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Appreciate Your (Un-Evolved) Body

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While awaiting Michael Behe’s new book, Darwin Devolves, please take the opportunity to marvel at why the human body hasn’t devolved completely by now, and works as well as it does. Here are things you probably didn’t even know about your body. They should arouse awe as we go about our tasks each day, and sleep each night. Scientists didn’t know some of these things either, until recently. They usually talk little about evolution when they examine the body in detail.

In the Blink of an Eye

Scientists have been aware of involuntary eye movements, called saccades, which continuously move our eyeballs with tiny jerks, even when we intently stare at something. Physiologists surmise that saccades keep our rods and cones from getting saturated, but the movements create a problem, too: how to keep our perceptions from jerking along with them. In PNAS, Jasper H. Fabius et al. describe “Time course of spatiotopic updating across saccades.” Whatever smooths our vision has to work fast:

Humans make frequent eye movements — about three to four times per second. Eye movements create changes in sensory input that the visual system should dissociate from changes in the outside world. Still, visual perception is introspectively undisrupted, but appears continuous. It has been hypothesized that the visual system anticipates the sensory changes based on a predictive signal from the oculomotor system. However, psychophysical studies suggested that this anticipation develops slowly: too slow for natural vision. Here, we examined the speed of this anticipation more closely using psychophysics and a motion illusion. We observed fast anticipatory updating, quantifiable in human behavior. The time scale at which the anticipation is reflected in behavior is compatible with typical fixation durations in natural viewing. [Emphasis added.]

The brain anticipates the movements and compensates for them, as if the muscles in they eyeball signal to the brain, “Get ready: I’m about to jerk by this much; you go left while I go right.” Physiologically, this shouldn’t be able to keep up with 3 or 4 jerks per second, but it works. They call it “fast anticipatory updating,” and measure it to be over 3 times faster (150 ms) than what the muscles of the eyeball should be able to accomplish (500 ms). They don’t exactly know how it works; they just know it does work.

Think what problems this means for evolution. Saccades are important to prevent blindness, so that system has to evolve. But the brain must simultaneously evolve the ability to anticipate saccades, needing a second lucky set of mutations has to be selected. Both systems undoubtedly require multiple lucky mutations, but without them all working in concert, the animal cannot survive.

Foot Notes

Upright walking remains a distinctive characteristic of humans. Many organs and systems are involved. We lack the stability of four feet, and being relatively thin and tall, we are at risk of falling over from gravity more than most animals. Staying upright requires many systems to work together: muscles, nerves, tendons, ligaments, the balance organs, the brain, and rapid feedback between all of them. Another paper in PNAS by Farris et al. probes one of these components: “The functional importance of human foot muscles for bipedal locomotion.” The paper begins,

Human feet have evolved

Now that the obligatory salute to Darwin is done, what are the facts? 

Here we show direct evidence for the significance of these foot muscles in supporting the mechanical performance of the human foot. Contrary to expectations, the intrinsic foot muscles contribute minimally to supporting the arch of the foot during walking and running. However, these muscles do influence our ability to produce forward propulsion from one stride into the next, highlighting their role in bipedal locomotion.

We are able to run and walk by a “windlass mechanism” in the longitudinal arch (LA). A windlass is used by engineers to lift heavy weights by wrapping a cable attached to the object to a rotating drum. It’s not an exact analogy, but it shows that the LA needs to store elastic energy like a spring for the push-off, and then stiffen on contact with the ground. Previously, this windlass mechanism was thought to reside primarily in the ligament running under muscles from heel to toes, the plantar aponeurosis, which wraps around the metatarsal heads and stiffens the ligament upon landing. Now, these anatomists give more credit to associated muscles of the plantar ligament, the plantar intrinsic muscles (PIMs). 

Thus begins a discussion of forces, costs, loads, materials and functions too detailed to go over here. Suffice it to say, those PIMs are more important than thought. They play an important role in standing, walking and running. Aren’t you glad that “the PIMs may have a fundamentally important role in the function of the human foot and our evolved specialism for bipedalism”? Evolution thought of everything!

How a Fever Functions

What’s the purpose of a fever? Nature says it helps the immune system battle an infection. How? Well, a rise in temperature triggers T cells to get on the move. They start producing heat-shock proteins (Hsps), and then systems really start moving in a coordinated way.

