The sloths we know and love today may be small and slow, but they're survivors. Unfortunately, the bulk of sloth species that once roamed the earth -- some of which grew to be the size of elephants -- cannot say the same.
Long ago, there was a dramatic uptick in the pace of sloth evolution. In addition to growing to substantial sizes, fossils suggest some could walk on two legs and others developed foot-long claws.
Exactly why or how the dramatic changes happened is unknown, but scientists are chalking it up to the usual favorable combination of factors: increased competition amongst species and an ideal environment.
The good times quickly came to an end, though, likely because of the arrival of humans or the animals' failure to thrive during the dawn of an ice age.
All but two families, consisting of six total species, were wiped out. At some point, being small was determined to offer a greater advantage, so the sloths shrunk over time.
Researchers discovered this by going beyond the usual practice of studying only living species. They also sought information through fossil examination. Doing so allowed them delve deep into millions years of sloth history.
Small or large, you have to admit they're pretty cute:
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