二足动物开始直立行走。
此外，专家还说，我们祖先在进化到二足动物时期的时候，凭借着“后肢动物”的优势更容易打败“前肢动物”，因为从上至下出拳力量会更大。
Standing up on their hind legs allowed our ancestors to fight with the strength of their forelimbs, making punching much more dangerous.
(古生)板足鲎目的：一种巨大的、有节的，属于板足鲎目的水生节肢动物，生存于奥陶纪到二叠纪时期。
Eurypterid: any of various large segmented aquatic arthropods of the order eurypterida that existed from the ordovician period to the Permian period.
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