Our Amazing Hands
The hand is where the mind meets the world. We use our hands to build fires, to fly airplanes, and to write. The human brain, with its open-ended creativity, may be the thing that makes the human race unique. But without hands, all the ideas we think up would come to nothing.
Study it carefully, you will find something interesting. The thumb (拇指) alone is controlled by nine separate muscles. The wrist is a group of bones and muscles connected with nerves (神经). The nerves send branches into each fingertip, which makes the fingers extremely flexible.
Early hands seemed more unusual and interesting than any hand today. Some animals had seven fingers. Others had eight. But by the time vertebrates (脊椎动物) appeared 340 million years ago, the hand had developed to only five fingers.
Nevertheless, there are still many different types of hands in living animals. After years of research, scientists are beginning to understand the molecular (分子的) changes in hands. This makes the hands of different animals very similar. There is a network of many genes (基因) that builds a hand, and all hands are built on that network.
The discovery has given scientists a deeper understanding of the development of hands. A bird's wing and a lion's paw (爪) may appear to have nothing in common. It may just be a little more of one protein (蛋白) here, a little less of another there. In the past, scientists could recognize only the outward signs that hands had developed from a common ancestor. Today scientists are uncovering the inward signs as well.
A. Hands can often be used for a number of different purposes.
B. They also see that all hands start out in much the same way.
C. It has kept that number for reasons scientists don't yet know.
D. No one would doubt that the five fingers are different with each other.
E. But the difference between them may come down to a tiny change in form.
F. The reason we can use our hands for so many things is their special structure.
G. So you can see a skilled watchmaker use his hands to set springs in place under a microscope.
