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Why are many rows of teeth formed in sharks and not in humans

We have received the following email from one of our readers:

Dear Professor Spuriousmonkey,

I have just been wondering why many rows of teeth are formed in sharks and not in humans. Because I am only a simple dentist, I think it would be quite useful to have a similar tooth-forming system in human beings. To my mind, because you are smart and a professor, you are the right person to give an introductory lecture about the subject.

A simple dentist

Dear simple dentist,

I sympathize with your choice of profession, not so much with you, but your patients. Of course we are quite willing to enlighten even the dentist on the intricate facts and theories of nature. We are not sure why a dentist would want patients with row after row of very sharp serrated teeth? Surely this will increase the risk of an accidental amputation of dentist's extremities!

Why is it that the shark has many rows of teeth and the human not? It is obvious that both animals have predatory tendencies. Humans as the top predators on land would surely be worthy of the splendid display of teeth that is present in the top predator of the ocean! Did nature play a dirty trick on the human species?

Yes nature did so. The cause lies in the developmental process that gives rise to teeth. The developmental process of creating nipples cannot be shut down in males because this would also affect other developmental processes and structures, such as hair and teeth. No nipples might therefore also mean no hair or no teeth. The sexes are unfortunately linked in this developmental process. Men get nipples because women do not want to have children by bald men without any teeth.

The other developmental problem is that because the same developmental processes are used for teeth, hair and nipples, the evolution of extra teeth, also meant the appearance of extra hair and nipples.

Women with shark's teeth must have been frowned upon to some degree since they were not allowed to hunt because feminism was not invented yet. Even if some desperate men could get past the teeth, they would be met with row after row of mammary gland and waves of extra hair. This certainly must have caused confusion and hindered reproductive success. Since the developmental process of making extra teeth is sex-linked this meant that men also couldn't evolve a shark's bite.

Why could sharks evolve so many teeth then? That is simple to answer: they have no nipples or hair.

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Professor at the UOL

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