The common structure of five fingers in mammals may stem from a shared ancestor, but experts have yet to determine the exact reason.
The paws of cats, dogs, and even kangaroos share a common trait with human hands: they all have five digits. Why does this similarity exist despite the fact that humans and these groups of animals evolved under different conditions?
Humans and many species of mammals have 5 digits on their forelimbs. (Photo: Sergey Ryumin).
To answer the question of why mammals (Mammalia) typically have five digits, we first need to understand why four-limbed vertebrates (Tetrapoda) have five digits. Mammals belong to Tetrapoda, a superclass that includes reptiles, amphibians, and birds. Even members of this superclass that do not have traditional limbs possess a skeleton with five digits, although they may only have four toes or fewer. For instance, whales, seals, and sea lions all have five digits in their front flippers.
Some “variations” like horses have only one toe, and birds have a fused finger bone at the tip of their wings. However, scientists have found that these creatures originally had five digits during the embryonic stage but regressed before birth.
This process is primarily driven by Hox genes, according to Thomas Stewart, an evolutionary biologist in Pennsylvania. Hox genes encode proteins that regulate the activity of other genes, turning them “on” or “off.” The Hox genes ensure that body parts are positioned correctly in the animal’s body as it develops from an embryo. Thus, they play a role in determining the bone structure of Tetrapoda.
Through this process, finger buds develop; depending on the species, these buds may either continue to grow or be reabsorbed. Subsequently, the cells surrounding the area where the fingers will appear will die, creating distinct digits.
The forelimb of a raccoon has 5 digits. (Photo: Riverotterecology).
Experts are still unclear about exactly when the structure of five digits first appeared. Stewart notes that the first group of animals to develop digits evolved from fish approximately 360 million years ago and had up to eight digits. However, the presence of the five-digit structure in most Tetrapoda species today suggests that this may be a “homology” — a gene or structure present in many organisms due to a common ancestor. Somehow, the common ancestor of all living Tetrapoda evolved to have five digits and passed this trait on to its descendants.
The common ancestor helps explain how mammals possess five digits, but it does not pinpoint the cause. One hypothesis regarding the cause is “canalization”: Over time, a gene or trait may become more stable and less prone to mutation. Stewart cites the example of cervical vertebrae: mammals typically have seven cervical vertebrae, even though this number does not seem to confer any particular advantage. According to this hypothesis, if a number has been effective for millions of years, there is little reason to change it.
However, not all scientists agree with the canalization hypothesis. Kimberly Cooper, an evolutionary developmental geneticist at the University of California, San Diego, asserts that polydactyly or having more than five digits occurs as a form of mutation in many mammals, including humans.
“Why don’t polydactyl species persist?” Cooper poses the question. She suggests that polydactyly is likely a disadvantage in the evolutionary process. Gene linkage may be a contributing factor. Specifically, as genes evolve over millions of years, some become linked, meaning that changing one gene (the number of digits) could lead to severe health issues. Yet, to date, no one has provided definitive evidence for this.
