脊椎动物包括鱼类和各种四足动物,它们的身体里都有由一连串脊椎骨构成的脊柱。我们人类也是这个家族的成员。距今4亿多年前,一些脊椎动物的头骨中出现了上下颌(下颌就是我们常说的下巴),这标志着脊椎动物由滤食性生活向更为主动的捕食性生活方式的过渡。颌的重要功能英国首相温斯顿·丘吉尔曾经说过:“动动下巴总好过动手。”他用下巴这个词表达的是无休止的外交谈判中的斗嘴。但是在远古的海洋中,直到4亿多年前,所有脊椎动物都是无颌的, Vertebrates include fish and various four-legged animals, all of which have spines in their bodies that consist of a series of vertebrae. We humans are also members of this family. More than 400 million years ago, the upper and lower jaws of the vertebrate skulls (the lower jaw is what we often refer to as the chin) appear to indicate the transition from vertebrate filter-feeding to more predatory lifestyle. Important function of jaws Winston Churchill, the prime minister, once said: “Moving your chin is always better than moving your hand.” He used the term chin to express bickering in endless diplomatic negotiations. But in the ancient oceans, all vertebrates were jawless until 400 million years ago,