Shouldering the Load

(Picture from here.)

Fourth of July weekend I separated my shoulder. Consequently, shoulders have been on my mind somewhat.

Turns out human shoulders are remarkably interesting.

The human shoulder is composed of three bones: the humerus (upper arm bone), clavicle and scapula. A projection of the scapula, called the acromion, serves as the fourth point of the joint. Unlike, say, the hip joint where the femur sits solidly in a cup of the pelvis, the bones of the shoulder are not articulated directly but held together by a complex of muscles and ligaments. This is easy to see in the naked skeleton. In the hip joint (see here) the head of the femur fits snugly in the cup. The head of the humerus sits some distance from a join of the acromion, scapula and clavicle.


A better picture of the shoulder is shown at left. Here, while the articulation of the bones is obscured it's more easily understood that the shoulder, as a joint, is far more a result of the interaction between muscles, bones and ligaments than a bone to bone combination. While this is true of every joint I tend to it is especially obvious in the shoulder.

(My injury involves the ligaments between the acromion and the clavicle.)

The clavicle first appears in bony fishes and associated with the pectoral fins along with the cleithrum. The cleithrum ran vertically along the scapula and formed part of the gill cavity. This arrangement persisted in some of the early land animals but disappeared early in the evolution of reptiles. The scapula, too, was found in bony fishes and attached to the upper surface of the pectoral fin. In early land animals a bone called the procoracoid was paired with the scapula.

Amphibians became reptiles. These evolved into two branches that we find important: sauropsids and therapsids. Sauropsids evolved into all existing birds and reptiles as well as dinosaurs.

Therapsids evolved into mammals.

By the time therapsids came around a third bone, the coracoid, formed just behind the procoracoid. Monotremes (platypus and the like) kept this three boned structure but in mammals the procoracoid disappeared and the coracoid fused with the scapula. The scapula of modern mammals is the result of this fusion and isn't exactly congruent with the bone of the same name in earlier animals. Such is the world of comparative anatomy.

The humerus has remained the upper arm bone since primitive fossil amphibians.

The shoulder, bearing a quarter of the weight of a four legged animal, changes based on the evolutionary strategy. We would expect the shoulder joint of an elephant, cheetah and zebra to be different based on their body types and the lives they lead. However, in these cases the forelimb bears significant weight. These are four legged animals.

The shoulders of animals that have evolved a significant difference between forelimb and hindlimb activities have a different story. Monkeys, dinosaurs, birds, bats and kangaroos have all separated forelimb and hindlimb function. Consequently, much of the weight bearing utility of the forelimbs have been abandoned. Not completely, of course. Gorillas bear significant weight on the forelimb (and hence the shoulder) but they descended from arboreal monkeys that used their forelimbs more for other things.

Fast forward a few million years to chimps and humans-- and possibly human forebears.

The shoulders of chimps (and australopithecines) is more cranially oriented than humans.

On the left here we can see a human shoulder joint. On the right is a chimp. (See here for the root article for these pictures.) Notice how long the clavicle is and how straight it is compared to the chimp. Not how the acromion butts up in the chimp and it's laid down low on the human. What has occurred here is in the human the joint has opened up. The chimp joint is more constrained than the human joint. Stronger but with fewer degrees of movement.

A result of this is a measurement of humeral torsion, the twist between the head of the humerus and the other end. Different primates have different twists. (Greg Laden has a good discussion here.) It is one of the anatomical idiosyncrasies that enable both human and Neanderthal (and Homo floresiensis and Homo erectus) to better manipulate things in front of them.

But the modern human shoulder is even more open than its ancestors. It is much less adapted to hanging from a tree than the shoulder of any of our relatives-- a side effect of our extreme verticality. As soon as we committed to walking on two legs we freed up the shoulder to adapt to new uses. First it moved to allow manipulation in the front. Then, about two million years ago, it started to open up further.

The modern shoulder has an ability to move in almost any direction. It can, for example, reach all the way back and swing all the way around to the front easily and quickly. It can do that holding on to a rock.

We could throw things.

About 400,000 years ago we could throw things we built-- like spears.

All of a sudden our reach truly did exceed our grasp.

Additional links:
William H. Calvin, a professor in Seattle, has even ascribed this ability with shaping how our brains work. (See here for The Throwing Madonna.)
Katsuya Nobuhara's lovely discussion of the shoulder here.