The Fascinating Evolution of Myelin Sheath and Its Origins
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The Evolutionary Leap of Myelin Sheath
Around 430 to 450 million years ago, marine organisms began to develop jaws, marking a significant evolutionary milestone. During this era, the first myelin sheaths emerged in jawed species, although the exact relationship between jaw evolution and the appearance of myelin remains unclear.
Have you ever considered the myelin sheath? This remarkable structure is essential for the nervous system's compactness and efficiency. Take, for instance, complex creatures like squids and octopuses, which lack myelin sheaths. Their unmyelinated nerve fibers occupy 15,000 times more space than a myelinated nerve in mammals, consuming an immense amount of energy.
To put this into perspective, without myelin sheaths, the human spinal cord would be nearly a meter wide! Imagine a lower back that exceeds 100 cm in width (not diameter)!
But how did myelin sheath evolve? Surprisingly, it traces back to a viral infection.
Understanding Myelin Sheath
At its core, the myelin sheath functions like insulation around electrical wires. It doesn't conduct electricity itself, which enables electrical signals to "jump" between gaps in the insulation, known as the nodes of Ranvier. This mechanism allows for rapid signal transmission; while unmyelinated fibers conduct signals at a sluggish pace of 0.5 to 10 meters per second, myelinated fibers can reach speeds of up to 150 meters per second.
Scientists have been puzzled by the sudden emergence of the myelin sheath in jawed vertebrates without any precursor structures. A breakthrough came in 2024, when researchers from the University of Cambridge, led by Ghosh et al., published findings in the journal Cell that traced the evolution of the myelin sheath through genetic studies.
They identified a gene, RNLTR12-int/RetroMyelin, that is crucial for myelin sheath formation. The team discovered that this gene is highly expressed during myelin sheath development in rats. When they silenced this gene, the formation of myelin was inhibited. Similar results were observed in zebrafish and frogs.
The role of RNLTR12-int was redefined as RetroMyelin, a remnant of ancient retroviral sequences that became integrated into the host genome. Retroviruses can reverse-transcribe their RNA into DNA, and if they infect reproductive cells, their genetic material can be passed down through generations.
As Ghosh et al. noted, "In the present study, we propose that in vertebrate species, RetroMyelin sequences originate due to germline invasion of ancient retroviruses carrying an RNLTR12-int-like sequence."
The Discovery of RetroMyelin
Ghosh et al. examined the presence of RNLTR12-int/RetroMyelin in various species, confirming its existence in all jawed vertebrates while noting its absence in jawless vertebrates and invertebrates.
This leads to two possible scenarios regarding its integration: 1. A single event in a common ancestor of jawed vertebrates before speciation. 2. Multiple independent events post-speciation.
To discern which scenario holds true, the researchers constructed a genetic tree of RNLTR12-int/RetroMyelin sequences across species. The close grouping of sequences within each species supported the idea of multiple independent integrations—an example of convergent evolution.
Why This Matters
The exact driving force behind the myelin sheath's evolution remains uncertain. However, its co-emergence with jaws may hint at a need for more sophisticated neural control over the rapid movements necessary for biting and chewing. The swift signaling enabled by the myelin sheath likely played a crucial role in these adaptations, allowing early vertebrates to become more efficient predators.
Prof. Robin Franklin, the study's director, emphasized the significance of retroviruses in vertebrate evolution: "If we didn't have retroviruses sticking their sequences into the vertebrate genome, then myelination wouldn't have happened, and without myelination, the whole diversity of vertebrates as we know it would never have happened."
The retrovirus-driven evolution of the myelin sheath showcases how viruses have profoundly influenced our evolutionary history. Notably, about 8% of our genomes consist of retroviral sequences called human endogenous retroviruses (HERVs), while another 40% consists of retrotransposon sequences, believed to have viral origins.
As an intriguing aside, this relationship between viruses and the myelin sheath has implications for diseases like multiple sclerosis, which involves the breakdown of the myelin sheath and has been linked to viral causes.
How an Ancient Virus Spread the Ability to Remember This video delves into the intriguing connection between ancient viruses and the development of memory capabilities in vertebrates.
The Two Viruses That We've Had For Millions of Years Explore the role of long-standing viral sequences in shaping vertebrate evolution and their lasting legacy.
In conclusion, the story of myelin sheath evolution highlights the unpredictable nature of evolutionary processes. It serves as a reminder of the intricate web of life and how even the smallest organisms can have a monumental impact on evolution. Thank you for joining me on this exploration. If you found this informative, feel free to subscribe or support my work.