Your teeth play vital roles in your daily life, despite often being overlooked. They help you articulate words, chew food for better digestion, and greet others without speaking. These unsung heroes typically receive attention in front of the bathroom mirror, post meals, and before bedtime. (Here are some mistakes everyone makes when brushing their teeth.) Moreover, your teeth can indicate surprising aspects about your health, signaling potential undiagnosed diseases or the need for a doctor’s visit.
Have you ever wondered how humans developed teeth initially? According to a 2009 article in the Journal of Anatomy, evidence suggests that humans and rodents share a common mammalian ancestor with teeth similar to modern humans’. But where exactly did these resilient yet small tools originate, and how did they end up in the mouth? This question has long been debated among scientists, with two prevailing theories on teeth’s origins being polar opposites.
Two tooth theories
Some experts propose that skin scales gradually evolved into teeth over prehistory (outside-in), while others argue that teeth developed inside the body, independent of external traits (inside-out).
A 2003 paper published in Evolution & Development provides insight into the inside-out theory. Fossil analysis led experts to hypothesize that teeth may have originated from one of the three embryonic layers (the endoderm), independently of jaw evolution. Proponents suggest that teeth initially formed in the pharynx of jawless vertebrates, later migrating inside the mouth.
Conversely, the outside-in camp, also analyzing prehistoric data, reached a different conclusion. They suggest that the armor-like scales of fossil fish—coverings external to the body—may have eventually “migrated into the mouth cavity,” as noted in a 2009 study in the International Journal of Biological Sciences.
How ancient fish may help us understand the possible origins of teeth
Two recent studies supporting the “outside-in” theory involve detailed examinations of some bizarre-looking fish.
Authors of a 2015 study in Biology Letters examined fossils of Romundina, an extinct scaly fish over 400 million years old, identifying it as the earliest-known teeth manifestation in the fossil record. Using X-rays and computer modeling, researchers formed a theory on how teeth developed in this armored Devonian-era swimmer, suggesting teeth evolved to enhance hunting efficiency.
Meanwhile, a 2022 study in the Journal of Anatomy focused on Ischyrhiza mira, a sawfish ancestor from the age of the last dinosaurs. It had sawtooth-like features akin to modern sawfish’s elongated snouts (rostral denticles), used for both food gathering and defense. Although not identical to actual teeth, they shared characteristics indicating a surprising link to teeth.
How skin scales became bone: a possible explanation
The 2022 study’s authors didn’t initially aim to investigate teeth origins, but their findings compelled them to explore further.
Examining Ischyrhiza’s rostral denticles under an electron microscope, they discovered that the internal crystalline structure resembled shark teeth more closely than expected. This differed from the dermal denticles (scales) seen on the sandpaper-like bodies of rays, sharks, or sawfish.
Constructed to endure stress like biting or chewing, Ischyrhiza mira’s rostral denticles offered support for the theory that these pseudo-proto-teeth migrated to the mouth cavity, evolving into an optimized system for breaking down food over time.
The tooth of the matter
Despite extensive data supporting both tooth evolution theories, we haven’t determined which is more accurate. The origin of teeth remains one of many masticulation-related mysteries.
For example, despite their critical role in survival, teeth often grow uncomfortably (e.g., crooked teeth, overbites, underbites, wisdom teeth needing removal). One might expect evolution to optimize such vital features over time.
Moreover, humans, like many mammals, replace primary teeth with a permanent set only once in their lifetimes, a pattern established over 200 million years ago. In contrast, reptiles, amphibians, and fish are polyphodonts, continually replacing teeth. This drives humans to maintain teeth’s strength and aesthetics, even though teeth, unlike bones, are the body’s hardest parts. (Here are some foods to eat—and to avoid—to whiten your teeth.)
Strangely enough, some vertebrates don’t have teeth
It’s hard to imagine life without teeth, yet a surprising number of vertebrates lack them.
Some mammals (e.g., aardvarks, armadillos, sloths) have teeth without enamel, allowing continuous growth and regrowth. Pangolins are toothless, relying on sticky tongues for sustenance. Among reptiles, turtles have been toothless for millions of years, utilizing strong beaks and jaws for feeding.
Birds, evolutionary heirs of ferocious dinosaurs, are completely toothless. Their beaks and digestive tracts facilitate digestion. According to a 2014 study in Science, birds likely lost teeth around the same time they developed beaks, a process that began over a hundred million years ago.
While we can’t trace teeth origins with certainty, examining dental evolution (or lack thereof) in other animals may eventually reveal the truth.
