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LIMBS,MOVEMENT,AND EXPRESSION

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modules) are involved. Nevertheless, the robot successfully demon- strates different styles of locomotion, including earthwormlike (the modules alternately squeeze and stretch), Slinky-like (end-over-end, like the childs toy), and caterpillarlike (many small feet). A PolyBot can also reconfigure itself, and not only for motion. It can create armlike limbs to deal with small objects.

A video designed to display PolyBots versatility shows it making its way through a military-style obstacle course while a bemused U.S. Marine watches (the military connection is that the project is partly funded by the Department of Defense through DARPA). Another video, this one computer-simulated, shows PolyBot successively adopting different shapes and motions for different terrains: a rolling tank-tread-like loop for a flat surface, an earthworm to crawl down a step, and a multilegged spider for rough terrain. The configuration change from worm to spider is especially striking because it dramati- cally illustrates the effective operation of a robot utterly different from anything human.

ARMS, HANDS, FINGERS, AND THUMBS

Walking on two legs, as humans do, might seem a disadvantage com- pared to the flexibility of a PolyBot or to the stability enjoyed by four-legged creatures. But walking on two legs rather than four frees our arms, hands, and fingers to carry out the complex functions of grasping, holding,and feeling.This extraordinary versatility is not only an essential part of being human; it is one reason we are human, because our flexibility in manipulating the world has improved our thinking capacities.

That flexibility comes largely from the opposability of the human thumbthat is, it can be brought into contact with the tips of any of the four fingers. Although some other primates can also do this, the crucial point is that humans have a large area of contact between the thumb and the sensitive skin of the fingertips. This gives enormous dexterity in dealing with objects and their textures,which has strongly affected the development of our species. Benjamin Franklin was

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