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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.
Except with dentures, missing teeth can be replaced with dental implants. People made and used dental implants since some 4000 years ago, using bamboo pegs and seashells instead of missing teeth.
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.
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.