通有两只脚的生物。从恐龙时代开始至今。至今人们还在寻找古生物的两足动物。
有一种理论认为，两足动物的进化与核心家庭有关：一夫一妻制的父母合作照顾他们的后代。
One theory is that bipedality evolved in conjunction with the nuclear family: monogamous parents cooperating to care for their offspring.
因为它们是两足动物，它们的手臂可以自由地进化出拍打的翅膀，这在当时的爬行动物中是不可能的。
Because they were bipedal, their arms were free to evolve flapping flight, which cannot be said for other reptiles of their time.
实际上，两足动物的行走可能比四肢的行走花费更少的力气。
Walking as bipeds might actually use less energy than movement on all fours does.
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