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Is the Human Shoulder Badly Designed?

Photo credit: West Point, U.S. Military Academy, via Flickr (cropped).

A few months ago, I fell and dislocated my left shoulder. My upper arm bone was put back in its socket the same day, but then I spent months in physical therapy to regain full function. In the process, I have learned a lot about an amazing joint that I previously took for granted.

The drawing below shows only part of the human shoulder’s anatomy. Not shown is the large deltoid muscle, which overlies the shoulder joint and connects the upper arm bone (humerus) to the collarbone (clavicle) and the shoulder blade (scapula). Also not shown is the trapezius muscle across the back, which connects the left and right scapulas. Both the deltoid and the trapezius play important roles in stabilizing the joint. 

Image source: National Institute Of Arthritis And Musculoskeletal And Skin Diseases (NIAMS); SVG version by Angelito7, Public domain, via Wikimedia Commons.

Anatomy of the Shoulder Joint

In the drawing, yellow indicates bone, red indicates muscle, blue indicates tendon, and purple indicates bursa (a fluid-filled cushion). The dashed black lines indicate the hidden ball-and-socket joint between the humerus and the scapula. Unlike the hip joint, in which the ball is deeper in the socket, the shoulder joint is more open. This means the shoulder joint is less stable than the hip joint, but it is also much more flexible. In fact, it is the most flexible joint in the human body. 

The biceps muscle at the lower left gets its name from the fact that it has two heads. One attaches, through a tendon and a small bursa, near the top of the humerus. The other head attaches to the coracoid process, an extension of the scapula. The lower end of the biceps muscle is attached to the forearm. Although it is primarily involved in moving the forearm, its divided head helps to stabilize the shoulder joint.

Both the flexibility and stability of the shoulder joint are due primarily to the muscles of the “rotator cuff,” listed on the left side of the drawing. All four of the listed muscles stretch across the scapula and attach to the top of the humerus. For a 10-minute tutorial on the rotator cuff, see here. For a longer (20-minute) tutorial on the movements, bones, and muscles of the shoulder, see here.

The more I have learned about the shoulder joint, the more I have been impressed by its specified complexity, which points to intelligent design. Imagine my surprise when I came across a six-and-a-half-minute video claiming that the human shoulder is a “design disaster.” The video was made by Cheddar News, which describes itself as “the only news network focused on the next generation of innovators and decision-makers[.] Cheddar News is where forward thinkers go to learn about the people, ideas and innovations that are driving change and creating what’s next.”

I am confident that a rigorous argument can be made for the intelligent design of the human shoulder. But that is not what I present here. In what follows, I examine the claims against design that are made in the Cheddar News video.

Proof that the Human Shoulder Is a Design Disaster?

The video’s producer is Natalia Ryzak, who has a master’s degree in journalism from Columbia University. At the beginning, Ryzak explains that “human shoulder blades tilt down and outwards, whereas chimps tilt up. Small variations like this are the reason humans have awful shoulders. And chimps, with whom we share nearly 99% of our DNA, don’t.” For that, Ryzak continues, “we can thank evolution — or more specifically, how we are outpacing it.”

But the tilt difference does not explain why the human shoulder is “awful.” If we spent most of our time swinging from tree branches, it might; but we don’t. And the claimed 99% similarity between human and chimp DNA has no bearing on the issue.

Ryzak goes on (from 0:47 to 0:59) to say:

Side effects of a human shoulder may include dislocation, separation, rotator cuff tears, bursitis, tendonitis, tendonosis, impingement syndrome, instability, arthritis, adhesive capsulitis (frozen shoulder), and fracture.

But these are not “side effects,” any more than getting a flat tire is a “side effect” of making an automobile. Or having a roof torn off by a tornado is a “side effect” of building a house. And these problems are not unique to humans: Chimps can also suffer from arthritis and fractures, among other things.

Enter Nathan Lents, professor of biology at the John Jay College of Criminal Justice in New York City. In 2015, Lents argued on his blog that the human eye is badly designed, primarily because “the vertebrate retina is wired in backwards.” Like Richard Dawkins and others before him, Lents based his claim on the fact that the light-sensing cells face away from the incoming light. But evidence published from the 1960s onward — and reported in standard textbooks — shows that this arrangement is far better than the one Lents favors.

