Therefore，if the Earth began as a superheated sphere in space, all the rocks making up its crust may well have been igneous and thus the ancestors of all other rocks.
(特殊结构may well have been——可能是)
因此，如果地球开始时是太空中一个过热的球体，那么所有组成地壳的岩石都可能是火成的，因此成为所有其他岩石的祖先。
解析：
主句：all the rocks may well have been igneous and thus the ancestors of all other rocks
从句1：if the Earth began as a superheated sphere in space
从句2：making up its crust
主句中两个宾语部分igneous和the ancestors of all other rocks并列
从句1作为条件状语从句，修饰主句
从句2非谓语动词后置修饰主句中的主语rocks
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