Teeth grew from the scales of primitive shark-like fish, new evidence has shown.
Old lineage cartilaginous fish such as sharks, skates and rays, lack bony skeletons and have skin containing small spiky scales called "dermal denticles".
Their tooth-like appearance is no accident, scientists believe.
Long ago during the early evolution of jawed vertebrates, evolution transferred dermal denticles from the skins of primitive fish to the mouth.
In the millennia that followed, the tiny appendages went on to produce the flesh-tearing six-inch long teeth of king dinosaur Tyrannosaurus rex and the formidable fangs of the sabre-toothed cat.
Our teeth could represent a direct link between us and our distant fishy ancestors, the research suggests.
Lead scientist Dr Andrews Gillis, from the Department of Zoology at Cambridge University, said: "Stroke a shark and you'll find it feels rougher than other fish, as shark skin is covered entirely in dermal denticles. There's evidence that shark skin was actually used as sandpaper as early as the Bronze Age.
"By labelling the different types of cells in the embryos of skate, we were able to trace their fates. We show that ... the denticle scales of sharks and skate develop from neural crest cells, just like teeth.
"Neural crest cells are central to the process of tooth development in mammals. Our findings suggest a deep evolutionary relationship between these primitive fish scales and the teeth of vertebrates."
The fact that teeth and sharks' denticle scales both arise from the same kind of embryonic cell suggests a common evolutionary origin, the team reported in the journal Proceedings of the National Academy of Sciences.
The skins of sharks are all that remains of armour plating that clad their jawless forbears 400 million years ago to protect against predators such as sea scorpions.