During the interview he mentioned that the guy had written a book about things getting better for humanity. That we've been getting less violent, from our early known history till now.

Steven Pinker? It sounds like there are two books it could be -- "The Better Angels of Our Nature" and his follow-up, "Enlightenment Now":

https://en.wikipedia.org/wiki/The_Better_Angels_of_Our_Nature
https://www.nytimes.com/2018/03/02/books/review/steven-pinker-enlightenment-now.html