What did people do before it was discovered?

The teacher was explaining to the class, "Oxygen is necessary to all life. Without it there'd be no life at all. Now just think - it was only discovered about 100 years ago." Jenny raised her hand and asked, "If that's true, what did people do before it was discovered?"

老师在课堂上讲道：“氧气对每一种生命都是不可或缺的。如果没有氧气，地球上将不会有任何生命存在。大家想想看，氧气的发现只有100多年啊！”

珍妮举手问：“如果真是这样的话，那么氧气还没发现以前，人们是怎么活的？”