Why Do Most Mammals Have 5 Fingers? Exploring the Homology of Living Tetrapods

Looking at the paws of a cat, dog, or even a kangaroo, one can observe that these animals have something in common with human hands. While some might be shrunken or positioned differently, all mammals possess five fingers. Why do we share this pattern with other vertebrates if we evolved under different conditions?

Animal Resemblance in Hand Structure

To find out why mammals have five fingers, it is essential to understand first why tetrapods or four-footed vertebrates possess five digits in each hand. Mammals belong to the superclass Tetrapoda, including other members like birds, amphibians, and reptiles. Even tetrapods with no traditional limbs also have fingers in their skeleton. For instance, sea lions, seals, and whales have five fingers in their flippers, even if they possess four or fewer toes.

There is some variation, however. Birds have a single fused finger bone at the end of their wing, while horses have only one toe. According to scientists, these animals start with as many as five fingers when they are still embryos, but these digits shrink away before they are born.

Experts are unsure when the five-finger plan first evolved. The first known species to develop fingers evolved from fish about 360 million years ago and possessed as many as eight fingers. Since the five-finger plan exists in most living tetrapods, it indicates that this trait is likely a homology, or a gene or structure shared between organisms due to a common ancestor.

While a common ancestor explains how mammals got five digits and passed this trait down to their descendants, it does not explain why. One theory that tries to explain this is canalization, a concept that a gene or trait becomes more stable and less likely to mutate over time.

However, the canalization idea was not supported by all researchers. Evolutionary developmental geneticist Kimberly Cooper from the University of California in San Diego argues that having more than five fingers (polydactyly) occurs due to mutation in many mammals, including humans. Many forms of mutations can cause polydactyly, but a recent study discovered that it can happen through the mutation of just a single nucleotide.

Role of Hox Genes in Finger Evolution

According to evolutionary biologist Thomas Stewart from Pennsylvania State University, Hox genes largely dictate this process. These are genetic materials responsible for encoding proteins that help regulate the activity of other genes. Turning them on or off helps ensure that specific body parts are appropriately located in an animal's body as it develops from an embryo.

As such, Hox genes influence the skeletal pattern of tetrapods. They do so by helping control proteins created by the sonic hedgehog gene to activate and block each other while forming tissues.

Through this process, the finger buds grow either continuously or by reabsorbing, depending on the animal. Then, the cells around the supposed location of the fingers die, forming separate digits. Exactly how this happens is still a complicated problem for scientists.