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Dental floss has a history that his maybe longer than of the toothbrush but we know for sure that it is in use since the 19th century. It is in the wider use since the invention of synthetic materials like nylon.
Since ancient times we had problems with crooked teeth. They interfered with proper eating and were not too aesthetically pleasing. Because of that, we tried to invent mechanical aids that helped us to fix them. Read about history of orthodontics.
Tooth decay is one of the oldest diseases of the human race. People had caries for millions of years and still have it today. With the appearance of sugar cane and other types of sugar prevalence of caries increased and until the modern treatments of caries, both preventive and remedial, lessen the number and seriousness of caries cases.
Dental instruments progressed for nearly 9000 years and from very primitive tools they developed into sophisticated equipment that can still seam scary but not so much when you compare them with tools people fixed their teeth some 100 years ago.