(Birds') 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，上面这个句子如果改为

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

句意会改变吗？是否第二种写法更合适？

希望老师解惑，谢谢！