A mere 13,000 years ago, much of the plant and animal life around Earth looked completely
alien.
For example, in my home state of Arizona, car sized armadillo like creatures known as
glyptodons roamed the landscape which was largely dense forest rather than the dry desert
seen today.
Other unusual creatures included the giant ground sloth which stood at 9 feet tall, elephant
like woolly mammoths, and a very large species of lion known as the American lion.
So, why are these megafauna no longer around today?
The answer is that these creatures were killed by one of the planet's most recent mass extinction
events which was caused by a massive comet impact event the likes of which have not been
seen in modern times.
This impact event created a devastating impact winter known as the "Younger Dryas" which
lasted for 1,200 years.
During this period, which lasted from 12,900 to 11,700 years ago, average temperatures
dropped by 10 degrees Celsius or 18 degrees Fahrenheit.
In what was before a planet slowly warming out of an ice age, instead ice age like conditions
returned over much of the landscape.
For decades, the existing notion that human overhunting caused this widespread extinction
was repeatedly challenged.
Some of the first evidence that humans were not the main cause of this extinction came
when magnetic microspherules were discovered at layers of ground around Clovis culture
sites in North America.
These microscopic spherules could only have one possible origin, a large meteorite impact
from an asteroid or comet, and were close in appearance to those found after the Tunguska
explosion in 1908.
During the Tunguska event, a 65 meter wide asteroid violently exploded in the atmosphere
at a height of only 13,000 feet, generating a powerful explosion known as an airburst
which flattened 2,000 square kilometers of forest.
Later analysis of similar sites across North America, Europe, the Middle East, and Greenland
all displayed these unique features at the same Younger Dryas boundary layer, indicating
that hundreds if not thousands of airbursts occurred over a brief timespan.
In addition, several of these areas had a definable black layer marking the beginning
of the Younger Dryas, hinting at a catastrophic and sudden change in much of the planet's
climate.
Another clue that this period was caused by an extraterrestrial collision came when these
layers were analyzed, revealing abnormally high levels of platinum which were 27 times
higher than background concentrations.
For context, platinum is the 8th rarest stable element in the Earth's crust, occurring at
a mere concentration of 3 parts per billion.
Platinum group elements such as platinum and iridium are far more common in large asteroids
or the cores of metallic comets than they are in the crust of Earth.
On earth, the planetary materials largely differentiated, causing the vast majority
of heavy elements such as platinum to sink into the planet's core.
Approximately 12,900 years ago, a large metal rich comet was on a collision course with
Earth.
This comet had abundant platinum in its stony core, and measured slightly more than 4 kilometers
or 2.5 miles wide.
As this massive object approached the inner solar system, large chunks of it began breaking
off, creating thousands of fragments in a large trail of debris.
This debris then entered Earth's upper atmosphere, and was travelling at speeds of 50 kilometers
per second or 112,000 miles per hour.
Almost all of these fragments began rapidly burning up, before violently exploding above
the surface, producing a planetary wide airburst where thousands of Tunguska sized events occurred
over the span of 180 seconds.
This released an immense amount of thermal radiation, causing any plants and animals
near an airburst to immediately ignite.
Over the next several hours, massive wildfires spread across the planet, burning 10% of all
of Earth's biomass across an area of 30 million square miles.
Large amounts of soot and sulfur dioxide were ejected high into the atmosphere, and soon
began reflecting significant quantities of sunlight, causing plants on the ground to
die.
As the 1,200 year long impact winter continued, numerous plants were unable to grow, and in
turn the herbivores and carnivores which directly or indirectly relied on those species to exist
almost all went extinct.
Finally, 11,700 years ago, temperatures became climbing once again, reversing the temporary
trend and creating a world largely absent of megafauna.
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