only if we act quickly.
To learn more about the Deep Time exhibit,
watch our other narrated virtual tours.
Or check out our webpage
to learn more about what our scientists do here everyday.