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."
The ends of its fingers were long, slender and curved - like claws.
"In life these probably would have been covered in a hard, keratinous coating, much like in modern-day birds," Dr Frobisch explained. These would have helped the small creature to grip and climb.
But the most significant observation the team made was that one finger on each hand and foot was "opposed" to the rest, much like a thumb.
"It's the first time in the fossil record that we've seen evidence of an opposable thumb," he said, adding that the creature was an early ancestor of mammals.
Between the time when Suminia lived, and the period to which fossils of the earliest-known tree-dwelling mammals have been dated, there is a gap of about 100 million years.
"It's exciting to see the evidence of an initial and successful evolutionary change or diversification that was successful and allowed these small animals to live in the trees," said Dr Frobisch.
The evolution of its specialised hands and feet, he explained, would have allowed Suminia to escape the fierce competition for food on the ground and avoid predators.
Simon Conway Morris, a palaeobiologist from the University of Cambridge described the finding as "very exciting".
"It shows once again that things in evolution can happen far earlier than might be expected," he told BBC News.
"In this case a vertebrate, specifically a synapsid - from which the mammals themselves emerged - was ahead of the game of climbing trees. In fact it was about 30 million years ahead of schedule."