...the hand or the brain?
Our hands are unique. Apes, monkeys, even raccoons have hands, but they don't have hands like we do. We have the best hands - in fact, hands are part of what makes us human. There are structures in our hands that don't exist in, say, a chimp's hands.
Our hands are perfectly adapted for tool use. And we just pushed the modern hand back to 1.42 million years ago.
There's a theory that the hand may have come before the brain - that we actually evolved our complex brains and, ultimately, our intelligence, in order to better manage our clever hands.