Back to the video on “Why the Human Shoulder is a Design Disaster.” Lents says (at 1:30) that the shoulder is “more of a floating joint than any other joint in the body.” Ryzak explains that the outer layer of muscles (consisting of the deltoid and trapezius) is stronger than the inner layer (the rotator cuff). Then Lents continues (from 1:59), “Having such an overlapping meshwork of muscles, what you’re inviting is pinching, and tearing, as the orientation can shift.” Lents compares the shoulder joint to the hip joint, in which “the relationship of the hip to the leg is fairly fixed in place.”

So far, the video has summarized the structure of the shoulder and its difference from the hip. The shoulder is more flexible than the hip. Good thing, too, or we wouldn’t be able to perform many of the actions we do. Just watch an acrobat performing on the parallel bars. Or a baseball player pitching a fastball. Or an athlete swimming the butterfly.

But journalist Ryzak confidently concludes ex cathedra (starting at 2:19) that “we’ve proven to you just how cr*ppy our shoulders are.” How so? Ryzak doesn’t say. Instead she simply suggests going “back into the evolution part.”

Does Evolution Explain It All?

According to Lents (starting at 2:28), “In our quadrupedal ancestors, in our deep past, really we had four legs, they weren’t really arms, to speak of. When you think of a dog and a cat, they don’t have arms, they have legs. But they still have a shoulder joint, as we can think of it.”

Then Ryzak says, “Our shoulders evolved for a life in the trees, swinging and hanging out. Then we left the trees behind and began to stand upright. This freed our arms up for other purposes, like hunting and gathering.” So from four-legged animals that walked and ran on the ground, we get animals that spend some of their time on the ground but mostly swing from branches to branches in the trees. Then those animals “evolved” into animals that stood upright and used their arms for other purposes. This is the standard Darwinian narrative. But how, exactly, did four-legged animals on the ground evolve into two-armed animals that swung on tree branches, which then evolved into two-armed animals that stood upright on the ground? The video offers no explanation; only an imaginative story.

Lents continues (starting at 2:54), “We are partially adapted for throwing, which is… no other animal in our group of animals throws anything.” This is not true: Chimps can throw, though not as far or as accurately as humans. Indeed, they are infamous for flinging feces at visitors to zoos.

But that’s a minor detail. Lents goes on to say, “So we believe that throwing was a very strong evolutionary pressure as we began to hunt — throwing spears, thrusting as well, so thrusting and throwing are very specific kinds of motion. And that required that floating nature to our shoulder.” But “evolutionary pressure” just means that throwing favored the survival of early humans. It does not account for the origin of the human shoulder. As Darwinian biologists wrote in 1996, adaptations “concern the survival of the fittest, not the arrival of the fittest.”

So the claim that “we left the trees behind and began to stand upright” does not explain the remarkable anatomy of the human shoulder. After all, chimps leave the trees on a regular basis (though they don’t stand upright). Yet their shoulder anatomy has not changed.

The Problem and Its Solution

According to Lents (at 3:57), “Part of the problem in present-day humans is not so much a bad shoulder design but a mismatch between what our shoulder is designed to do and how we use it on a daily basis.” Of course, Lents doesn’t think the shoulder was intelligently designed. As a Darwinist, he believes that the shoulder evolved through accidental variations and survival of the fittest. And in our immediate ancestors, the shoulder was adapted (“designed”) to swing through trees.

Most of our modern activities are very different. Ryzak adds (starting at 4:16), 

It might surprise you, but simply sitting at your desk is a major contributor to shoulder problems. When we hunch forward for days, hours, months, years on end, we end up causing unnecessary pulls and strains on our rotator cuff muscles. That can lead to injuries.

Lents explains (starting at 5:20) that you can minimize shoulder problems by “changing the way you eat, changing the ways you use your body.” And, Ryzak adds (from 5:34 to 5:52), “pay attention to basic posture.” So after all the talk about bad design and evolutionary mismatch, the solution to our “design disaster” is for us to pay attention to dietexercise, and posture

I think I could have figured that out without all the anti-design rhetoric and Darwinian storytelling. Oh, and I would add: Be careful not to fall in such a way as to dislocate your shoulder.