What is dumbing us down in recent years is the change in death rates of the young. In earlier times many children did not grow up to reproduce. Death might result from defective genes, of many sorts. Heredity illnesses might be one cause, some lack of intelligence might lead to some stupid accident, or one might be unable to resist some form of illness.
My mother in law prepared family lists going back to the early 1800s. One family in mid 1800s had 9 children, but either 4 or 5 of them died around age 2 in August or September. They may have died because that was the time that family stopped nursing their babies and they began to eat contaminated food and developed dysentery which killed them. Were some of the children weaker physically and unable to survive contamination of food, or were some smarter and kept themselves cleaner or avoided food that did not smell right. At any rate, in those ages the weaker and the more stupid were more likely to die before reaching breeding age. Perhaps a smarter set of parents would have figured out that the change of diet was causing the problem and changed the weaning age, or kinds of foods allowed. Certainly some of the Jewish food rituals or taboos had some survival value before people knew about germs.
https://biblehub.com/leviticus/11-7.htm
The real reason:
Trichinella species can infect swine, wild omnivores (foxes, wolves, bears, skunk, raccoons, rats, and other small mammals), and humans.
Trichinella is the smallest human nematode parasite, yet it is also the largest of all intracellular parasites.
Oral ingestion of cyst- or larvae-contaminated tissue is the usual route of infection, but congenital and mammary transmission can occur in rats.
https://en.wikipedia.org/wiki/Trichinella