麻烦帮忙分析一下这个句子的结构～Why was the origin of life a sample step in

Why 疑问副词作状语
was 谓语
the origin of life主语
a sample step 表语
in the Earth's history 状语
把句子换成陈述句,成分会更加明了：
The origin of life was a sample step in the Earth's history
再问： 那麻烦问一下这个表语是个什么用法啊
再答： a sample step 是名词作表语 a sample step的核心词是step，a 和 sample都是定语修饰step