Ancestral Origin
Mammals' common five-fingered structure likely stems from a shared ancestor, but the precise reason remains a subject of scientific inquiry.
Tetrapod Inheritance
To understand why mammals, a subclass of Tetrapoda, possess five fingers, it's necessary to recognize the five-fingered nature of Tetrapods. Tetrapods include mammals, reptiles, amphibians, and birds. Even members without traditional limbs, such as whales, seals, and sea lions, possess skeletal structures indicating the presence of five digits, though they may only develop four or fewer toes.
Developmental Factors
Exceptions like horses with a single toe and birds with a single fused finger bone at the wingtip initially have five digits in their embryonic stages, which later regress before birth.
This process is largely controlled by Hox genes, explains Thomas Stewart, an evolutionary biologist at Penn State. Hox genes encode proteins that regulate the activity of other genes, turning them "on" or "off." In Tetrapods, Hox genes ensure that developing limbs form with the correct skeletal pattern.
During this process, finger buds arise, and depending on the species, they either continue to develop or are reabsorbed. Surrounding cells then die, creating distinct fingers.
Evolutionary Hypothesis
While the exact timing of the five-fingered structure's first appearance is still uncertain, Stewart notes that the earliest animals with fingers evolved from fish around 360 million years ago and had as many as eight fingers.
The presence of the five-fingered structure in most Tetrapods today, however, suggests it may be a "homology"—a gene or structure found in multiple organisms due to a common ancestor. The common ancestor of all living Tetrapods may have somehow evolved to have five fingers and passed this trait to its descendants.
The Reason Why
While the shared ancestry explains how mammals acquired five fingers, it does not fully illuminate the reason why.
One hypothesis is that of "canalization": over time, a gene or trait becomes more stable and less likely to mutate. Stewart cites the example of neck vertebrae: mammals typically have seven, even though that number seems to provide no particular advantage. Under this hypothesis, if something has worked well for millions of years, there is no evolutionary pressure 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, points out that polydactyly, the condition of having more than five fingers, occurs as a mutation in many mammals, including humans.
"Why don't we see polydactyly persist in species?" Cooper asks, suggesting that extra fingers may be detrimental during evolutionary selection. The reason could be genetic linkage. As genes evolve over millions of years, some become linked together, meaning that changing one gene (finger number) could lead to other severe health problems. However, definitive evidence for this has yet to be found.