The honing canine disappeared in our ancestors because they stopped needing large, dagger-like teeth for fighting rivals and processing food. The earliest known hominins, dating back roughly 6 to 7 million years, already show reduced, blunter canines, making this one of the very first traits that separated our lineage from other apes. The shift was likely driven by changes in social behavior, mating strategies, and eventually the use of tools.
How the Honing Canine Works in Other Primates
In nearly all non-human primates with large canines, the upper canine, lower canine, and a specialized lower premolar work together as a self-sharpening system called the honing complex. Every time the jaw closes, the back surface of the upper canine slides against the front surface of a single-cusped lower premolar. That premolar has a tall, centrally placed cusp and a broad sloping face covered in enamel that acts like a built-in whetstone, keeping the upper canine’s edge sharp from tip to base. At the same time, the lower canine scrapes along the outer face of the upper canine, honing itself as well.
This system requires a specific dental arrangement. The upper jaw has a gap, called a diastema, just in front of the upper canine so the long lower canine can slot into it when the mouth closes. The lower premolar is distinctly different in shape from the premolars behind it: elongated, single-cusped, and tilted to serve as a sharpening blade rather than a chewing surface. In chimpanzees and gorillas, this whole setup keeps the canines pointed and effective throughout life.
What Changed in Early Hominins
Modern human canines look almost nothing like the canines of a chimpanzee. They are short, shaped more like incisors, and lack a pointed tip. Because there is no elongated premolar to sharpen against and no diastema to accommodate a long lower canine, human canines wear down at their tips instead of being honed along their edges. The lower premolar that once served as a sharpening tool became two-cusped and flat, built for chewing rather than honing.
These changes did not happen recently. The oldest known hominins already show them in progress. Sahelanthropus tchadensis, a species from Chad dating to roughly 6 to 7 million years ago, had small, flat canine teeth that were unusual for a male primate. The Smithsonian Institution identifies small canines as one of only two defining human anatomical traits visible in this species (the other being evidence of upright walking). Orrorin tugenensis from Kenya, at about 6 million years old, and Ardipithecus kadabba, between 5.8 and 5.2 million years old, show similarly reduced canines. By the time of Ardipithecus ramidus around 4.4 million years ago, canine size differences between males and females were nearly as small as in modern humans.
The Weapon Hypothesis
The most widely discussed explanation centers on aggression and mating competition. In living primates, large canines function as weapons. Males use them in fights over mates and territory, and these confrontations can be lethal. Species where males compete intensely for females tend to have the largest, most sexually dimorphic canines: males’ canines are dramatically bigger than females’. A tall, sharp canine has been shown to significantly enhance a male’s reproductive success in these species.
If our ancestors shifted toward a social structure with less violent male competition, the selective pressure maintaining large canines would have weakened. Research on Ardipithecus ramidus supports this idea. Its canine sexual dimorphism was nearly human-like, suggesting males were not relying on large teeth to compete for mates. Some living primates offer a parallel: spider monkeys in the genus Brachyteles live in multi-male, multi-female groups where males are extremely tolerant of one another and compete only weakly for mating access. These monkeys have reduced canine dimorphism compared to closely related species with more aggressive males.
The pattern is not perfectly clean, though. Woolly monkeys (Lagothrix) also live in tolerant multi-male groups but still have large male canines and moderate body size differences between sexes. So reduced aggression alone may not fully explain canine shrinkage. Other factors likely contributed.
Tools as Replacement Weapons
One compelling theory ties canine reduction directly to bipedalism and tool use. In every early hominin specimen found so far, bipedal walking and reduced canines appear together. The idea is straightforward: once ancestors began walking upright, their hands were freed to carry and wield objects. A pointed stick used for defense or for digging up underground storage organs like tubers could replace the canine’s role as both a weapon and a food-processing tool.
If even a crude handheld tool was more effective than a tooth for stabbing or threatening a rival, individuals with slightly smaller canines would not have been at a disadvantage. Over many generations, the teeth could shrink without any cost to survival or reproduction. This would also explain the tight correlation between bipedalism and canine reduction in the fossil record: the two traits enabled each other. Upright posture made tool use practical, and tool use made large canines expendable.
Dietary Shifts and Jaw Mechanics
Changes in diet may have added further pressure. As hominins began eating tougher, more varied plant foods, especially underground tubers and roots, the jaw needed to generate strong, even grinding forces. Large, projecting canines interfere with the side-to-side chewing motion needed to process tough foods because they lock the jaw into a narrow range of movement. Smaller, flatter canines allowed the jaw to move more freely, improving chewing efficiency.
The lower premolar’s transformation reflects this shift clearly. In apes, the front lower premolar is tall, single-cusped, and tilted, optimized for sharpening the upper canine. In early hominins, this tooth gradually became shorter, broader, and two-cusped, taking on a shape better suited for crushing food. Studies of premolar crowns in early hominins show systematic variation in cusp number, cusp placement, and the size of the chewing basin across different species, with the chewing basin expanding significantly in some lineages, particularly in the robust australopithecines.
Why It Happened So Early
What surprises many people is the timing. Canine reduction is not a late development linked to big brains or stone tools. It predates the oldest known stone tools by at least 3 million years. It predates significant brain expansion by even longer. Small canines and bipedalism are the founding traits of the human lineage, present from the very beginning of the fossil record.
This timing suggests the social and ecological pressures driving canine reduction were among the very first forces shaping our ancestors after they diverged from the lineage leading to chimpanzees. Whatever combination of reduced male aggression, early tool use, and dietary change was responsible, it was already well underway by 6 to 7 million years ago. By the time hominins like Ardipithecus ramidus appeared around 4.4 million years ago, the honing complex was essentially gone: canines were small, the diastema had closed, and the lower premolar had lost its role as a sharpening blade. The self-sharpening tooth system that works so well for chimpanzees and gorillas had become irrelevant to a lineage finding new solutions to old problems.