The Hsps travelled to the inner surface of cells’ outer membranes, where they bound to the tails of membrane proteins known as integrins. This process pulled integrins together, and the integrin sections jutting from the cells’ outer surfaces formed complexes that stuck to blood-vessel walls. The formation of integrin complexes also triggered the migration of T cells to infection sites.

That’s a lot of parts needing coordinated plan, as if to form a network and a road as the T cells switch on and move to the site. Researchers found that mice with mutant integrins unable to bind to Hsps died from fever and infection. 

Antennas Up

In honor of Dr. Behe, who used molecular machines as examples of irreducible complexity in his groundbreaking ID book, Darwin’s Black Box, let’s revisit one of his examples: the cilium. Twenty-two years after Behe wrote about cilia, the University of Geneva explains “How our cellular antennas are formed.” Yes; we have tens of trillions of information-gathering antennas.

Most of our cells contain an immobile primary cilium, an antenna used to transfer information from the surrounding environment. Some cells also have many mobile cilia that are used to generate movement. The ‘skeleton’ of the cilium consists of microtubule doublets, which are ‘pairs’ of proteins essential for their formation and function. Defects in the assembly or functioning of the cilia can cause various pathologies called ciliopathies.

As Behe wrote, cilia are not just extensions of the cell membrane. They are complex and fascinating structures filled with molecular machines that carry building materials, called tubulins, to the tip and return waste materials back. The system resembles a series of trucks in a mine shaft bringing in timbers and carrying out ore. The trucks must travel at the proper speed with the proper amount of cargo, or else you have a ciliopathy. Nobody wants those. They include “brain malformations, retinal or fertility disorders, kidney or liver diseases, recurrent respiratory infections and skeletal anomalies.” That such a tiny little antenna can lead to grief when it fails tells a lot about irreducible complexity: get it right, or die.

The axis of the cilium or flagellum also serves as a “rail” for the movement of “trains” of molecules from one end of the organelle to the other, propelled by real molecular motors.

The researchers’ claim to fame was assembling tubulin building blocks into doublets in vitro. That’s all, but they are proud of their little bit of progress in understanding cilia: “The doublet is a structure essential for the formation and functions of cilia, but its assembly was unknown until now.” They found that the tubulin regulates itself in a way that prevents runaway assembly of doublets elsewhere. Everything is under tight control, and now we can breathe, think, and walk upright.

Expelled: No Mucus Allowed

What does it take to cough? That’s an unusual question Burton F. Dickey addresses in PNAS, “What it takes for a cough to expel mucus from the airway.”

Cough is one of the most common symptoms for seeking medical care. If cough is going to cause that much trouble, it better be worth it, and the clinical evidence is that indeed it is. Patients with impaired cough due to neuromuscular disease or postoperative sedation suffer high rates of atelectasis and pneumonia due to the failure to clear secretions from the airways, and there is evidence that a heightened cough reflex improves health.

Now that we have the right perspective on the cough reflex, let’s see it basically as a matter of generating force sufficient to expel unwanted material. Mucus is good in its place, Dickey says. “Mucus is a remarkable and protean substance, with properties on the border between a viscous fluid and a soft elastic solid.” It can hold microbes for the immune system to reach. And remember those cilia? The motile kind are always at work in the airways, moving dust and germs to where they can be swallowed (stomach acid takes quick care of that). A cough is needed when there is too much foreign material, but the body must maintain a balance between mucus clearance and mucus dysfunction. Now, let’s see what it takes to cough:

Cough has been studied extensively, so its mechanism and the forces it generates are well known. A cough begins with a rapid inspiration to fill the lungs with air, followed by closure of the glottis, contraction of the expiratory muscles of the chest and abdomen to generate a high intrathoracic pressure, and the sudden opening of the glottis to forcefully expel air from the mouth. During coughing, intrathoracic pressure can reach 200 cmH2O, which both provides the motive force for airflow (up to 8 L/s) and narrows the central airways by compression (Fig. 1, Middle Right) to maximize velocity (up to 28,000 cm/s or 626 mi/h). This expels secretions from the airways and into the throat (pharynx) (Fig. 1, Left), where they can be either swallowed or expectorated.

Eight liters per second moving at 626 miles an hour! That’s incredible, comparable to the speed of a bullet. Keep that in mind this cold and flu season.

These are just samples of the countless wonders of bodily design. Considering the specifications for proper function in each of them, we shouldn’t be surprised that we get sick. We should be surprised that we are ever well. In an intelligently designed body, everything has its place. When it all works together, life can be beautiful.

Photo credit: Vidar Nordli-Mathisen via Unsplash.