According to a new study published in BMC Evolutionary Biology, the sloths that grew instead of shrinking did so at an impressive rate. Some lineages may have been growing at rates of nearly 300 pounds every million years. That may sound slow, but it's some of the fastest scientists have seen in mammals.
For growth that speedy, these giant sloths must have had a strong advantage over their smaller relatives. In each generation, the very biggest must have survived, passing on their ginormous genes. A much smaller collection of species shrank down in size, presumably so that they weren't competing for the same resources as the giants. And 11,000 years ago, whatever changes killed the giant sloths -- probably climate change, human hunters, disease, or some combination of those -- spared the little guys.
It's important to remember that today's animals are just a curated selection of their ancestors. "There are many other groups, such as hyenas, elephants and rhinos, that, like sloths, have only a few living species," University College Dublin Earth Institute professor and study co-author John Finarelli said in a statement. "But if we look into the distant past, these groups were much more diverse, and in many cases very different to their current forms."
Here's to giant sloths, which we can only hope were massively adorable.