Michael H. answered • 06/18/19
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I've often wondered about this myself. I suspect it is a compromise between some advantage of having more than five fingers and a different advantage conferred by having fewer than five. There is evidence that having five digits is an old adaptation: many orders of mammals have five. The fossil record shows that horse ancestors used to have multiple hooves, but that must not have been an advantage to their form of locomotion because it gradually gave way to one.
Michael H.
update: This week's science news features a relevant article, quoted below: Extra fingers, often seen as useless, can offer major dexterity advantages An extra digit proves useful for texting, typing and eating, a case study shows By Laura Sanders 7:00am, June 12, 2019 An extra finger can be incredibly handy. Two people born with six fingers per hand can tie their shoes, adroitly manage phones and play a complicated video game — all with a single hand, a study shows. These people’s superior dexterity, described June 3 in Nature Communications, suggests that instead of being seen as aberrations that ought to be surgically removed, extra fingers can bring benefits. The results also highlight how flexible the human brain can be, a feature that will be central to the design of brain-controlled robotic appendages. For the study, bioengineer Etienne Burdet of Imperial College London and colleagues worked with a 52-year-old mother and her 17-year-old son, both born with six fingers on each hand. These extra fingers, positioned between the thumb and index finger, resemble thumbs in the versatile ways that they can move. Brain scans and anatomical MRI scans revealed that the extra fingers are controlled by a dedicated brain system, along with muscles and tendons. That means that these extra fingers aren’t just along for the ride, controlled by the muscles that move the other fingers, as some doctors had thought. SEEING SIX An extra finger on each hand, thought by some scientists to be useless, can allow people to single-handedly tie shoelaces, as well as to type and play video games in innovative ways. These people’s brains had no trouble directing their extra fingers, the results show. Extra robotic fingers or other appendages controlled by a person’s mind could bring similar increases in neural workloads, though the challenge would be greater for a person not born with the extra digits. Burdet says that these participants live in a world designed for people with five fingers, which can lead to interesting adaptations. Eating utensils are too simple for them, he says, “so they constantly change the posture on the utensils and use them in a different way.” After spending time with the participants, “I slowly felt impaired with my five-fingered hands,” he says. The results may not extend to other people with extra digits, Burdet says. In some cases, extra fingers may be less well developed. Citations C. Mehring et al. Augmented manipulation ability in humans with six-fingered hands. Nature Communications. Published online June 3, 2019. doi:10.1038/s41467-019-10306-w. Further Reading L. Sanders. Learning takes brain acrobatics. Science News. Vol. 192, September 16, 2017, p. 22. L. Sanders. The brain set free. Science News. Vol. 182, August 11, 2012, p. 18.07/09/19