Our grandparents and great-grandparents who were raised in the late 1800's and early 1900's lived a primary agricultural existence with 80% of Americans living in a small family farm in rural areas. Before 1910 heart attacks were virtually unheard of. By 1930, heart attacks accounted for no more than 3000 deaths a year.
By 1960, that number had risen to over 500,000. Genetics cannot explain this huge increase over one or two generations. However 100 years ago the average person consumed 3,500 calories a day which is approximately the same as we eat now, but in the early 1900's most of the calories consumed were carbohydrates, and there was considerable less fat.
In addition in the 1900's folk did a lot of exercise and their work was more active and hobbies also made them exercise more. As our working habits have changed, so have calorie requirements. The average person now needs around 2000-2500 a day to keep at their present weight.