JERRY BERGMAN It has long been believed that the sperm tail is a very simple structure, mostly a tail that wags to propel the sperm forward to reach the egg. Textbooks pictured this structure as a simple string like filament much thinner than a human hair. It was observed that the sperm tail produced harmonic bending waves, thus they were said to cause the sperm to swim like a tadpole. The bigger question then was, what was in the sperm head?
It has long been believed that the sperm tail is a very simple structure, mostly a tail that wags to propel the sperm forward to reach the egg. Textbooks pictured this structure as a simple string like filament much thinner than a human hair. It was observed that the sperm tail produced harmonic bending waves, thus they were said to cause the sperm to swim like a tadpole. The bigger question then was, what was in the sperm head?
Ideas about how sperm cells work date from their discovery by Leeuwenhoek in the 17th century. It was once widely believed that there exists a little person inside of every sperm cell head that traveled to the mother’s womb. The womb’s only role was to feed the child as it grew into a baby. This view was called preformationism. Another preformationist idea held that it was the egg that contained a little person that would begin to grow into a baby once the sperm initiated conception.
When it was recognized that the formation of a child required both a sperm (the smallest cell in the body) and an egg (the largest cell in the body), it was realized that reproduction was far more complex than once thought. Next, the parts of the sperm were studied. Researchers observed that the head contained mostly 23 haploid chromosomes. The first part of the tail, they found, contained mitochondria, the powerhouses of the cell that produce ATP. The rest of the tail consisted of a long thread-like structure. That ‘simple’ thread, the tail, is at last now being revealed by more detailed observations, and it turns out to be amazingly complex!
Research around 1968 proved that the sperm tail—properly called a flagellum—consists of a complex system of sliding filaments that are connected by elastic springs resembling cylinder-like structures. The cross-linked filament bundles, found both in cilia and flagella, are ubiquitous in life. Scientists also found evidence that this system contains a scaffold, the function of which is to enable the sperm to swim in the hostile environment necessary to reach the egg in the Fallopian tube. It was also discovered that the basic sperm tail design is very similar in most animal species. No evidence was found for it having evolved from a simple structure in primitive animals to a complex structure in higher animals.
The Latest Wiggle
New research published in 2017 has discovered that the tail system is far more complex than thought 50 years ago. The front design is vital to transmit information to distant parts of the tail to enable it to function as an effective unit for steering and propelling the sperm to its end goal.[i] The system works by complex elasto-hydrodynamics that we can only briefly outline here. Each tail is programmed to produce slightly different movements in order for the sperm to reach the egg. Scientists learned some of the details of this system partly by physically moving parts of dead sperm tails and designing mathematical models of the motion.
Any one movement in this complex sequence appears to be able to trigger motion right through to the distant parts of the tail.
One mechanism is counter-bending of a passive flagellum that instigates a compensatory counter-curvature in the section that it is connected to.[ii] The researchers found that movement, which begins near the sperm’s head, is transmitted along the interconnected system, creating controlled oscillatory movement along the tail’s full length.[iii] The tail “utilizes interconnected elastic spring structures to transmit mechanical information from the front to the distant parts of the tail, helping it to bend appropriately and ultimately swim toward an egg.”[iv]
The sections within the tail communicate by sensing the front of the tail near the section that contains the mitochondria. Each section in turn responds to information in front of it to synchronize their motions. The sperm tail mechanism first creates a sliding motion between the filaments inside the cylindrically arranged structure, producing the tail bending. Movement in this complex sequence triggers motion down to the distant parts of the tail. Even with these revelations, researchers are only beginning to understand what goes on. Phys.org says,
“The mechanism of a sperm tail first creates a sliding motion between filaments, inside this cylindrically arranged structure, finally resulting in a tail bending, a bit like the piston that converts back and forth motion in to [sic] rotation of the wheel on a train. Any one movement in this complex sequence appears to be able to trigger motion right through to the distant parts of the tail.
“The big question now is, are particular springs in the tail coupled-up to transmit specific biomechanical information, and just [do] these ‘rowers’ self-organize?
It turns out that the design of the sperm tail is just as complex as the design of the bacteria flagellum, which is well known to Darwin skeptics as an icon of intelligent design. Both are complex well-designed structures, one located on a so-called simple primitive organism, and the other on the sperm of the most complex organism known, namely humans.
This example illustrates a trend in biology. When structures do not seem very difficult to evolve, further research shows them to be far more complex and, consequently, much more difficult to explain by unguided evolution. What appear to be simple structures, such as sperm tails, on further investigation are shown to be far more complex, requiring a good understanding of biomechanics, especially elasto-hydrodynamics.
If Darwin were alive today, would he still try to explain such complex phenomena with his theory of gradual transmutation via natural selection? In 1859, when he suggested that all life was the result of slow, gradual variations (now thought to come from genetic mistakes), he could not have known the intricacies of sexual reproduction down to the tip of a sperm tail. Those who know today have no such excuse.
[i] “The mechanical properties of sperm tails revealed,” https://phys.org/news/2017-05-mechanical-properties-sperm-tails-revealed.html
[ii] Coy, Rachel and Hermes Gadêlha. 2017. The counterbend dynamics of cross-linked filament bundles and flagella, Journal of the Royal Society Interface. 14(130):1-10. May 31.
[iii] Phys.org, Ibid.
[iv] Phys.org, Ibid.
Resource: Creation-Evolution Headlines https://crev.info