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among the first to realize that the ability to manipulate is essential to human culture; it was he who defined humanity as the tool-making animal.The physician and primatologist John Napier enlarges on this comment in his book Hands, explaining that the opposable thumb

promoted the adoption of the upright posture and bipedal walking, tool- using and tool-making that, in turn, led to enlargement of the brain through a positive feed-back mechanism. In this sense it was probably the single most crucial adaptation in our evolutionary history.

But the power of a hand cannot be brought to bear without immense flexibility from shoulder to wrist, which in humans draws on six degrees of freedom.Two of these degrees represent rotation at the shoulder, raising the arm higher or lower, and rotating it backward and forward. A third is found at the elbow joint, and three more operate at the wristrotation around the axis of the forearm, move- ment of the hand up or down, and movement left and right. Design- ing so versatile a jointed system for an artificial being is an engineering challenge, and designing it to move properly is a geometric and com- putational one, as complex as programming a robot to walk on two legs, and sometimes requiring advanced mathematics. But the hu- manoid robots H6, H7, P3, and ASIMO all have arms and wrists as multijointed as human ones and are programmed to carry out some manipulations. H7 can reach under a table and grasp an object on the floor, and P3 can turn a handle to open a door.

However, what passes for hands on these units are poor substi- tutes for human hands. Rather than flexible thumb and fingers, they have a gripper design that grasps an object between stiff tines, with a squeezing action like a big pair of tweezers. Moreover, the robotic hands lack any sense of the weight or texture of an object.This lim- ited design appears in prosthetic hands as well; Paolo Dario and his colleagues of the Mitech Laboratory, Scuola Superiore SantAnna, in Pisa, Italy, note that current prosthetic hands are simple grippers with one or two degrees of freedom, which barely restores the capability of the thumbindex [finger] pinch.

Nevertheless, even limited capability is worth a great deal in a prosthetic replacement and is not necessarily a problem for many

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