Unravelling sloth extinctions
What is often called the Sixth Mass Extinction refers to the elimination of way too many animal and plant species over the last few decades. Primarily, this has been due to the staggering growth in the number of we humans who are increasingly altering habitats and overconsuming the Earth's natural resources.
But we often overlook the fact that mass extinctions started long ago as Homo sapiens spread across the globe. In the Americas, the first invaders arrived between 25,000 and 21,000 years ago, most likely by crossing the Bering Land Bridge from Asia into Alaska. Though dating is still being investigated, they'd have advanced to central North America by at least 15,000 years ago, and South America around 15,000 years ago as well. These people had to eat, and large animals would have provided large energy sources requiring a limited amount of work to kill them.
It didn't take long for much of the megafauna to disappear. There's even a name for it: the Quaternary extinction event. In both North and South America, it peaked between 13,000 and 11,000 years ago. (Megafauna is usually defined as animals weighing over 45 kg = 100 pounds.)
Surprisingly, among those extinctions were many species of sloth - more than 100 species in 7 or 8 families have been identified from fossils. Mostly quite large - up to the size of elephants - and ground dwellers, they would have been easy to find and kill by the newly arrived humans.
Today, of those 100-plus sloth species that once roamed the Americas, only six species of tree-dwelling sloths survive.
Note - Some scientists suggest that climate change also contributed to the extinctions. After all, the last Ice Age ended about 11,700 years ago. That theory sounds unlikely because, going further into the past, those species of sloth that most recently went extinct would have previously survived one or more glacial periods. As well, the six remaining sloth species would also have been affected by climate change - yet they're still with us. Finally, you might wonder why the timing of sloth extinctions on the Caribbean islands wasn't till later on, about 6,000 years ago. Well, humans didn't colonize the islands till later on.
As one scientist succinctly put it: "Hunting these large mammals tracks global colonization perfectly. Humans move to a new place, megafauna suffer extinction."
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