How objective are books like "Sapiens: A Brief History of Humankind"? How are they "proven"?
How can one assess the objectivity of historical literature?
Why is it still not subjective, if not even personally biased?
Start with whether or not the book gets the basic facts correct. Not the most obvious ones, but the more subtler ones. If those are correct, then one can start to presume a basic competence, and understanding, by the author.
For this book, a pdf file exists online. I started reading, and before finishing the first page I found two glaring errors:
Secondly, the muscles atrophied. Like a government diverting money from defence to education, humans diverted energy from biceps to neurons. It's hardly a foregone conclusion that this is a good strategy for survival on the savannah. A chimpanzee can't win an argument with a Home Sapiens, but the ape can rip the man apart like a rag doll.
The statements "the muscles atrophied" and "humans diverted energy from biceps to neurons" are both incorrect. It is correct that chimpanzees "can rip the man apart like a rag doll", which hides the error as to why - which has nothing to do with humankind's greater brain size. The real reason is that our hand and lower arm joints are constructed differently from our ape relatives. Ape joints are constructed to maximize leverage, to obtain the benefits of raw power. Human joints are constructed with less leverage, to the benefit of much finer motor control for tasks such as tool building. Additionally, humans have evolved a greater percentage of slow-twitch muscle compared to fast twitch muscle, for increased endurance at the cost of raw strength.
This leads into a further error on the subsequent page, where the author infers that humans evolved two hand from ape-like relatives with four feet. In fact, the opposite occurred. The proper sequence of evolution is that monkees evolved four hands from four feet, which trait is still possessed by our ape relatives today.
Our ape relatives are almost as nimble with their "feet" as with their hands. Humans evolved two feet from the rearmost two hands in order to obtain the benefits of standing upright.
When one starts to see such errors of fact pile up in a book, one can dismiss any claim to serious scholarship on the part of the author. No author can expect to be perfect, but the reader has the right to expect an honest and earnest striving for perfection.