The three-toed sloth is tiny, but most of its extinct relatives were racing to be the biggest. (Bigstock)

Until around 11,000 years ago, sloths were incredibly diverse. While some species shrank down to the 13 pounds or so of our modern equivalent, others were as big as elephants. Even though our two living groups (two-toed sloths and three-toed sloths) are similar to each other in size and appearance, they're actually more closely related to many larger, now extinct species than they are to each other. This means that sloths were doing a lot of size-shifting from species to species, and that some were outgrowing their close cousins by orders of magnitude.

Megatherium was the size of an elephant. (Bjarte Rettedal)

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.