The first documented case of down syndrome is about 1500 years ago. https://www.newscientist.com/article/dn25848-oldest-case-of-downs-syndrome-from-medieval-france/

As for arguing about the cause of birth defects, we do know some toxins can increase rates, but birth defects are documented going back to ancient history. There are writings in cuneiform, meaning the earliest civilizations to develop writing were already familiar with them. This would predate many of the toxins you most likely care about by millennia, and it would have been limited to a small subset of the population working the most toxic jobs, so a link probably would have been established somehow if they ONLY resulted from toxin exposure. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1746-1561.1969.tb07057.x

On allergies, I already commented in response to the other peanuts comment, but food allergies were described but poorly understood going all the way back to ancient Greece.

On our bodies always having been able to protect us, that's not quite correct. Our bodies have always been able to protect *enough of us to sustain the species.* Malaria has been documented for over 2000 years in ancient Greece and China. The black plague killed as much as fucking HALF of Europe in the 14th century. Disease has ravaged humanity as long as there has been humanity.

Our bodies make mistakes. They ARE fallible. They succumb to disease. They form incorrectly. They develop allergies to safe, mundane foods.