Animals do feel PTSD, see https://www.frontiersin.org/articles/10.3389/fnbeh.2012.0002... (an evolutionary advantage, to avoid things that an organism barely survived). There are rat models, in which there are similarities up to particular neurobiological changes. Anecdotally, quite a few friends who took a dog from a shelter, and it had PTSD (likely due to maltreatment by the previous owners, or abandonment) triggered by certain actions (leaving along, shouting, raising one's hand, the smell of alcohol, etc).
In this context, the presumption that humans a few hundreds, or thousands, years ago didn't experience PTSD is... strange.
In this context, the presumption that humans a few hundreds, or thousands, years ago didn't experience PTSD is... strange.