他们的老祖先是海洋中的鱼类,在那之前是各种或多或少像虫子一样起源于最原始细菌的生物,一直生活在海洋中.
经过一系列的研究后,我个人理解的是:
and before that .与 marine fish 并列.before that 是插入语,修饰creatures.
still in the sea 是插入语,修饰creatures.
简化一下是
ancestors were marine fish,and,ancestors were creatures before fish.
creatures were various,and more or less worm-like.
creatures stretched back to the primeval bacteria.
creatures were still in the sea.
stretch back to,起源于.stretch back,追溯到.