A popular YouTube science channel called SmarterEveryDay has 11.5 million subscribers. The channel recently posted a fantastic video about the bacterial flagellum titled, “Nature’s Incredible ROTATING MOTOR (It’s Electric!).” It has been up for less than a week and already has over 1.9 million views. In the video, engineer Destin Sandlin explains how he became captivated after watching an online animation of the bacterial flagellum. He notes that the flagellum “is a really big topic, not only in biomechanics” but also in “philosophy.” That’s because “the complexity of the flagellum implies many things about the origin of life” and “raises questions that people are debating and they’re talking about how can this be?” Sandlin says that he’s “not going to answer” those deeper questions in this video and he doesn’t explicitly endorse intelligent design — but he clearly appreciates the importance of this tiny molecular machine.

Flagellar Similarities to Human-Designed Machines

If you’re not familiar with the bacterial flagellum, here’s some background from the textbook Discovering Intelligent Design:

The flagellum is a micro-molecular propeller assembly driven by a rotary engine that propels bacteria toward food or a hospitable living environment. There are various types of flagella, but all function like a rotary engine made by humans as found in some car and boat motors. 

Flagella contain many parts that are familiar to human engineers, including a rotor, a stator, a drive shaft, a u-joint, and a propeller.

In a similar way, the video notes that the flagellum has a “biochemical motor,” an “axle” or “shaft,” “gears” or “pinions,” and a “propeller.” Sandlin says the flagellar motor is “amazing” and “incredible” and its complexity “reminds [me]… of an electric motor.” But the similarities between flagella and human-designed machines go much deeper.

How Does It Turn On?

Sandlin interviews Vanderbilt researcher Prashant Singh, an enthusiastic and knowledgeable scientist who explains how the flagellum is powered. Within the bacterial cell membrane is a buildup of protons (hydrogen ions), and inside the bacterial cell there is a lack of hydrogen ions. This leads to an energy gradient, and the flow of protons through that gradient powers the bacterial flagellum — effectively converting chemical potential energy into kinetic energy through an “electrochemical force.”       

This leads to the question: “How does it know when to turn the motor on?” Dr. Singh provides a clearly stated reply:  

There are sensors on the outside of the bacteria. Once it knows that there’s a threat, or there’s more energy near, it senses that it gets a chemical signal, and there’s a cascade of signals that go through. And one of the proteins well known for this is called CheY. The moment it senses that I need to run away from this location or I want to go to a different location, that protein comes and binds to it, and it encourages the protein to turn in a clockwise direction.

Sandlin appreciates the complexity that must exist to make all this possible. He says there must be a “coordinate system inside the bacteria,” and “sensors on the outside of the bacteria,” and “the bacteria knows where a sensor is triggered” and then “how to trigger what motor on what side of the bacteria.” This process by which the bacteria determines the direction in which to swim is called chemotaxis.

Getting “Emotional” at Chemotaxis

Many bacteria have multiple flagella that can spin together in unison in a bundle. When this happens, the bacterium swims straight and fast — but this bundled, fast, straight swimming only happens when the flagella spin in a counterclockwise direction. But you don’t want to swim too fast in one direction for too long or you might end up going in the wrong direction. So the bacterium regularly stops swimming and checks to make sure it’s still swimming in the “right” direction. To do this, the flagella switch directions to spin clockwise — and when this happens, Singh explains, “the bundle opens up.” This “pauses the bacteria” and it no longer swims forward. Instead, it starts “stumbling around.” This leads to a “biased random walk” where it can swim straight, and then stumble, and at each stumble it is checking if it’s going in the “right direction.” Over time it moves in the right direction.

At this point, Sandlin discovers more similarities with human-designed technology. He explains that he worked on a missile guidance system that would use a “biased dithering” to steer it in the right direction. It doesn’t necessarily go perfectly “straight” but it does correct itself to ensure that ultimately it finds the target. This is roughly similar to chemotaxis. Sandlin realizes the flagellum contains an “operating system,” “sensors,” “effectors,” “actuators,” and “feedback” — all features commonly found in human-designed technology. He says these similarities between the guidance systems on the missile and the flagellum make him “emotional.”

Changing Directions

Crucial to this process is the ability of the flagellum to change directions. How does this happen? A flagellar protein complex called MotAB spins via the flow of proteins through the bacterial cell membrane. MotAB directly engages with a basal ring of the flagellum and acts like a gear or pinion to turn the flagellum. MotAB is embedded in the inner membrane, and it is “only turning in one direction, it cannot go in two directions.” So how does it change the direction of the flagellum? The MotAB gear can “shift in and out” — either spinning the outside of the ring or shifting in to spin the inside of the ring. Singh further explains that if you need more torque on the flagellum you can add more MotAB complexes.

Again, Sandlin sees the similarity to human-designed technology: “It’s like shifting into reverse in a manual transmission car.”  Singh agrees: “It’s almost like you have a reverse gear.”  

Sounds Like Irreducible Complexity

Sandlin closes by noting that the complexity of the flagellum raises the question, “How can something this complex come to be out of nothing?” He then outlines an argument that sounds a lot like irreducible complexity:

The logic goes like this: If this motor system is composed of complex individual parts, and all these parts work together to perform the overall function of rotating, then how did the individual parts come to be? Did it all have to happen at the same time? Or is there some evolutionary advantage to the cell for every intermediate stage of development? Is 15 percent of this motor advantageous to the cell? What function would 50 percent of the structure perform? What were the steps these components took to assemble into such a complex molecular machine in the first place?

Sandlin doesn’t answer this question and doesn’t explicitly say that it could not have evolved. He notes that some scientists have cited the type 3 secretory system (T3SS) as part of an evolutionary pathway to the flagellum — but he immediately cautions, “This device [the T3SS] looks quite similar [to the flagellum], but it’s quite different in its protein structure. The complexity and origin of the bacterial flagellar motor is a really interesting conundrum.” We have made similar points in the past about crucial differences between the T3SS and the flagellum.  

From our vantage, the evidence suggests that the flagellum is indeed irreducibly complex. Again, from Discovering Intelligent Design:

Genetic knockout experiments by microbiologist Scott Minnich show that the flagellum fails to assemble or function properly if any one of its approximately 35 genes is removed. In this all-or-nothing game, mutations cannot produce the complexity needed to evolve a functional flagellum one step at a time, and the odds are too daunting for it to assemble in one great leap.

But not everyone agrees. Dr. Singh opines: “over the years the design has evolved to be so perfect.” But where are the details showing that this really happened?

Small Thing, Big Impact

In the end, Sandlin expounds upon his emotional reaction to seeing the complexity of the flagellum. He says its complexity gives him “joy” and makes him feel “awe and reverence,” and even brings him to give thanks to God. What a beautiful reaction to such a little thing!