Why do humans grow two sets of teeth? These marsupials rewrite the story of dental development

Teeth for children and adults from Tamar Walabi. The scale bar is 1 cm. Credit: Nasrallah et al.

You only get 52 teeth in your life: 20, followed by 32 for adults.

This is not the case for all animals. Some, like rodents, never replace their teeth. Others, like sharks, keep replacing them again and again.

So why do we humans replace our teeth only once? and how it all age Replacement work?

These are tough questions, and we don’t have all the answers. but new Discovery About the strange tooth-replacement habits of Tamar Wallaby, a tiny Australian marsupial, may help shed some light on this dental puzzle.

Not everyone replaces teeth the same way

It has long been assumed that modern mammals replace their teeth in the same way. However, the progress in 3D scanning and modeling Revealed mammals with unusual tooth replacement, such as Tamar Walabi (Macropus Eugenius) and the fruit bats (Idolon Helfoam).

These mammals have given us important clues about how humans and other mammals evolved from their ancestors continuous Tooth replacement.

How does a person make teeth and replace them?

Human teeth begin to grow between the sixth and eighth weeks of fetal development, when a group of tissue within the gums called the primary dental lamina begins to thicken. Along this range, clusters of special stem cells appear at the sites of future teeth, known as ‘plaques’.






The plaques then begin to grow into teeth, going through the bud, cap, and bell stages along the way. It takes shape in its final form and hardens with layers of dentin and enamel. Eventually, it will burst through the gums. The incisors are the first to erupt, since 6 months of age, which is why they are calledteethingStage!

This generation of teeth, which grow from the primary dental lamina, is known as “primary teeth” or baby teeth.

Secondary teeth or adult teeth grow slightly differently. A branch of tissue called the successive lamina grows out of a child’s tooth, and this tissue develops the alternate tooth like an apple on a tree branch. Adult teeth begin to grow in before we are born, but it takes many years for the full set to form and eventually emerge.

Replacement occurs when the adult teeth are large enough that the deciduous teeth eventually push out and the permanent set of teeth remain for the rest of our lives. Usually the first molar erupts between 6 and 7 years of age, while our wisdom teeth are the last to erupt (between about 17 and 21 years of age).

Most mammals replace their teeth once during their lifetime, as do we. This is known as “bidental” (two sets of teeth).

Some groups of mammals, such as rodents, do not replace their teeth at all. These “monophiodonts” deal with the same set of teeth throughout their lives. There are also a few unusual mammals, such as the echidna, which do not grow any teeth at all!

Learning from Wallaby

Tamar Wallaby is also a bi-dental, replacing his teeth only once.

Why do humans grow two sets of teeth?  These marsupials rewrite the story of dental development

Tooth development in premolars in Tamar Wallaby in 2D and 3D, ‘P3’ late baby teeth appear 47 days after their siblings ‘dP2’ and ‘dP3’

Scientists have long assumed that they replaced their teeth in the same way that humans do, although historical observations dating back to 1893 noted unusual things about the evolution of these marsupial teeth. For starters, while we replace our incisors, canines and premolars, the wallaby tamar only replaces premolars.

Recently my colleagues in Monash University and the University of Melbourne And I noticed the teeth of Tamar Wallab from the fetus until the age of adulthood. We used a technique called diceWhich combines staining and a CT scan, and has found something surprising.

Instead of replacing the premolar teeth growing from the successive lamina, it was actually a delayed growth of the baby teeth from the primary dental lamina.

This means that Tamar Walabi It does not have any traditional alternative to teeth. This discovery opens a huge set of new questions. What exactly are these teeth?

One explanation for these late deciduous teeth could be a link to our ancestors from the constant replacement of teeth.

Your teeth formed millions of years ago

Unlike mammals, most other animals, including fish, sharks, amphibians, and reptiles, have their teeth replaced. multiple times (they are “polyphyodonts”). Mammals Lost This ability is about 205 million years old.

The reason we stop making teeth is because our dental plate degrades After making our second group, while still active in polyphyodonts.

Why do humans grow two sets of teeth?  These marsupials rewrite the story of dental development

In reptiles, the teeth are replaced by waves, or “Zahnreihen”. Each blue line shows one wave. Credit: Whitlock & Richman

Interestingly, in modern and fossil polydentalism, replacement teeth often develop in groups of alternating waves, known as ‘rows of teeth“.

And while dates only replace your premolars, these are delayed baby teeth It could represent the presence of choriocarcinoma that still occurs in modern mammals.

This gives us an idea of ​​how we evolved from ancestors with continuous tooth replacement: by modifying and reducing a system hundreds of millions of years old.

The research also found that fruit bats (Idolon Helfoam) make the substitution teeth In unusual ways, including developing it in front of, behind, next to the child’s tooth, or separated from it.

This is exciting because, along with dates, it shows that there may be a wealth of diversity of tooth replacement across mammals taking place under our noses or gums!


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