Homologous vs. Analogous
Let's take a look at the world, specifically the mammals, around us. Look closely. What similarities do you see? Well, the first one that strikes me is we all have a similar shape. We all have a head, a body and some form of arms and legs, right? In some cases, those parts are similar in structure, function or both.
For example, the bones in the legs of a horse and donkey are similar in both structure and function. Both animals use them primarily for walking and running. However, the bones in the wing of a bat and the pectoral fin of a whale, even though they're similar in structure, are very different in function - one uses them for flying and the other for swimming.
When these traits share a common evolutionary origin - meaning they develop from the same embryonic tissue in the fetus - and when they share a common ancestor, they are called homologous structures. In the case of the donkey and the horse, it's easy to see these two share a common ancestor; however, it's a little more difficult when looking at a whale versus a bat, isn't it? Well, in this case, way, way, way back in time, these two shared a common, four-legged ancestor, but over time, their pathways have diverged greatly, each adapting to totally different environments.
But, in some cases, even though they are similar in function, like the wing of a bat and the wing of an insect, they do not share a common evolutionary origin. In these cases, they are called analogous structures. Now, note that I said similar and not identical in structure. What this means, for example, is that all vertebrates share the commonality of having, say, upper and lower limbs. Those limbs are made up of similar bones and muscles between species. The general function is the same, even if the specific functions vary slightly.
Think about it this way: all animals use their arms and legs to move; however, in some animals, they are used primarily for walking and running, while in others, they are used for swimming, climbing or even flying.
Upper Limb Variations
For example, you have probably noticed that your arms are shorter than your legs. However, this is not the case in all species. Many species of monkeys, in particular, have longer arms than legs, or some, like dogs and cats, have four limbs all of similar length. This is because the specific function of those limbs differs from humans. Monkeys do a lot more climbing than walking, while dogs and cats use all four limbs to walk and run.
However, despite small differences in function, they still have one upper arm bone, followed by two lower arm bones, wrist bones and finger bones - just like humans, although, sometimes the length or number of the bones and the shape of the muscles used to move their arms and legs may vary slightly. But, despite slight variations, the order in which they are put together and the general function of motion are similar to those in humans and other mammals.
For example, let's look at a dog, a cat and a human. Pretty different species, right? But, in addition to similar bone structure as we saw earlier, we also have similar upper limb muscles, such as the deltoid, biceps, triceps and extensor muscles. We determine whether a muscle is homologous by looking at which bones it attaches to and what function it performs. For example, the muscles in your arm, the biceps and triceps, help to flex and extend your arm. What about in cats? Or horses? Or other animals? Do they perform the same function? Do they attach to the same bones?
In humans, the triceps is attached to the scapula and the ulna. In dogs and cats, the triceps are attached to these same bones, giving them similar origin and insertion points. The extensor muscles in humans allow us to extend our wrist. In dogs and cats, these same muscles allow them to extend their paws. In all three species, the deltoid helps move the upper limb; however, in humans, it produces a greater range of motion than in dogs, making its function slightly different.
Or, let's look at even greater differences in function by going back to the whale. You may not think that a whale's pectoral fin looks anything like your arm, but if you go inside and look at the bones, you will see they are organized in a similar way. It's just that their size and shape varies. We already talked about how we both have one upper, two lower and multiple wrist and finger bones, but now, look at how the shape varies.
While the pectoral fin of a whale is organized similarly to a human arm, the shapes of the bones are different.
In whales, their version of the humerus, radius and ulna is a lot shorter and fatter than a human's, but their finger bones are a lot longer. This makes them more adapted to swimming, while we humans are more adapted to, say, playing video games with our hands.
Lower Limb Variations
Just like the upper limbs of many mammals are similar, the lower limbs are also homologous. Let's start by looking at two species that can stand on two legs - humans and gorillas. We both have large upper leg bones followed by a kneecap, two lower leg bones and ankle and toe bones. Hmm, sounds pretty similar to the arm structure, doesn't it?
While humans and gorillas have similar lower limb structures, they are different because of different functions.
However, while our legs allow us to stand pretty upright, the gorilla's legs are shaped a little differently, due to the fact that even though they can stand on two legs, usually, they use their arms and legs to walk. Notice above, how the pelvis of the gorilla is a little longer than that in humans. Similarly, the shape of the muscle on top of the pelvis is also a little longer in gorillas than in humans.
So, even though we may walk a little differently, we still have the same bones and the same muscles in our legs. We also see some of these muscles in other species like cats, dogs and horses. See that muscle below? The biceps femoris? Humans, gorillas, cats, dogs, horses and tons of other species all use that muscle to move their legs.
Both humans and gorillas use the biceps femoris to move their legs.
Back and Chest
These similarities aren't just limited to our arms and legs. You can also see similar bones and muscles on the rest of the body, like the trapezius, that large diamond-like trapezoid shaped muscle on your back, which moves and stabilizes the shoulder blades and neck. Underneath that is the spine, or vertebral column, common to all vertebrates.
Humans and gorillas have similar trapezius muscles and spines.
Moving to the front of our body, we have the pectoralis muscles, or pecs for short. Those are the ones body builders like to show off! They help with shoulder rotation and arm movement. The abdominal muscles and the obliques, all of these can be found in multiple species of animals, including humans.
Pectoralis and abdominal muscles are both found in gorillas and humans.
So now, I hope you have a better idea of all the similarities that exist in the animal kingdom. Even though we all look very different when we are born, we start out looking very similar. And, from that similarity comes similarities in development, structure and function. When these traits are similar in structure and share a common evolutionary origin, they are called homologous structures. However, sometimes we see similarities in function but without similarities in origin. These are called analogous structures.
Let's take a quick look at the limbs of a few animals to review this idea. You can see below that each animal represented has a humerus (the upper arm bone), a radius and ulna (two lower arm bones), carpal bones in the wrist and phalanges in the fingers.
Each of these different species have the same bones in the upper limbs.
Now, see how they are all kind of similar, but also kind of different in structure? Maybe the length or size of the bone changes? Well, that's because each structure has adapted to the primary movement of that particular animal. The shape of a bat's bones are longer, skinnier and lighter, aiding in flight. The whale's phalanges are long and spread out, and when covered in skin and muscle, aid in swimming.
Humans and other primates, on the other hand, have skin separating their fingers, allowing them to grasp objects like trees for climbing or pens for writing. While the shortness of, say, a cat or dog paw prevents this ability. These similarities and differences are found all throughout the animal kingdom and all throughout the body, not just in the arms and legs.
After watching this video lesson, you'll be able to:
- Differentiate between homologous and analogous structures
- Identify both upper and lower limbs of different species that are similar but different in shape or function