Three-rooted teeth range from common to genuinely rare depending on which tooth you’re talking about and what your ancestry is. For the tooth most associated with this trait, the lower first molar, a global meta-analysis estimated that about 10% of the world population has a three-rooted version.1PubMed Central. Three-Rooted Permanent Mandibular First Molars: A Meta-Analysis of Prevalence But that global figure hides enormous variation: in some East Asian and Indigenous American populations the rate exceeds 40%, while in people of European or African descent it dips below about 3.5%.2PubMed Central. Rare dental trait provides morphological evidence of archaic introgression in Asian fossil record Other teeth, like premolars, almost never sprout a third root, making those cases genuinely unusual finds.
Which Teeth Are Supposed to Have Three Roots
Upper molars normally have three roots: two on the cheek side and one on the palate side. This is the standard anatomy dentists expect, and it’s true across virtually all human populations. So if your dentist mentions a three-rooted tooth and you have an upper molar, that’s completely normal and not what anyone means when they talk about an “extra” root.
The conversation about three-rooted teeth being rare almost always centers on the lower first molar. Lower molars are supposed to have two roots, one toward the front of the mouth and one toward the back. When a third root shows up, it typically appears on the tongue side of the tooth, behind and to the inside. Dental literature calls this extra root a “radix entomolaris” when it sits on the tongue side toward the back or a “radix paramolaris” when it sits on the tongue side toward the front. The entomolaris variant is far more common of the two. In an Iranian study, the paramolaris version turned up in only about 1.2% of lower first molars.3PubMed Central. Prevalence and Distribution of Radix Paramolaris in the Mandibular First and Second Molars of an Iranian Population A micro-CT study of three-rooted lower first molars found the extra root was located on the tongue side toward the back in the majority of specimens.4PubMed. The radix entomolaris and paramolaris: a micro-computed tomographic study of 3-rooted mandibular first molars
Ancestry Is the Biggest Factor
Population ancestry is, by a wide margin, the single strongest predictor of whether your lower molars have three roots. This trait is sometimes called the “three-rooted lower molar” in dental anthropology, and researchers have used it for decades as a marker to trace population movements and relationships.
A review of both clinical and bioarchaeological evidence found that in non-Asian populations the three-rooted lower molar occurs in fewer than 3.5% of people, while in populations with East Asian or Indigenous American ancestry the rate can exceed 40%.2PubMed Central. Rare dental trait provides morphological evidence of archaic introgression in Asian fossil record Chinese populations consistently show some of the highest rates. One study using cone-beam CT imaging in a sample that included 1,444 lower first molars found that a quarter of them were three-rooted.5Scientific Reports. Detection of three-rooted mandibular first molars on panoramic radiographs using deep learning Meanwhile, an Indian study found the rate was about 6% of patients and roughly 4.5% of teeth.6PubMed. Prevalence of three-rooted mandibular permanent first molars among the Indian Population In a Saudi Arabian sample, the overall rate was about 3% for lower first molars.7The Saudi Dental Journal. Prevalence of three-rooted mandibular permanent first and second molars in the Saudi population
These aren’t small differences. The gap between a rate below 4% and a rate above 40% is tenfold, and it tracks cleanly with continental ancestry. Populations with roots in sub-Saharan Africa and Europe sit at the low end; East Asian, Southeast Asian, and Indigenous American populations sit at the high end; and South Asian and Middle Eastern populations fall somewhere in between.
A Trait Inherited From Denisovans
One of the more surprising discoveries about the three-rooted lower molar is where it may have come from. In 2019, researchers examining a hemimandible (half of a lower jaw) from a Denisovan individual found in Xiahe, China, noticed that it had a three-rooted lower second molar.2PubMed Central. Rare dental trait provides morphological evidence of archaic introgression in Asian fossil record Denisovans were archaic relatives of modern humans who interbred with the ancestors of present-day East Asian and Oceanian populations. The researchers argued that the high frequency of three-rooted lower molars in Asian-derived groups, combined with its rarity outside those populations, is consistent with the trait being introduced through interbreeding with Denisovans rather than arising independently in modern humans.
