But the first people who were like ourselves lived so long ago that even their sagas, if they had any, are forgotten.

1. 这里的 that 作用，什么从句？

【答】此为so...that...句型，that 引导结果状语从句。 ... so long ago that...

2. 这里的 if 又是什么？

【答】if 引导条件状语从句，插在自己的主句中，或者说把主语even their sagas 和谓语are forgotten 分隔。

3. are forgotten?

【答】even their sagas are forgotten。