Nothing strange about hunter gatherers evolve eating a lot of fiber. Think about the other extreme, what if you took in all your energy and nutrients through fluids. How well would your bowels work with nothing in them but liquids? I know my bowel movements are much easier after I've eaten some salad than after a low fiber meal.
There's plenty of articles about fiber, here's just a short list I just found.
I'm talking about whether or not they are essential. I eat very little plants and don't have any problems. I also practice fasting from time to time, and I don't have any problems with no food of any kind for days at a time. Go figure.
I've never heard of anyone dying from lack of fiber. Have you? Is there a medical condition for this?
I'm sure that some hunter gatherers ate plants containing fiber. We are omnivores and able to get calories from many different sources depending on availability in our environment. That still doesn't mean that fiber is a necessity or requirement.
"This article reviews the physiology of ingestion of fiber and defecation. It also looks into the impact of dietary fiber on various colorectal diseases. A strong case cannot be made for a protective effect of dietary fiber against colorectal polyp or cancer. Neither has fiber been found to be useful in chronic constipation and irritable bowel syndrome. It is also not useful in the treatment of perianal conditions. The fiber deficit - diverticulosis theory should also be challenged. The authors urge clinicians to keep an open mind about fiber. One must be aware of the truths and myths about fiber before recommending it.
... a strong recommendation cannot be made for a protective effect of dietary fiber against colorectal polyp or cancer. Despite a lack of evidence however, current recommendations are still to increase dietary fiber. In the latest position statement of the American Dietetic Association[3], increasing dietary fiber is still promoted to protect against colon cancer despite stating that there is no proof of efficacy in this regard
...
Whilst it is not the intention of the authors to totally discourage fiber in the diet and the use of fiber supplements, there does not seem to be much use for fiber in colorectal diseases. We, however, want to emphasize that what we have all been made to believe about fiber needs a second look. We often choose to believe a lie, as a lie repeated often enough by enough people becomes accepted as the truth. We urge clinicians to keep an open mind. While there are some benefits of a diet high in natural fiber, one must know the exact indications before recommending such a diet. Myths about fiber must be debunked and truth installed."
Nope. Lack of fiber is not a cause of colon cancer.
A more objective view of the science is:
“...the interactions between meat, gut and health outcomes such as CRC [colorectal cancer] are very complex and are not clearly pointing in one direction... Epidemiological and mechanistic data on associations between red and processed meat intake and CRC are inconsistent and underlying mechanisms are unclear... Better biomarkers of meat intake and of cancer occurrence and updated food composition databases are required for future studies.”
There's plenty of articles about fiber, here's just a short list I just found.
https://www.pharmacytimes.com/news/how-fiber-helps-prevent-c...
https://www.health.harvard.edu/newsletter_article/Dietary_fi...
https://www.bmj.com/content/343/bmj.d6617
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4588743/