I'd risk saying tree dwellers, maybe some type of theropod before the loss of 2 of its fingers. You see, this design could be considered as zygodactyl as seen in these images:
Note that the external toe of the owl can swivel to switch between a zygodactyl foot for perching and a "eagle" foot for grabbing prey.
Now, why is this beneficial? The fact that it's common in tree-clinging birds should say enough: it's a great design for climbing and perching. So your hand could perhaps appear in a 5 digit reptilian that went somewhat along the theropod evolutionary path, eventually becorming a feathered creature with traits more found in tree-dwelling primates, maybe backed by a tail for additional support and improved balance. This hand would help it climb trees with ease and wrap its hands around branches, and would likely have sharp claws at the end of each finger. This path wouldn't be too different from ours, as our own opposing digit comes from our tree dwelling ancestors, except yours might also be able to have more controlled descends, going down trees head first due to having backwards facing claws, a trait which would stick around even after they left the trees.