The teeth you use to eat may have been fish scales a long time ago!

Did you know that the emergence of teeth is one of the greatest advances in the history of vertebrate evolution?

Without teeth, it is hard to imagine how fragile life would become. As the hardest organ in the body of jawed vertebrates, teeth have been preserved in billions of years of evolution since their first appearance. So where do teeth come from?

Before answering this question, let us first re-understand teeth.

Part 1

Do you really know teeth?

What we usually call teeth are a kind of hard calcified tissue that grows on the jawbone. They mainly consist of three basic structures: the innermost is the pulp cavity , which contains the dental nerves and blood vessels, the middle is a layer of dentin , and the dentin is covered by a layer of white and bright enamel , which is extremely hard and plays a protective role for the teeth.

The structure of teeth

Image source: Sparks

According to this definition, teeth are an organ unique to vertebrates, but not all vertebrates have teeth . For example, modern birds do not have teeth.

In order to reduce weight and adapt to flight, birds' teeth have degenerated during the process of evolution.

In addition, some more primitive jawless vertebrates also do not have real teeth. For example, the teeth of lampreys, although they grow inside the mouth and look scary, are actually keratin teeth with the same composition as fingernails.

The keratinous teeth of lampreys are not true teeth

Image source: Reference [8]

Part 2

The world's first tooth

All modern vertebrates, from sharks to humans, have very neat and symmetrical teeth and the same pattern of tooth replacement, where new teeth grow from the inside of the jaw and move outward to replace old ones.

However, there are some differences between the teeth of bony fish (a group of land vertebrates that includes crocodiles, dinosaurs, pterosaurs, dogs, humans, etc.) and the teeth of cartilaginous fish, represented by sharks. For example, in all bony fish, teeth are always attached to the jawbone, while sharks have no bones. Their skeletons are all made of cartilage, so that the scales on their bodies and the teeth in their mouths are directly embedded in the skin. In addition, bony fish lose their teeth by dissolving the tooth base, while sharks simply replace their teeth by shedding the worn teeth through the skin.

These similarities and differences between bony and cartilaginous fish teeth raise many questions about the origin of teeth, such as: When did such an orderly tooth replacement system evolve? And which species has a more primitive tooth attachment state, sharks or bony fish?

Previously, researchers had focused on a group of arthrodires that lived between 430 million and 360 million years ago because they were the only known fish to have teeth before the origin of bony and cartilaginous fish.

A typical representative of arthropods is the marine overlord of the Devonian period - Dunkleosteus , which has a terrifying big mouth with huge and sharp plate-like teeth.

Dunkleosteus, the Overlord of the Devonian Ocean

Image source: Photo by Zhikun Gai at the Swedish National Museum of Natural History

However, it has been difficult for scientists to understand how they evolved into the teeth of modern vertebrates because the position of teeth and the way they were added are so different in arthropods compared to bony fish and sharks.

In fact, arthropods do not have real teeth. The teeth on their upper and lower jaws are actually extensions of the skull and are completely different from real teeth.

Scientists from Uppsala University in Sweden then turned their attention to the fossils of acanthothoracids, an early type of armored fish that was more primitive than the arthropods.

Scientists used powerful X-ray imaging technology from the European Synchrotron Radiation Facility (ESRF) to perform high-resolution 3D imaging of a 409-million-year-old spiny-breasted fish fossil discovered in the Czech Republic in the late 20th century.

A spiny-breasted fish hides in a giant nautilus shell in search of food in a tropical coral reef 409 million years ago

Image credit: Jan Sovak

The scan results were shocking. The fossil unexpectedly preserved a well-preserved tooth row, and even intact cell spaces could be seen within the dentin of these ancient teeth.

In terms of tooth growth patterns, the teeth of spiny-breasted fish were attached to the bone, with new teeth appearing inside the jaw and old teeth located at the edge of the jaw, which is strikingly similar to the tooth pattern of modern humans .

Replacement teeth inside the jaws of spiny-breasted fish

Image source: Reference [6]

This shows that in terms of the way teeth are attached, bony fish and their terrestrial vertebrate descendants have retained the characteristics of their ancestors, while the state of shark teeth attached to the skin is specialized. This further proves that cartilaginous fish represented by sharks are not the ancestors of humans, but a side branch in the evolutionary chain from fish to humans.

