What makes our hands different to other animals? We have an opposable thumb. We have four fingers and a thumb on each hand. We are not the only animals to have this many fingers, and we are not the only animals to have an opposable thumb, but the reach of our thumb sets us apart.
A five-fingered arm is called a pentadactyl limb, and five fingers is the norm, rather than the exception. Almost all mammals, birds, reptiles, and amphibians have five fingers or toes because our shared ancestor had five digits. We all share an ancestor that lived around 420 million years ago. It came out of the sea and its fins became limbs and it grew five digits on each limb. That animal was able to reproduce and five digits was passed on to all future animals. The fossil record shows that there were creatures with six or seven digits, even one with thirteen digits, but, for whatever reason, they were not able to pass that trait on as well as the creature with five digits. No one knows why.
Five digits must have an evolutionary advantage for it to be so widespread, but nobody knows what that advantage is. Even animals that don’t appear to have five digits do. Horses have one digit on each foot, but when they are still embryos, they have five that then fuse into one before they are born. Bats have five fingers across their wings and even whales have the remains of five fingers in their flippers. Nobody knows why, but the other fossils found seem to have been unable to survive on land. Their ribs were too weak for them to breathe, or their shoulder and hip joints prevented them from walking properly. Is this connected to the number of digits? Nobody knows. However, the gene that is responsible for the number of digits, SOX9, is responsible for the development of the body during the embryonic phase. Maybe if the gene causes more digits to grow, it messes something else up in the process. Maybe that is something that people will know in the future.
If the number of digits we have doesn’t set us apart from the other animals, then what does? It is our opposable thumbs. A thumb is generally a fifth finger that bends in a different direction to the other fingers and can be used to pin or grip things. We are not alone in having thumbs. Most primates have thumbs. Pandas, koalas, and opossums have thumbs. Chameleons and one species of frog have thumbs. They all work in a slightly different way.
Panda thumbs don’t bend. It is an extra digit that the panda can use to grip bamboo with to make it easier to eat. Koalas and opossums have an opposable toe on their hind feet. This allows them to grip smaller branches. Most primates have opposable thumbs, which they can use to grip branches and climb. They can also use them to manipulate simple tools. However, most primates aren’t able to manipulate tools in the same way that we are, and this is where the main difference in our hands come from. We have longer thumbs in relation to our hand than any other animal. Our thumb rotates and stretches far enough that we can touch every other finger on our hand with the tip of the thumb. That may not sound like much, but it is enough to give us the dexterity to manipulate complex tools with our hands.
We couldn’t evolve this type of thumb until we came down out of the trees because a longer thumb is a hindrance for a tree climbing primate. Most climbing animals need short thumbs and long fingers. However, a longer thumb is an advantage for a land-dwelling primate that has learned how to use tools to hunt animals. Early humans with more dexterous hands would have been able to kill more animals, have access to more meat, and reproduce more. Dexterity was passed on. Having access to more meat gave the brain the protein and energy it needed to expand as well. Our long thumb means we can pinch things by putting the pad of the thumb against the pad of the index finger. This is a technique that would have helped tremendously in making stone tools, and it could be that action that made sure the ability was carried on.
Things like our opposable thumb didn’t evolve on their own. At the time we evolved these thumbs, we started using stone tools, killing more animals, having more protein and fat, evolving bigger brains, gaining language, starting to learn, remember, and plan. Each one of these things reinforced the others and they all helped us evolve to the point where we are today. However, it is our opposable thumb that makes our hands different to other animals. And this is what I learned today.
Sources
https://www.scientificamerican.com/article/why-do-most-species-have/
https://www.nationalgeographic.com/science/article/how-did-you-get-five-fingers