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:
Sloths were once as large as elephants
Baby Three-toed Sloth seen in Coca, Ecuador - Donate to SlothSanctuary.org!
A sloth peeks out from behind a door on a floating house in the 'Lago do Janauari' near Manaus, Brazil, Tuesday, May 20, 2014. Manaus is one of the host cities for the 2014 World Cup in Brazil. (AP Photo/Felipe Dana)
In this May 20, 2014 photo, a sloth holds itself on the deck of the Silva family's floating house on the Lago do Janauari, or Solimoes River, near Manaus, Brazil. Between the parents, the four kids, the dog, the two sloths, the python and the baby caiman, itâs a tight fit in the Silva familyâs two-room floating home in the Amazon rainforest. The exotic houseguests help 35-year-old fisherman Evandro Correia da Silva and his family eke out a living on their native Solimoes River, on the opposite bank from Manaus, a World Cup host city where the U.S., English and Italian teams will be among those battling it out. (AP Photo/Felipe Dana)
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Claire, a worker at the Sloth Sanctuary in Penshurt, some 220 km east of San Jose, Costa Rica, holds baby sloths on August 30, 2012. AFP PHOTO/Rodrigo ARANGUA (Photo credit should read RODRIGO ARANGUA/AFP/GettyImages)
A baby two-toed sloth (Choloepus) eats fruit at the Aiunau Foundation in Caldas, some 25 km south of Medellin, Antioquia department, Colombia on September 15, 2012. Croatian scientist Tinka Plese created the foundation 10 years ago, where sloths --which have been captured by illegal wildlife traffickers and then sold to people between US40 to 150 dollars-- are rescued, rehabilitated and released. More than 700 sloths have been released to date. AFP PHOTO/Raul ARBOLEDA (Photo credit should read RAUL ARBOLEDA/AFP/GettyImages)
Jutta Hoyer from the zoo in Halle, eastern Germany, feeds sloth Paula on May 17. According to the zoo, Paula lives there since 40 years and is the oldest sloth living in a European zoo. In the wild, sloths live mainly in tropical rainforests of Central and South America. AFP PHOTO WALTRAUD GRUBITZSCH GERMANY OUT (Photo credit should read WALTRAUD GRUBITZSCH/AFP/Getty Images)
COSTA RICA - MARCH 03: Pale-throated sloth (Bradypus tridactylus), Bradypodidae, Cahuita National Park, Costa Rica. (Photo by DeAgostini/Getty Images)
This photo released by Six Flags Discovery Kingdom shows Chewy, a four-year-old female two-toed sloth, at her home at Six Flags Discovery Kingdom in Vallejo, Calif., on Tuesday, Oct. 15, 2013. The third annual International Sloth Day takes place on Saturday, Oct. 19. The day was established by the Columbia-based AIUNAU, a conservation and rehabilitation organization aimed at protecting sloths, anteaters and armadillos ? small mammals endangered by human threats such as illegal trafficking, injury and mistreatment. Sloths are native to Central and South America. (AP Photo/Six Flags Discovery Kingdom, Nancy Chan)
A Southern two-toed sloth is seen with her four-month old baby at the Singapore Zoo on Friday March 5, 2010 in Singapore. The Singapore Zoo is actively involved in wildlife preservation efforts through zoo exchange programs as well as public education campaigns. (AP Photo/Wong Maye-E)
UNSPECIFIED - AUGUST 14: Illustration of Smilodon attacking Ground Sloth (Photo by De Agostini Picture Library/De Agostini/Getty Images)
THE TONIGHT SHOW STARRING JIMMY FALLON -- Episode 0025 -- Pictured: (l-r) Animal expert Jeff Musial with a baby sloth during an interview with host Jimmy Fallon on March 21, 2014 -- (Photo by: Lloyd Bishop/NBC/NBCU Photo Bank via Getty Images)
Eight months old baby sloth Camillo yawns as he is pictured on May 17, 2011 at the zoo in Halle, eastern Germany. In the wild, sloths live mainly in tropical rainforests of Central and South America. AFP PHOTO WALTRAUD GRUBITZSCH GERMANY OUT (Photo credit should read WALTRAUD GRUBITZSCH/AFP/Getty Images)