As almost the most primitive group of all jawed vertebrates, the tooth growth pattern of spiny fish is surprisingly similar to that of modern humans, which shows that when the first jawed vertebrate appeared, the human tooth growth pattern had already been established.

In September 2022, Nature magazine published a cover article on the 439-million-year-old jawed fossil reservoir discovered by the team of Academician Zhu Min from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, in the Silurian Rongxi Formation in Shiqian, Guizhou.

The research team selected 23 jawed tooth specimens from nearly 4 tons of fish microfossil samples collected from the wild. Through high-precision CT, three-dimensional restoration, and histological sections, it was found that these teeth belonged to the 439 million-year-old Diplodocus.

Diplodocerus

Image source: Simulstone Technology

This discovery pushed the earliest fossil record of teeth back 14 million years and is the "first tooth in the world" discovered so far.

Part 3

Two hypotheses about the origin of teeth

Now that we have figured out the approximate time when teeth appeared, how did teeth originate and how did they become what they are today?

Currently, there are two hypotheses in academia about the origin of teeth, namely the "inside-out" hypothesis and the "outside-in" hypothesis .

The “outside-in” and “inside-out” hypotheses of the origin of human teeth

Image credit: Gai Zhikun and Zhu Min

The "inside-out" hypothesis holds that teeth gradually moved from the pharynx into the mouth . This hypothesis is mainly based on an extinct group of conodonts, also called toothed animals .

Conodonts appeared in the early Cambrian period 500 million years ago and became extinct in the late Triassic period about 200 million years ago. These eel-like creatures have no internal or external skeletons and are almost all soft, except for the hard, tooth-like spines in their throats, which are considered to be the pharyngeal teeth of conodonts.

Conodonts and conodonts

Image credit: Gai Zhikun and Zhu Min

The material that makes up the conodonts looks very similar to the dentin and enamel of modern teeth and was once thought to be a homologous structure. Therefore, a long-standing hypothesis is that the conodonts in the pharynx of conodonts are the oldest teeth.

Comparison of tooth histology of euconodonts (left) and sarcopterygians (right)

Image credit: Gai Zhikun and Zhu Min

With the origin of the jaw, the conodont spines of these pharyngeal teeth expanded outward, attached to the jaw and eventually formed teeth. This is the "inside-out" hypothesis of the origin of teeth.

The "outside-in" hypothesis holds that the teeth of jawed mammals evolved from the scales of ancient fish , and that the scales of ancient fish entered the mouth and became teeth with the origin of jaws.

This hypothesis was mainly inspired by the scales on sharks, because the basic composition structure of shark teeth and scales is surprisingly similar!

Although shark scales look smooth, they feel rougher to the touch than those of other fishes. This is because the surface of shark skin is completely covered with dermal denticles, which are hard and sharp like teeth and are regularly arranged on the shark's skin.

The spotted cat shark and its shield scales. The shark's skin is covered with tooth-like small teeth arranged in the same direction, which helps to reduce the resistance of the shark when swimming.

Image source: Smithsonian

Shark scales have three basic structures similar to its teeth: pulp cavity, dentin and enamel (yes, very similar to the structure of human teeth). These two seemingly unrelated organs have highly similar basic organizational structures. It can even be said that scales are teeth that do not grow on the jaws, which undoubtedly suggests that teeth and scales are likely to develop in the same way.

Cross-section of shark skin (top) and basic structure of scales (bottom right) and shield scales under a scanning electron microscope (bottom left)

Image source: Reference [5]

Based on this, scientists speculate that part of the scales around the mouths of primitive fish may help them hunt. With the origin of the jaw, this part of the scales was retained in evolution and gradually moved to the mouth, attached to the jaw, and eventually evolved into the teeth of all jawed vertebrates. This is the "outside-in" hypothesis of the origin of teeth.

So which hypothesis is correct?

In 2013, Nature magazine reported a research result from Professor Philip Donoghue's team at the University of Bristol in the UK. They used the most advanced X-ray tomography microscope and synchrotron radiation technology at the time to reveal the internal structure and composition of conodonts.

