Edit: thank you all for your responses. Rather than asking “why are there two bones in the forearm?”, a better way to ask is “what functionality does two bones do for the forearm?”. From your answers, I realize that that not only can I move my forearm up and down and sideways like I, somewhat, can do with the arm, but I can also rotate the forearm itself up and down unlike the arm. I guess there is a similar difference in the leg between the thigh area and the calves/shin area since the latter has two bones two. Again, thank you all for your responses.
In: Biology
It’s all about stability. In the forearm, the radius allows you to flip your arm so that your palm can point both up and down. The ulna allows you to flew and extend your arm. If it was just one bone facilitating all of those movements, the joint would be unstable. For example, the humerus in the only bone that moves in the shoulder joint, and it has a huge range of motion. What you gain in range of motion, you lose in stability. Thus, you see a lot more shoulder dislocations than elbow dislocations. And like another user has said, evolution plays a role as well. It must just be more evolutionarily beneficial to have a super flexible shoulder joint, and not so much to have an equally flexible elbow joint
Oooh! So many better replies already.
Can you imagine seeing a bone capable of rotating only on one end? The bone would have to break to rotate itself because bones like to stay straight and aligned. The two forearm bones can stay intact while the arm rotates because the bones remain outside the center of rotation and don’t experience that twisting stress.
Hold the ends of 2 pencils side by side and twist one end. Then try again with 1 pencil. Imagine the pencils are bone and the ends where you’re holding them is where they connect to the body.
I will leave the anatomical considerations for someone who might better be able to discuss the functionality of those specific bones. The exact physiological implications are not an area of expertise for me.
But the evolutionary implications are an area of expertise for me. Here is the key takeaway: evolution does not create that which is ideal. It starts with what already exists and tweaks it, with the useful tweaks becoming more common. So even if you could theoretically design a skeletal structure like the one you described, and even if it would work perfectly well for humans, it doesnt change the fact that evolution has been working from that basic skeletal structure for hundreds of millions of years. There are literally hundreds of millions of species, most extinct, that had that skeletal structure. We inherited it from ancestral species.
When you ask “why” it is that way, the causal explanation is because it is coded in our DNA, and evolution does not have any conscious ability to decide to just start fresh and rebuild the arm. It just tweaks what is already there.
Your forearm twists a lot more than your upper arm. Two bones provide a more stable joint both on the elbow and wrist ends.
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