[Bit#135] Why Humans Can’t Synthesize Vitamin C: The Secret of Genetic Mutation

Introduction

The fact that humans cannot produce Vitamin C on their own is not merely a nutritional deficiency. It is a fascinating scientific topic intertwined with human evolutionary history and genetic events. Behind the single supplement pill we take every day lies a profound question about why we survived in our current form. Are we incomplete beings with degraded functions, or are we winners of evolution who made an efficient choice for survival?

1. Vitamin C Production Shutdown, a Broken Gene Switch in the Human Body

Countless animals on Earth make Vitamin C on their own in their liver or kidneys. Without having to eat fresh fruits, they synthesize as much as they need in their bodies. But humans are different. The function to produce Vitamin C is completely stopped. We cannot survive unless we consume it through food or supplements every day. Why did only humans end up with such an inefficient body?

The secret lies in the traces remaining in our DNA. Originally, human ancestors also had a complete gene that could directly make Vitamin C. To synthesize Vitamin C, a series of chemical reactions must occur continuously in the body. At each step, a dedicated enzyme works to convert substances step by step.

There is one key enzyme that must work in the final step. It is the L-gulonolactone oxidase enzyme. The command device that produces this enzyme is the GULO gene. However, the human GULO gene is completely broken. A mutation occurred in the middle of the gene, leaving the switch turned off.

Scientists call such a gene a pseudogene. It is a shell of a gene that retains its shape but cannot function. It is like the factory building and machinery are installed as they are, but the main power line is cut off, stopping operation.

Then when and why on earth was this power line cut off? Was it really a simple genetic accident, or was there another reason? To find the answer to the question, we must turn time back to the very ancient past of humanity.

2. A Genetic Reversal Caused by the Rainforest Environment 60 Million Years Ago

Then when and why did our ancestors lose this useful function? We need to turn the clock back about 60 million years.

At that time, a huge genetic accident occurred in the ancestors of higher primates. The GULO gene that synthesizes Vitamin C broke down. What normally happens when an essential gene breaks down in a living organism? It is the law of nature that entire groups go extinct by suffering from fatal diseases. But our ancestors survived just fine. How on earth was this possible?

The secret was in the abundant environment where our ancestors lived. At that time, primate ancestors lived in a rainforest overflowing with fresh fruits and tree nuts all year round. The entire surroundings were a giant Vitamin C buffet. There was no need at all to directly make Vitamin C in my body using tremendous energy and nutrients. Just climbing a tree and picking fruit allowed them to fill tens of times more than the daily required amount.

In the end, even though the body’s Vitamin C factory closed its doors, no problem occurred. Rather, they could save the massive energy that was used to make Vitamin C.

Natural selection likes efficiency. An individual that saved that energy and put it into survival and reproduction was much more advantageous than an individual that wasted energy uselessly making a substance easily obtained from the outside. When an accidental gene breakdown met an abundant environment, it turned into an evolutionary advantage that saved energy rather than a fatal defect.

3. Survival Advantages Humanity Gained by Giving Up Vitamin C Synthesis

The event of giving up self-production of Vitamin C was not a simple loss. Behind it was a surprising survival strategy hidden. For a living organism to directly make a specific substance, surprisingly massive energy goes in. The process of breaking down nutrients and turning chemical reactions itself is considerable physical consumption.

Primate ancestors boldly closed this production line. And they began to invest that remaining energy in a completely different place. They used it to grow their brains and precisely develop physical functions. Instead of giving up internal production, they chose intelligent activities to actively seek food outside.

There is also another meticulous evolutionary advantage. There is a byproduct that occurs when synthesizing Vitamin C internally. It is toxic reactive oxygen species and metabolic waste. By closing the Vitamin C production factory, humanity completely escaped the risk of exposure to these toxic byproducts.

Furthermore, humanity absorbed powerful antioxidants in the process of filling Vitamin C from the outside. They are numerous phytochemicals and flavonoids contained in fruits and vegetables. Instead of ingesting just Vitamin C alone, they gained various antioxidants good for the body as a package.

Thanks to losing the function to make Vitamin C on our own, we became smarter and became clever predators seeking a wider variety of food. The fatal gene breakdown served as the best turning point that developed the human brain.

4. External Intake Dependence Changing Modern Human Eating Habits and Body Structure

The shutdown of internal Vitamin C production fundamentally changed human eating habits. Unable to make Vitamin C on their own, humans had to constantly move to find fresh vegetables and fruits. This external dependence became a powerful motive for humanity to develop gathering and collecting activities. Ingesting fresh food was directly linked to survival.

Furthermore, this characteristic left deep traces on human body structure and dietary culture. Vitamin C is a water-soluble vitamin that is not stored in the body and is easily excreted. Therefore, humans had to receive nutrition periodically. Techniques to explore and store various food ingredients naturally developed. Modern human food culture of eating various foods evenly actually started from a survival effort to fill a genetic defect.

What if humanity could still synthesize Vitamin C on their own internally? Perhaps we might not have strived to find fresh fruits or vegetables. The diversity of dietary life would likely have fallen much lower than now. In the end, the genetic limitation of not being able to make Vitamin C completed humanity as a being that eats more diligently and variously.

Humanity’s internal body structure also changed accordingly. Unlike other animals that synthesize Vitamin C, humans developed the transporter system of the intestinal tract more precisely to maximize internal absorption rate. They came to possess a high-efficiency absorption mechanism that quickly transports even a tiny amount of Vitamin C to the blood without missing it.

Conclusion

Thanks to losing the self-production function, humans learned how to constantly communicate with the external ecosystem. Deficiency was not a simple absence, but a powerful evolutionary driver leading us to a wider world. The appearance of humanity living dependent on different living organisms and food ingredients because we are not perfect gives a deep resonance. What new possibilities might another defect or lack in our lives be creating?