The study found that although the late types of conodonts grew in a similar way to the teeth of modern animals, that is, they grew by covering the enamel on the dentin, the most primitive conodonts had no enamel-like layer at all.

This means that although the basic structure of late conodonts is completely indistinguishable from modern teeth, they are not the same thing at all, but the result of independent evolution .

With the denial of the homology between conodonts and jawed teeth, the "inside-out" hypothesis based on their homology has lost its basis.

Did our teeth really evolve from the scales of ancient fish?

Part 4

Fish scales or teeth?

Although the "inside-out" hypothesis has been questioned, scientists' studies of species such as zebrafish have shown that fish scales and teeth develop from distinctly different cell clusters in fish embryos, pouring cold water on the "outside-in" hypothesis that teeth evolved from fish scales .

To further clarify the relationship between teeth and fish scales, researchers from the University of Cambridge focused their research on sharks and rays because they represent a fairly ancient lineage - cartilaginous fish, which differentiated 400 million years ago.

Using fluorescent markers to track cell development in shark embryos, the researchers found that shark scales and teeth actually arise from the same type of cells, called neural crest cells.

This shows that the scales and teeth of these primitive fish do have the same embryonic origin and there is a deep homology between them.

So, if teeth really evolved gradually from scales, is there any fossil evidence of scales gradually transitioning into teeth?

Let's go back to the spiny-breasted fish mentioned above. In addition to discovering a dentition and tooth replacement pattern similar to that of modern jawed fish, scientists also found tooth-like tooth units near its jaws. Not only are these tooth units almost identical to teeth in shape, but the tooth units near the jaws gradually transitioned into teeth.

This suggests that teeth most likely evolved from dental units on the skin .

In fact, these tooth units are some tooth-like patterns or modified pointed scales on the edge of the mouth , which provides strong fossil evidence for the "outside-in" hypothesis of the origin of teeth.

Tooth units near the jaws of spiny-breasted fish

Image source: Reference [6]

Part.5

Conclusion

After reading the evolution of teeth, do you have such questions: Where did humans come from, and where will they go in the end? The starting point of evolution is always fascinating. If teeth originated from the scales of ancient fish, then where did the scales come from? Perhaps in the future, scientists will solve this mystery, and then we will have a better understanding of the question of "where did humans come from?"

References:

[1] García-López, S. and Sanz-López, J. 2002. Devonian to Lower Carboniferous conodont biostratigraphy of the Bernesga Valley section (Cantabrian Zone, NW Spain). In García-López S. and Bastida F. (eds.), Palaeozoic conodonts from northern Spain. Eight International Conodont Symposium held in Europe. Publications del Instituto Geológico y Minero de España (Serie: Cuadernos del Museo Geominero, 1), Madrid: 163-205.

[2] Qu Bing, Wang Jing, Pan Chao, Zhou Xiaohui, & Pan Lanlan. (2011). Research on the preparation of shark skin-like surface microstructure materials. Journal of Dalian Ocean University, 26(2), 3.

[3]Sparks, KJ 2013. Encyclopaedia Britannica. Encyclop©Œdia Britannica, Incorporated.

[4] Gai Zhikun, Zhu Min. 2018. The evolutionary history of jawless fish and the fossil record of China. Shanghai Science and Technology Press.

[5]De Iuliis G., Pulerà D.. 2007. The dissection of vertebrates. Academic press.

[6] Vaškaninová V, Chen D, Tafforeau P, et al. 2020. Marginal dentition and multiple dermal jawbones as the ancestral condition of jawed vertebrates. Science, 369(6500): 211-216.

[7] Qu Q, Haitina T, Zhu M, et al. 2015. New genomic and fossil data illuminate the origin of enamel. Nature, 526(7571): 108-111.

[8]Potter IC, Gill HS, Renaud CB, et al. 2015. The taxonomy,

phylogeny, and distribution of lampreys[M]//Docker M F. Lampreys: Biology, Conservation and Control: Volume 1. Dordrecht: Springer Netherlands: 35—73.

Produced by: Science Popularization China

Author: Shan Xianren Gai Zhikun (Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences)

Producer: China Science Expo

This article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)