I just broke my lower leg while skiing (both the tibia and the fibula). It's recovering well, having rest as my most prominent medicine. While thinking about my fracture, it appeared to me that we only have one bone in our upper arm (the humerus) and upper leg (the femur) and two bones in the lower arm (radius and ulna) and lower leg (tibia and fibula). Why is this? Or maybe better: what is the function or use of a second bone in the lower arm/leg?
The reasons for 2 bones in your lower arm can be to help your hand turn and give you more control over it. The shoulder joint is a ball and socket joint and has large muscles to help control it. Since the movements and actions of a wrist and shoulder are different, different joints were needed. Wrist are more delicate and do a lot different precise task. This distribution of bones also helps keep the body strong and keep balance. More bones always equals more flexibility. We need more flexibility distal of our joints than at the joints.
For more data try reading more on the biomechanics of bones and the skeletal system.
Why do we have 2 bones in the forearm and only one in the upper arm? Not a good source, however I think their explanation is helpful. This is Yahoo answers, so if you don't care for their opinion please don't click.