Bad teeth like this are because our modern diet tends to be bad in certain nutrients, particularly vitamin d.

Dr Weston Price traveled around the world at the beginning of the 20th century and found that tribal people who were starting to switch from their usual high nutrient diets over to westernized diets suddenly began to show all the types of bad teeth problems like the ones in this picture.

The older members of those tribes who never ate western food had perfect teeth. A google search will bring up Dr. Price's book which is free online and gives the diet he suggests to prevent these kinds of problems.

Dr Gerald Judd also put a free book online on how to remineralize tooth damage by fixing your diet, especially by including enough phosphorus, vitamin c, calcium, vitamin d, vitamin k and avoiding refined sugars and refined grains.