刚开始肯定是不适应的。以前它们是用四肢爬树的。到了地上前肢就可以解放出来，做一些觅食，防御攻击敌人的工作。慢慢地，前肢就进化成了手，由后肢独立承担起了行走的责任，也就是能直立行走了。
gāng kāishǐ kěndìng shì bù shìyīng de 。yǐqián tāmen shì yòng sìzhī pá shù de 。dào le dìshang qiánzhī jiù kěyǐ jiěfàng chūlái ，zuò yīxiē mìshí ，fángyù gōngjī dírén de gōngzuò 。mànmàn de ，qiánzhī jiù jìnhuà chéng le shǒu ，yóu hòuzhī dúlì chéngdān qǐ le xíngzǒu de zérèn ，yě jiùshì néng zhílì xíngzǒu le 。
At first, they definitely weren't used to it. In the past, they had used all four limbs to climb trees. After arriving on the ground, their front limbs were free to do the job of foraging for food, and defending from enemy attacks. Slowly, their front limbs evolved into arms, which caused the back legs to take the sole responsibility of walking about. This was also the ability to walk upright.