From a methylation perspective:
I think it's probably always existed, but was much rarer. The stressors of today's society are like a perfect storm for all methylation based diseases - there are about 30 of them including autism, cancer, Parkinson's and Alzheimers, and of course, ME.
I've gotten into doing geneology the past year or so and have looked at many death certificates from 100 years ago or more. You see stuff like strokes, heart attacks, tuberculosis, alcoholism, infectious diseases. You don't see any "modern diseases" like cancer, ME, etc., even "reading between the lines".
I can trace the MTHFR genes in my family because the same genes that cause strokes and heart attacks can also cause ME.
Great grandparents generation - strokes, heart attacks, infectious disease.
Grandparents - strokes, heart attacks, diabetes, infections.
Parents generation (the Greatest Generation) - cancer, cancer, cancer, cancer, cancer, diabetes, diabetes, depression, anxiety, ME, thyroid, Parkinson type syndrome.
My generation (Baby Boomers) - arthritis, cancer, diabetes, thyroid, adrenals, ME, ME, FM, ADD, high blood pressure, high blood pressure, depression, anxiety, bipolar, OCD, etc. etc.
Next generation - still young, but already a thyroid problem. Thankfully, no autism.
Stressors = everything mentioned by the posters above = non-nutritious food, food additives, "folic acid", cleaning products, paints, perfumes, personal care products, plastics, car exhaust, water pollution, air pollution, vaccines, mercury fillings, GMOs, hormones, antibiotics, pesticides, herbicides, etc. etc. etc. etc. etc.
I mean, what isn't poisoned in our society? Those with MTHFR mutations (us and our families), are truly the "Canaries in the Coalmine".