If you look at the paws of a cat, of a dog or even a kangaroo, you will notice that they have something in common with our hands. While some may be smaller or positioned differently, all of these mammals have five toes. Why do we share this pattern with our furry friends, even though we evolved under different conditions?
Mammals belong to the superclass Tetrapoda
To answer the question of why mammals have five toes, we must first understand why tetrapod (Greek for “four-legged”) vertebrates have five toes. Mammals belong to the superclass Tetrapoda, which also includes reptiles, amphibians and birds.
Even members of this group without traditional limbs have five toes in their skeleton – whales, seals and sea lions have five toes in their flippers – even if they have four or fewer toes.
There is some variation: horses have only one toe, and birds have a fused bone at the end of the wing. However, the scientists have discovered that these animals start out with five toes as embryos, but shrink before birth.
This process is dictated largely by Hox genes, said Thomas Stewar, an evolutionary biologist at Penn State. Hox genes encode proteins that help regulate the activity of other genes, turning them on or off.
They help ensure that body parts end up in their correct place in an animal’s body, as it develops from an embryo. As such, they are involved in dictating the skeletal pattern of tetrapods, and they do so by helping to control proteins created by the Sonic Hedgehog gene (yes, that’s the real name) to turn on and off each other during tissue creation.
Fingers grow through this process. Depending on the type of animal, these sprouts may continue to grow or reabsorb. Then, the cells around where the fingers should be die, creating separate digits. Exactly how this happens is “certainly a pretty complicated problem,” Stewart said. The details vary depending on which scientist you ask.
No one is sure when this five-finger plane first evolved. The first known animals to develop fingers evolved from fish about 360 million years ago and had up to eight toes, Stewart said.
The existence of the five-toed plane in most living tetrapods indicates that the trait is likely a “homology” – a gene or structure shared between organisms because they have a common ancestor. The common ancestor of all living tetrapods must have somehow evolved to have five toes and passed this pattern on to its descendants.
A common ancestor explains how mammals had five toes, but doesn’t tell us why. One theory is canalization: the idea that over time a gene or trait becomes more stable and less likely to mutate.
Stewart gave the example of the cervical, or neck, vertebrae: mammals almost always have seven of these vertebrae, although that number does not appear to offer a particular advantage. If the number has been working for millions of years, according to this theory there is no reason to change it.
not all researchers agree with the idea of channeling. Kimberly Cooper , an evolutionary developmental geneticist at the University of California, San Diego, points out that polydactyly, or having more than five toes, occurs as a mutation in many mammals, including humans.
There are multiple mutations that can cause polydactyly, but a recent study published in the journal Nature found that it can occur through the mutation of just one nucleotide in the sonic hedgehog gene.
“If it’s that easy,” Cooper asked, “why aren’t there polydactyl species?” He argued that it must be because polydactyly is an evolutionary disadvantage. Some speculate it could be due to genetic linkage: as genes evolve over millions of years, some become linked, meaning that changing one gene (the amount of fingers) could lead to other health problems more serious. But so far no one has offered concrete evidence, Stewart told LiveScience.
“We can ask a very simple question about why we don’t see more than five fingers, and it seems like we should arrive at a simple answer,” he said. “But it’s a really deep problem. That makes this field really exciting.”
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