A 260-million-year-old fossil is the oldest known tree-dwelling creature, according to researchers.
Scientists described the finding as the earliest evidence in the fossil record of an “opposable thumb”.
In the Royal Society journal Proceedings B, they described how the animal’s elongated hands and fingers would have helped it to grip and climb.
This, they say, shows an evolutionary change that allowed animals to live in trees, away from terrestrial predators.
The fossilised creature, named Suminia getmanovi, has been dated to late Permian period, 100 million years earlier than the first known tree-dwelling mammal.
It was first discovered in Russia in 1994.
But for lead author Jorg Frobisch, from the Field Museum in Chicago, US, said this study was the first opportunity to examine its whole skeleton.
He told BBC News that he and his colleagues looked in most detail at the fossil’s hands – comparing them to other, living terrestrial and tree-dwelling animals.
Suminia, he explained, was a small animal – about 50cm (20 inches) from its nose to the tip of its tail. “But for the size of its body, it had relatively long limbs, and very long hands and feet,” said Dr Frobisch.
“The hands and feet made up almost half of the length of its whole limb,” he continued. “That’s humungous, if you compare it to your own arm.”