Most mammals have 5 fingers

CNN

Even if some might be shrunken or differently positioned, all of these mammals have five digits, or fingers.
To answer the question of why mammals have five fingers, we must first understand why tetrapod (Greek for “four-footed”) vertebrates have five fingers.
Mammals belong to the superclass Tetrapoda, which also includes reptiles, amphibians and birds.
Even members of this group without traditional limbs have five fingers in their skeleton — whales, seals and sea lions have five fingers in their flippers — even if they have four or fewer toes.
The first known animals to develop fingers evolved from fish around 360 million years ago and had as many as eight fingers, Stewart said.
The common ancestor of all living tetrapods must have somehow evolved to have five fingers and passed that pattern down to its descendants.
A common ancestor explains how mammals got five fingers, but it doesn’t tell us why.
Kimberly Cooper, a evolutionary developmental geneticist at the University of California, San Diego, points out that polydactyly, or having more than five fingers, occurs as a mutation in many mammals, including humans.

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A common feature among the paws of cats, dogs, and even kangaroos is that they are similar to human hands. All of these mammals have five digits, or fingers, though some may be smaller or arranged differently. Why, despite the fact that our environments of evolution were different, do we share this pattern with our animal companions?

It is necessary to comprehend why tetrapod—Greek for “four-footed”—vertebrates have five fingers before we can respond to the question of why mammals have five fingers. The superclass Tetrapoda, which also includes amphibians, reptiles, and birds, is comprised of mammals. Whales, seals, and sea lions, for example, have five fingers in their flippers, even though they only have four or fewer toes. Even members of this group without traditional limbs have five fingers in their skeleton.

A single fused finger bone is present at the end of a bird’s wing, whereas horses have only one toe. But as embryos, these animals can have up to five fingers, but before they are born, those fingers disappear. This is something that scientists have discovered.

Hox genes play a major role in this process, according to Penn State evolutionary biologist Thomas Stewart, who spoke with Live Science. Hox genes encode proteins that aid in controlling the on/off state of other genes. As an animal develops from an embryo, they aid in making sure that its body parts land in the proper places. As a result, they help regulate the proteins produced by the sonic hedgehog gene (yes, that’s the actual name) to activate and block each other during the creation of tissues, thereby dictating the skeletal pattern of tetrapods.

The process results in the growth of finger buds, which may reabsorb or continue to grow based on the kind of animal. Afterwards, distinct digits are formed when the cells surrounding the finger positions die. Stewart acknowledged that the specifics of this phenomenon are “admittedly a pretty complicated problem.”. Which scientist you ask will tell you different things.

It’s unclear whence this five-finger strategy originally originated. According to Stewart, the earliest animals known to have fingers were fish, and they may have had as many as eight fingers when they first evolved. But since most extant tetrapods have the five-finger plan, this suggests that the characteristic is probably a “homology”—a gene or structure that all organisms share because of a common ancestor. Somehow, the common ancestor of all extant tetrapods had to evolve five fingers, which it then passed on to its progeny.

Mammals’ five fingers can be explained by a common ancestor, but this explanation does not explain why. The idea that a gene or trait becomes more stable and less likely to mutate over time is known as canalization theory. Stewart used the example of the cervical, or neck, vertebrae, pointing out that although having seven of these vertebrae doesn’t seem to provide any particular advantage, mammals nearly always have seven of them. Under this theory, there’s no reason to change the number if it has functioned for millions of years.

Related: Which animal species is the oldest?

All researchers do not, however, concur with the canalization theory. Evolutionary developmental geneticist Kimberly Cooper of the University of California, San Diego notes that many mammals, including humans, are genetically predisposed to polydactyly, or having more than five fingers. One nucleotide in the sonic hedgehog gene can cause polydactyly, although there are several other mutations that can also lead to it, according to a recent study that was published in the journal Nature.

Cooper inquired, “If it’s that simple, why don’t polydactyl species exist?” She reasoned that polydactyly must be an evolutionary disadvantage. Some believe it may be due to gene linkage: as genes change over millions of years, some become linked. This means that altering one gene (the number of fingers) may cause other, more serious health problems. However, no one has provided verifiable evidence as of yet, Stewart told Live Science.

“It seems like we should come up with a simple answer to a very basic question: why don’t we see more than five fingers?” he stated. “However, the issue is quite complex. That’s what makes [this field] so fascinating. “.

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