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Alexandra Bruce
January 23, 2015

99% of all life that has ever existed on Earth has gone extinct – and as many of you may already know, we are currently smack in the middle of a mass extinction event, colloquially known as the Sixth Extinction, which began around 12,000 years ago, initially marked by the rapid demise of many of the world’s megafauna (large land animals).

The general consensus is that our current mass extinction event has always been caused by human activity, such as the over-
hunting of mammoths and other megafauna, which had long lifespans and which were slow to recover from great losses to their
numbers – but even in Hawaii, it is estimated that 2,000 species of birds, alone have become extinct, since the arrival of humans
there 30,000 years ago.

There is also a good argument that a low-density cometary impact occurred around 12,900 years ago over the present-day Great Lakes region of the US and Canada, which at that time was covered with a massive, three-mile-thick glacier, known as the Laurentide Ice Sheet. No major impact crater has been found, either due to this glacial shield or that the event, itself was a
massive airblast, similar but larger than the Tunguska Event over Siberia in 1908. There is evidence that its gargantuan shockwave triggered fires throughout the continent, leaving behind a uniform “black mat.”

In addition, layer of hexagonal nano-diamonds can be found in the Northern Hemisphere, as far away as Germany, indicating an impact of enormous pressure.

The impact also resulted in the end of the Clovis Culture of Paleo-Indians throughout North America, as well as a population
bottleneck among the Paleo-Indians who survived this cataclysm. Still, 80% of Native Americans are descended from the original
Paleo-Indians and the rest descend from peoples who arrived in more recent migrations from Northeastern Siberia, according to DNA tests.

Two other population bottlenecks have been discovered by Cambridge University scientists; one 50,000 years ago in Eurasia and another, more recent one, around the Bering Strait. These bottlenecks account for Africa’s much higher genetic diversity than that of any other continent.

Although most of us already know that North America was home to millions of mammoths and saber toothed tigers, it was also the land of the giant short-faced bear (the largest carnivorous land mammal in North America, 11-12 feet (3.4-3.7 m) tall on its hind legs, with a 14 foot (4.3 m) vertical arm reach, and 5-6 feet (1.5-1.8 m) high at the shoulder when walking on all fours. The American lion roamed wild and free with the stilt-legged horse and the western camels, 7 ft/2.1 m tall.

My favorite American megafauna were the gigantic ground sloths, which reached the size of modern-day elephants, the last one of which is said to have expired in Cuba in the 1500s. I would also love to have gotten to see the giant moas of New Zealand, which were 12 ft/3.6 m tall and hunted into extinction by the Maoris, 300 years prior to the arrival of Europeans.

In this episode of SciShow, Hank takes us on a trip in his nifty time travel machine to revisit the five previous major mass extinction events that have impacted species over the Earth’s history and he invites us to give a proper name to the current one, which, if not solely caused by human activity at its start, long ago has without question been rapidly accelerating since the Industrial Revolution. One commenter befittingly offered: “The Anthropic Extinction Event”.


References for this SciShow episode can be found in the Google document here:

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Alexandra Bruce

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