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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