This isn’t the only physical trait linked to Denisovan ancestry, but it’s one of the most concrete morphological examples. Most Denisovan-derived traits identified so far are genomic, involving immune genes or high-altitude adaptations. Finding a visible, structural trait like an extra tooth root tied to archaic interbreeding gave anthropologists something tangible to work with in the fossil record, where DNA often doesn’t survive but teeth do.
What Controls Root Number During Development
Tooth roots form after the crown is already largely shaped, and the developmental machinery that determines how many roots a tooth grows is partly distinct from what controls the crown. A key player is a structure called Hertwig’s epithelial root sheath, a collar of tissue that wraps around the developing root area and guides root elongation and branching. Several signaling molecules expressed in and around this sheath help determine whether a root splits into two or three separate prongs.8Development. Cellular and molecular mechanisms of tooth root development
Genetic studies in animals have confirmed that root and crown development are controlled by overlapping but distinct sets of genes. One gene in particular, Wnt10a, has been linked to the formation of the furcation, which is the branching point where a single root trunk divides into separate roots.9PubMed Central. Signaling Pathways Critical for Tooth Root Formation Mutations in these pathways can lead to teeth with fewer roots than expected, or in some cases more. The fact that root-controlling genetics are partially separate from crown genetics explains why you can have a perfectly normal-looking crown sitting on top of an unusual number of roots, something that only shows up on an X-ray.
If One Side Has It, the Other Probably Does Too
An extra root doesn’t just pop up randomly on one tooth. If your lower first molar on the right side has three roots, there’s a strong chance the one on the left does too. The meta-analysis of three-rooted lower first molars found that more than 45% of people with the trait had it on both sides.1PubMed Central. Three-Rooted Permanent Mandibular First Molars: A Meta-Analysis of Prevalence A Saudi Arabian study using cone-beam CT reported 100% symmetry in the number of roots between right and left lower first molars in the same individual.10The Saudi Dental Journal. Anatomical variations and bilateral symmetry of roots and root canal system of mandibular first permanent molars in Saudi Arabian population utilizing cone-beam computed tomography
This high bilateral symmetry makes sense genetically. The developmental program that determines root number operates on both sides of the jaw, and whatever genetic variant causes the third root is present in every cell, not just on one side. The practical takeaway is useful: if your dentist finds a third root on one lower molar, they should check the matching molar on the opposite side before performing any procedure there. Assuming the standard two-root anatomy on the other side would be a mistake.
Do Men and Women Differ
Some studies have found a difference between the sexes, though the pattern isn’t consistent across populations. In a Saudi Arabian sample, women had three-rooted lower first molars at a higher rate than men, about 5.7% versus 3%, a difference that was statistically meaningful.7The Saudi Dental Journal. Prevalence of three-rooted mandibular permanent first and second molars in the Saudi population Interestingly, the pattern reversed for lower second molars in the same study, where men had the higher rate. Other population studies have not consistently replicated a sex difference in either direction, so the evidence here is thin enough that you shouldn’t read too much into it. The takeaway is that population ancestry matters far more than sex in predicting this trait.
Why Extra Roots Matter for Root Canals
An extra root isn’t a health problem on its own. People with three-rooted lower molars chew, speak, and live just fine without ever knowing. The issue arises when that tooth needs treatment, particularly a root canal. Each root typically houses at least one canal, and the whole point of a root canal is to clean and fill every canal in the tooth. If the dentist doesn’t know a third root exists, they’re going to miss its canal entirely.
That matters more than you might think. A cross-sectional study found that teeth with missed canals had periapical problems, essentially infections at the root tip, in roughly 83% of cases. Maxillary molars with a missed canal were about three times more likely to develop these infections than molars where all canals were found and treated.11PubMed. The Influence of Missed Canals on the Prevalence of Periapical Lesions in Endodontically Treated Teeth: A Cross-sectional Study A separate study looking at early treatment failures confirmed that untreated additional canals were a significant predictor of the root canal failing within the first few years.12PubMed Central. Predicting early endodontic treatment failure following primary root canal treatment
The third root on a lower molar often curves or tilts inward toward the tongue, which makes cleaning its canal trickier than the main two. If the practitioner doesn’t account for the root’s curvature, there’s a risk of perforating the root wall or breaking an instrument inside the canal. These aren’t just theoretical concerns; endodontic case reports routinely flag the lingual curvature of the extra root as a source of procedural complications.
