Defined Words: vertebrates, spine, ribs, limbs, digits, opposable thumb
You have a spine. You have a head with a brain in it and a bunch of sensory organs on it. You have four limbs each with five digits and six joints (at the most) that have developed over generations to perform specific functions. We’re about to talk about why.
Humans took quite a while to develop and needed a specific set of circumstances to become what we are now. We’ll start before humans at the beginning of multicellular life on Earth. Multicellularity meant that an individual cell could allocate resources away from once vital tasks in order to perform more specific tasks, counting on the other cells of the organism to provide for the whole.
More cells growing on an organism and reproducing meant that the organism had to expand outward. Diffusion causes organisms to spread and branch off. Sharing the same DNA meant that the Organism only “knows” how to form one type of overall structure. But we’ll have to skip through some of that to hone in on our biological line.
We’ll fast forward to the annelids, you know of this group of organisms as worms. I know, they seem a little gross, but these guys are kind of like our great grandparents. They are the reason we’re here today, even if they aren’t the most pleasing to look at. These guys are mainly just a tube for food to pass through and they move around by contracting and relaxing their movement cells antagonistically.
Organisms that produced denser structures (bones, shells, etc.) were better able to survive. Some produced the structures on the outside; others produced them on the inside. Regardless of the structural adaptation, the mechanism of movement was the same: antagonistic groups of cells contract and relax, respectively. We’ll focus on the ones that developed an internal structure, let’s call them vertebrates.
Let’s take a moment to talk about what’s going on with the sensory organs. Movement means you’re going in a single direction, so the sensory organs were allocated to the front where it could collect the most information. The collected information helps the organism to survive and gather other organisms to put in its food tube, also at the front; isn’t that a helpful coincidence. The food tube would have to eventually expel waste, so structures to expel waste were allocated farther away; not the best smells or sights down there.
The fish are our evolutionary grandparents: you have a skull with two eye holes because your bony fish ancestor had two eyes. A pair of eyes was most efficient to see depth. A separate bone structure allows the organism to better control the opening of the food tube, we call this a jaw.
Over time, a dense structure formed around the communicative cells sending and receiving signals from the rest of the organism. We call this a spine. The bony structures were mostly separate but connected loosely to allow organisms to move at several points, we call these vertebrae. Eventually, mutations and adaptations allowed for other dense structures to branch out from the spine.
Some of those structures wrapped around other organs to provide more protection, we call these ribs. Other bone structures branched and adapted into limbs. The reason you, your dog, even your iguana have five digits on your limbs is because the first organism to explore the dry land niche had five digits. We are all descendants of that early amphibian.
Although insects and spiders have more than four limbs they are not vertebrates. These organisms are on a different evolutionary line than us; they are still animals, but they split from us before spines developed. The reason we have only four limbs was a matter of efficiently allocating resources.
Think about the most stable structure: a triangle. Three points of contact for a three dimensional world gives a lot of stability. Organisms with four limbs had the ability to maintain three points of contact while moving one limb. Four limbs was the most energy efficient means to moving around on land, extra limbs just weren’t necessary so any mutation with more wouldn’t have much of an advantage.
Each limb has five digits; however there are several examples of organisms that have adapted to not grow all five. Some have fused digits together for greater stability (hooves) and some have stopped producing limbs altogether (whales/dolphins)! All of these adaptations allow organisms to better fit their specific niches.
Over generations, our tendency of walking upright has fused two bones in our ankle; less range of motion is a trade-off for greater stability. Our line has also adapted the ability to press our digits opposite each other; we called this an opposable thumb. Think about all that we’ve accomplished because we had the ability to pick up rocks and sticks and turn them into anything we needed.
Your specific shape, among humans, is often decided by the muscles you most commonly move and which ones you’ve automated to lock. When you explore your body, you’ll find fat only has the capacity to build up on top of underused muscles. Get to know the muscle and implement it into your movements to watch the fat disappear. You might even get taller out of it!
Your body is a machine; one that moves in specific motions due to your ancestors’ necessity to stay alive. Your body is “designed” to function in a specific way. Remember to use it correctly so that you don’t harm yourself. Your core developed first and it should therefore be the strongest. A boost in sedentary culture has led us to forget how to stand up straight. Learn about your body so that you can learn how to respect it, you’ve only got one!