Thomas Jefferson received a large fossil of what he thought was an enormous American lion. He even warned Lewis and Clark of the potential carnivore that they might encounter on the journey west. Soon after, scientists comparing anatomical features between modern and fossil sloths realized it was indeed a ground sloth, which was later named Megalonyx jeffersonii.

In the Origin of Species, Darwin pointed out that fossils of giant armadillos (a sloth cousin) and ground sloths are found only in the Americas, not in the Old World or Australia. He thought that it was probably not a coincidence that modern-day armadillos and tree sloths are only found naturally in the Americas as well. The term for this concept is biogeography.

Most ground sloths walked on the sides of their hind feet, much like their modern cousins, the anteaters and tamanduas, walk today. Some may have used their tails to balance. There is also fossil evidence that some species evolved denser bones, allowing them to become aquatic.

These marine sloths, like modern day marine iguanas, could have used their long claws to hook onto the corals and graze on seagrasses. The sloths' transition from land to sea probably resulted from a lack of food along the coast of Peru. The Teacher Institute of Evolutionary Science provides free resources to learn more about phylogeny and how organisms have changed over the years.

The educator, Kenny Coogan has worked with modern day sloths for several years.

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