What it was so become was uncertain until between 4.5 and 3.8 billion years ago when the dust

问题描述:

What it was so become was uncertain until between 4.5 and 3.8 billion years ago when the dust
settled into a solid globe.请问when在这里引导的是什么从句?为什么?

是状语从句,when引导的从句修饰4.5 and 3.8 billion years ago