Why do mammals usually have 5 fingers?

The common five-finger structure in mammals may be due to a common ancestor, but experts have yet to find the exact reason.

Cat, dog or even kangaroo paws have one thing in common with human hands: they have 5 fingers. Why is this commonality present even though humans and the above animal groups evolved under different conditions?

Humans and many mammals have five fingers on their forelimbs. (Photo: Sergey Ryumin).

To answer the question why Mammalia usually have 5 fingers, we must first understand why 4 - legged vertebrates ( Tetrapoda ) have 5 fingers. Mammals belong to Tetrapoda , a superclass that also includes reptiles, amphibians, and birds. In this superclass, members without traditional limbs also have skeletons with five fingers, even though they have four or fewer toes. For example, whales, seals, and sea lions all possess five toes on their front flippers.

Some "variants" like horses have only one toe and birds have one finger bone fused at the end of the wing. However, scientists discovered that these creatures initially had five fingers as embryos, but atrophied before they were born.

This process is largely determined by Hox genes , according to Thomas Stewart, an evolutionary biologist at Penn State. Hox genes encode proteins that help regulate the activity of other genes , turning them "on" or "off" . Hox genes ensure that organs stay in the right place in an animal's body as it develops from an embryo . Therefore, they participate in determining the skeletal pattern of Tetrapoda .

Through this process, finger buds develop; depending on the animal, the buds may continue to grow or reabsorb. Then, the cells around the spot where the fingers will appear will die, creating separate fingers.

The raccoon's forelimbs have 5 fingers. (Photo: Riverotterecology).

Experts are not sure exactly when the 5-finger structure first appeared. The first group of animals to develop fingers evolved from fish about 360 million years ago and had up to eight fingers, Stewart said. However, the existence of a five-finger structure in most modern Tetrapoda species suggests that this may be a "homology" - a gene or structure present in many organisms because they have a common ancestor. It's possible that somehow, the common ancestor of all living tetrapods evolved to have five fingers and passed this trait on to their descendants.

Common ancestry helps explain how mammals came to possess five fingers, but doesn't help show why. One theory about the cause is "channelization": Over time, a gene or trait becomes more stable and less likely to mutate. Stewart gives the example of cervical vertebrae: Mammals typically possess seven cervical vertebrae, although that number does not seem to provide any special benefit. According to this hypothesis, if a number has worked for millions of years, there is no reason to change it.

However, not all scientists agree with the canalization hypothesis. According to Kimberly Cooper, an evolutionary developmental geneticist at the University of California, San Diego, polydactyly, or having more than five fingers, occurs as a mutation in many mammals, including even humans.

"Why don't polydactyl animals exist?", Cooper asked. She believes that it is likely that polydactyly is an evolutionary disadvantage. Gene linkage may be a cause. Specifically, as genes evolve over millions of years, some become linked to each other, meaning that changing one gene (number of fingers) can lead to other serious health problems. But until now, no one has yet provided solid evidence for this.