How Dentists Spot Extra Roots
A standard two-dimensional X-ray (periapical or panoramic radiograph) can reveal a third root, but it doesn’t always. Because the extra root tends to sit behind or overlap with the other roots in a flat image, it can be hidden. One study that used deep learning to detect three-rooted lower first molars on panoramic radiographs relied on cone-beam CT as the gold standard for confirming whether a third root was really there.5Scientific Reports. Detection of three-rooted mandibular first molars on panoramic radiographs using deep learning
Cone-beam CT (CBCT) has become the go-to tool when unusual anatomy is suspected. It produces a three-dimensional image of the tooth and surrounding bone, making extra roots visible from any angle. Research has shown that CBCT measurements of root length and trunk dimensions closely match actual physical measurements.13Journal of Hard Tissue Biology. Analysis of the Length and Types of Root Trunk and Length of Root in Human First and Second Molars and to the Actual Measurements with the 3D CBCT The practical benefit is that a dentist planning a root canal on a tooth with an unexpected third root can see exactly where the extra canal is, how it curves, and how long it runs before ever picking up an instrument. Not every patient needs a CBCT for routine dental work, but when a standard X-ray hints at an extra root or when a root canal fails unexpectedly, the three-dimensional scan often reveals the problem.
Three-Rooted Premolars and Other Oddities
While three-rooted lower molars are the most discussed, extra roots can show up in other teeth as well, though far less frequently. Upper first premolars normally have one or two roots, but in one study of an Emirati population, about 1.9% had three roots.14The Open Dentistry Journal. Evaluation of Variations in Root Canal Anatomy and Morphology of Permanent Maxillary Premolars among the Emirate Population using CBCT Lower premolars normally have a single root, and finding three roots in one is rare enough to warrant a published case report. One such report described a patient with bilateral three-rooted lower premolars alongside four-rooted lower molars, calling it a rare anatomical variant.15PubMed Central. Bilateral Three Rooted Mandibular Premolars and Four Rooted Mandibular First and Second Molar: A Rare Anatomical Variant
These unusual configurations in premolars are genuinely rare in a way that three-rooted lower molars are not. A three-rooted lower molar in a Chinese patient is statistically ordinary. A three-rooted lower premolar in anyone is a case report. The clinical implications are similar, however: unexpected roots mean unexpected canals, and unexpected canals mean potential complications during treatment if they go undetected.
Humans Versus Other Primates
The variation in root number that we see in human teeth is unusually high compared to our closest living relatives. A study of over 400 chimpanzee specimens from a single population found that root number was remarkably consistent: more than 97% of specimens had the same number of roots for a given tooth position, with only a few teeth showing any variability at all.16PubMed. Variability in premolar and molar root number in a modern population of Pan troglodytes verus Humans, by contrast, show substantial variation in root number across multiple tooth types, and the difference between human and ape root patterns turns out to be significant across the board.
This comparison tells us something about how root anatomy evolved in our lineage. The researchers suggested that the evolutionary changes in premolar root number between humans and apes didn’t happen all at once but in a mosaic pattern, with different teeth evolving their root configurations at different times. Combined with the Denisovan interbreeding evidence, it paints a picture of root anatomy as a trait that has been actively shaped by both natural selection and gene flow from archaic human species, not just a random quirk of individual development.
When to Actually Worry About It
If you’ve been told you have a three-rooted lower molar, you don’t have a dental problem. You have a dental variation, and it’s one shared by roughly a billion people worldwide given the global prevalence data. The tooth functions normally, supports normal biting forces, and doesn’t increase your risk of cavities or gum disease on its own. The situation where it becomes relevant is procedural: root canals, extractions, and periodontal surgery all require the dentist to understand the root anatomy they’re working with.
For extractions, an extra root means one more point of anchorage in the bone, which can make the tooth harder to remove. Sectioning the tooth into pieces before extraction is more common when extra roots are present. For periodontal work, the furcation area where roots branch apart is a known trouble spot for gum disease in any multirooted tooth, and an additional root creates one more furcation zone to manage. If you know you have the trait and ever need one of these procedures, mentioning it to your dentist is worthwhile. They can order a CBCT scan ahead of time rather than discovering the third root mid-procedure. Given the high bilateral symmetry, volunteering the information about one side alerts the clinician to check the other side too.