Why is "what" used here, not "who"?
We might expect that "who" is used to refer to people. "What," however, is not only used in reference to objects, but also when asking about a specific characteristic or attribute of a person. The speaker is asking "what" has two thumbs that pulled up the sky (this might be super strength), and that can be considered an attribute - hence the "what." Ex: Who is Mary? My sister. (Who the person) Ex: What is John? He is a teacher. (What is the attribute of the person)