美國(guó)英語(yǔ)聽(tīng)力80篇3 Lesson43
Wrist Structure
For years, many scientists have considered walking on two legs as one sign of being human.
Early humans were not thought to have used their knuckle bones or other parts of their hands in walking.
Now, however, American scientists have found evidence that humans developed from creatures that walked on their knuckles, much like gorillas and chimpanzees do today.
Researchers Brian Richmond and David Strait are from the George Washington University in Washington, D.C.
They say they made their discovery almost by accident during a visit to the Smithsonian Institution in Washington.
They compared old reports about the biology of apes with a copy of the wrist bone from Lucy, the most famous fossil remains of an early form of human.
Their findings are reported in the publication Nature.
The researchers examined how the wrist structure of chimpanzees and gorillas differs from other apes.
They also examined the wrist bones from Lucy and another human-like creature that lived in Africa more than 3,000,000 years ago.
The researchers say the ancient creatures had the same wrist structure as modern chimpanzees and gorillas.
They found structures in the wrist bones that restricted the movement of the wrist.
However; the researchers do not believe the creatures used their knuckles in walking because they had longer legs than the apes.
The researchers say a creature with longer legs would have a difficult time
supporting its weight on its lower legs and hands at the same time.
They suggest that the wrist structure for knuckle-walking may have developed even earlier -- as much as 5,000,000 years ago.
David Strait says the development of walking upright required major changes in the structure of the early human ancestors.
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