A vestigial structure is an organ or physical attribute that has lost some, most, or all of its original or ancestral function but yet is still retained by members of a given species. An example of a vestigial structure in humans is the appendix (at least, to the best of our knowledge). Suggesting that the little toe is a vestigial structure implies that the human foot has somehow changed over the last many centuries or millennia and that the fifth toe no longer serves a useful role or function. The big problem with this argument is that nothing at all has changed about the structure and function of the foot itself (including the little toe) and that the pinky toe is still very much an important part of the foot.
Indeed, our foot anatomy, including that of the little toe, remains unchanged. The little toe still has extensor and flexor tendons, a bone structure that’s built for weight-bearing activities (i.e., the bones of the fifth toe are themselves shaped like arches, which are meant to bear weight), and the ability to participate in balance and propulsion. The pinky toe is still an important player in walking, running, and other dynamic movements, and it still provides valuable feedback about the condition of the terrain through sensory feedback. The little toe also helps disperse impact forces during gait, which is another important and necessary function that it continues to serve.
It’s true that the appearance of many of our feet has changed, including the alignment of our fifth toes, but that’s because the footwear we tend to wear in industrialized societies binds and squeezes our feet in unnatural ways, causing passive toe deformities such as bunions, bunionettes, and various types of crooked toes. Because the big and little toes are subjected to the most pressure when squeezed into narrow toe box footwear, their shape and orientation is changed the most over time, resulting in toes that deviate inward, toward the foot's mid-line, and lose their ability to function optimally. Without the deforming effects of conventional footwear, our feet would look identical to those of our ancestors, who enjoyed strong, healthy arches and significant natural toe splay—each individual toe in line with its corresponding metatarsal bone.
This ancestral foot shape can still be observed in newborn babies and infants and in unshod (or minimally shod) adults around the globe, including throughout South and Central America, Africa, Asia, the Middle East, and anywhere else that people habitually go barefoot or wear only minimalist footwear. Many of us in industrialized, shoe-wearing nations can restore this ancestral foot shape and function, at least to some degree, by adopting natural approaches that include using Correct Toes toe spacers (to realign the toes in their proper anatomical position), choosing men’s or women’s footwear that’s flat and flexible in the sole and widest at the ends of the toes, and by performing helpful stretching and strengthening exercises to restore proper tone in the toe flexor, extensor, abductor, and adductor muscles and tendons.
Even though much can be done to rehabilitate the toes using natural methods, the belief that the little toe serves no useful purpose (and is, in fact, an impediment to be dealt with) persists. An extreme example of this is how some people undergo elective surgery to remove the fifth toe. Most folks who opt for this procedure do so to achieve a more comfortable fit for their foot within conventional footwear, especially high heels and other fashion footwear. Some even elect to have their fifth metatarsal bone shaved down so that they can slide their foot into even narrower shoes. This surgical alteration is, surprisingly, not uncommon. It may be medically necessary in some rare cases, but it is, by and large, a cosmetic procedure driven by the desire to slim the foot, and it is based on the false assumption that the little toe is dispensable.
So, for the pinky toe to be a vestigial structure, we’d need skeletal evidence that its structure or function has changed over time. Absent that, and with the knowledge that the little toe is still very much involved in the functioning of the foot (particularly when it is properly aligned), it’s clear that the fifth toe is anything but vestigial. A healthy, functioning little toe that possesses the ability to splay and grip surfaces is integral to the overall health of the foot and a useful structure for the rest of the musculoskeletal system.