from Michael Hamm - The World’s First Shoulder
Shoulder as modified gill arch, Andrew Gillis 2016
The world's first shoulder was a wrinkle in an embryo's neck, floating in the oceans of Ordovician Earth, ~480 million years ago. The wrinkle, fickle in form, did not follow a single plan. But soon it stumbled on a great purpose -- to steer water around the bodies of ancient fish.
It became a simple fin, with a structure resembling long thin spars anchored to a rocky shoreline. Its nerves and vessels rode the spars out to the fin's tip, while its muscles hugged close around the anchoring bones, swiveling the spars as needed.
The “shoulder” fin skeleton of Abudefduf saxatilis, from Westneat et al 2004. CL= Cleithrum, SC=Scapula, CO=Coracoid, RA= Radials, FR=Fin Rays.
Then around 418 million years ago, a splinter faction of fish devised a new kind of fin. These embryos grew their neck-wrinkles not into a fan of spars, but a 2D tree, with a strong central trunk and smaller branches of bone-nerve-vessel radiating toward the edge. New movements in the water became possible -- swirls and twists and pumps dazzled the shallows of Silurian seas, outmaneuvering predators and extending the new fishes' range.
Besides near-term success this new fin also held a great potential --- to grow other sorts of 'trees' from the central axis. Such as a tree that can plant its tip on waterless ground, with its water-swivel modified to push the whole animal along:
Fin-to-Limb Homology, Woltering et al 2020
With the new stressors of walking (and later others, like digging, climbing, flying, carrying, throwing), bone lattices bloomed with solutions: Stubby scapulae stretched into plates and valleys, and the nubs of humeri/ulnae/radii became long obelisks holding a deft splay of digits at their tips. Nerves and vessels devised ways to slide in nested sleeves, helping them tolerate greater pressures and tensions.
Tetrapod Limb, basic themes Hirasawa and Kuratani 2020.
Throughout these profound changes, many old traditions held: Red blood cells, action potentials, immune systems, endocrine feedback loops, inflammatory cascades. Central nervous systems with aversions, attractions, memories, desires.
Each new version of shoulder did its best to propel and protect these animals in their environment. Each shoulder had deficiencies and failure modes. And when it became overwhelmed, each did its best to regenerate.
From fins to hands, (Carl Zimmer/NYT 2016)
So we can think of our human shoulders as being a recent flowering of this fin-limb continuum. And for those of us working with shoulder function and injury recovery, our ancient forebears may offer us a few lessons:
• 'Neutral' position still resides in the fin-plane, i.e. the plane of the scapula, 30-40deg forward from the frontal plane. So any assessments or movements we make might usefully begin from that neutral zone.
• Every shoulder is still neck-connected. To the degree we have irritation/dysregulation to our mid-to-lower neck it will tend to affect our shoulders. This means someone evaluating a shoulder issue should at least briefly screen the neck’s structural behavior and inflammatory state.
• The arch of clavicle-to-scapula, may structurally still behave like a gill arch – a sequence of horseshoes running down through jaw, hyoid, thyroid, cricoid, etc. (Remember that odd muscle, omohyoid?). So we should look for possible cross-influences between shoulder and breath/pharyngeal function.
• A shoulder’s versatility is always in tension with its stability. (For example, Homo Erectus’ shoulder was much more stable, but much worse at throwing things.) Each shoulder is a study in trade-offs. Before trying to ‘fix’ a shoulder problem, we should appreciate the tradeoffs a given individual has made in their chosen activities.
• The forelimb is still our protector, our catcher of food, our clever shaper of outer reality. Damage to our shoulders is an injury to these fundamental drives, and thus to our sense of personal agency. Let’s appreciate how deep the shoulder’s roots run, and how many layers of a person can feel the impact when it doesn’t function well. It may be that shoulder recovery must first involve the reclamation of a meaningful task, even before the tissue strength or range-of-motion returns to normal.
Michael Hamm LMT CCST teaches online & in-person courses in anatomy, embodiment, and clinical approaches to manual therapy, and is based in Seattle, WA. Background at: https://www.integrativebodyworkeducation.com/
