本句中的only修饰的是谓语动词还是because of ?

Living on the ground may only have become an option in the first place,because of one key event: the extinction of dinosaurs.

本句中的only是修饰谓语动词 may have become 还是修饰because of ?感觉从句意上理解应该更接近于后者，上面这个句子假如改为

Living on the ground may have become an option in the first place,onlybecause of one key event: the extinction of dinosaurs.

句意会改变吗？是不是第二种写法更适合？

期望老师解惑，谢谢！

Living on the ground may only have become an option in the first place, because of one key event: the extinction of dinosaurs.

因为because 之前有了逗号，only 只能修饰主句。假如没逗号，则可以修饰 because 从句。only 的这种使用方法跟 not 一致，限制范围可以是转移到状语或状语从句上面。