Miley’s front foot is very typical of a dog foot. It’s designed for running long and hard over open ground, but you’ll note that she does have dewclaw on the foot.
Because dogs are digitigrade, it looks like her dewclaw is half way up her wrist, but that’s not actually her “wrist” at all. That’s the part of her foot that doesn’t touch the ground.
Although dogs do use their dewclaws, their exact utility is pretty limited. They can’t grab things with them, and in many breeds, it is still traditional for people to remove them on very young puppies. It’s done to prevent tearing, which is actually a pretty rare occurrence, even among dogs that still have their dewclaws on the front legs.
All species of dog have dewclaws on the front legs, except for the African wild dog. It is so well-adapted to cursorial hunting, that the dewclaw has already disappeared in that species.
But where would such digit come from?
Well, if you really want to know, you better understand evolution.
Just as the big toe on humans is a severely modified great ape “thumb,” the dewclaw started out as a fully functional fifth digit.
To understand this a bit better, one must look at an animal that retains many features of the primitive carnivorans.
Probably the best example I can think of is the kinkajou. Kinkajous are procyonids that are almost entirely arboreal in their habits, and in this way, they strongly resemble the ancestral carnivoran in both habits and phenotype.
Indeed, they are so primitive in appearance that their bodies actual remind meany people of very primitive primates, which led many people to deem the kinkajou a type of monkey. In some Latin American countries, kinkajous are still called “night monkeys.”
And they actually look more like some kind of primate than many lemurs and bush babies do! That’s because if you go far back enough in the lineage of placental mammals, you will find ancestral arboreal animals that look something like this.
Kinkajous and dogs last shared a common ancestor some 40 or 50 million years ago. Their common ancestors were creatures called miacids, and these particular miacids were the ancestral caniforms. The big split in caniforms happened the dog family split off from the rest to go chasing things on the ground.
The ancestors of kinkajous stayed in the trees, and as predators, they are pretty poor at the game. Indeed, they live almost exclusively on fruit and nectar from flowers.
In the trees, their feet have never undergone the selection pressures that have made the dogs’ running shoes. Their feet have stayed very much like those primitive carnivorans.
In the photo above, you can see that a kinkajou has hands that almost resemble our own. They can actually grab things and hold them in those hands, as you can see in this video:
Although dogs can turn door knobs with their paws, they really can’t grab things in quite the same way.
Now, dogs have certainly not gone as far as horses have on their digitigrade anatomy. Horses run around on a single toe with a very hard and thick nail on it.
But they have lost their ancestral hands.
Their feet are now runner’s cleats.
The only vestige that they were once designed quite differently is that dewclaw.
And even if dewclaws are not entirely useless, they aren’t nearly as functional to a dog as that fifth digit is to a kinkajou.
In retrospect, it’s probably a good thing dogs don’t still have those kind of feet. Just imagine what kind of trouble they could cause if they were able to manipulate objects as well as a kinkajou can!