Our ancestors didn't write a lot of stuff down. True, language wasn't invented yet, so we give them a bit of a break.
Science gave modern people a break with the ability to sequence the human genome. That turned up a lot more information about the human family tree (or bush, really).
Adam Rutherford, both a geneticist and a science journalist, celebrates the recent discoveries in his book A Brief History of Everyone Who Ever Lived: The Human Story Retold Through Our Genes.
We move quickly through human history in this chat from 2017.