Sloths: how did two different animals wind up looking so similar?

March 12, 2019 by Richard Gray, From Horizon Magazine, Horizon: The EU Research & Innovation Magazine

Sloths and guppies appear to have little in common – one is an arboreal mammal living in the slow lane, while the other is a tiny tropical fish with a frantic existence. Yet both could hold the key to better understanding a fundamental process of evolution.

Hanging lazily from tree branches where they barely move for hours on end, sloths hardly seem like born survivors. Yet the sloths that creep through the rainforest canopies of Central and South America are the last remaining members of a group of animals that has roamed the Earth for at least 30 million years and once boasted hundreds of species.

Many of their extinct cousins were giants – some larger than elephants – that spent most of their time on the ground before they died out 10,000 years ago at the end of the last ice age. Modern sloths, however, are thought to have avoided the extinctions that wiped out their larger relatives because they ascended into the trees.

But what makes their survival more extraordinary is that the two types of modern sloths – the two-toed and the three-toed sloths – are thought to have adapted to life in the trees entirely independently of each other. They are an example of what is known in biology as convergent evolution – a process where species develop similar adaptations to a habitat through different evolutionary routes.

"When you look at them hanging in the trees, the two look very similar," said Dr. Marcela Uliano da Silva, a computational biologist at the Leibniz Institute for Zoo and Wildlife Research in Berlin, Germany.

'(But) they have different numbers of toes on their forelimbs and the way the muscles connect from the body to the bones in the arms is also different," she said.

Evolutionary tree

These physical differences are signs that two-toed and three-toed sloths found their own ways to adapt to a lifestyle in the trees. They are thought to have last shared a common ground-dwelling ancestor about 30 million years ago, which is around the same time that humans split from baboons on the evolutionary tree.

In most cases, evolution tends to follow a classic route where species diversify from a common ancestor to adapt to different habitats and challenges. Modern sloths, however, are an example of where evolution has gone in the opposite direction by separately adapting to have very similar lives in the trees.

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