[Bit#30] Why Humans Can’t Survive on Grass: The Secret of Cellulose Digestion

1. Why Cows Can But Humans Can’t: The Fortress of Cellulose

Cellulose, the primary component of grass, is actually a massive energy source composed of thousands of glucose molecules linked together. Its raw material is identical to the starch found in potatoes or rice. However, the structure makes all the difference. While starch is a flexible chain, cellulose is tightly twisted like a steel cable. This structural difference dictates the fate of digestion.

The human digestive system lacks the scissors required to cut these sturdy bonds. This is because our bodies do not secrete a specific enzyme called cellulase. On the other hand, how do cows manage it? Interestingly, cows cannot produce this enzyme on their own either. Instead, they have hired powerful partners: tens of trillions of microorganisms living in their stomachs.

Cows consume grass and send it into a massive fermentation tank known as the rumen. Here, microbes break down the cellulose on the cow’s behalf, converting it into a form the cow can absorb. To humans, grass is merely dietary fiber that passes through undigested, but to a cow, it is a magnificent feast prepared by microorganisms.

Why then did humans fail to cultivate these useful microbes? If our bodies possessed a giant stomach for fermenting grass, we would look very different today. The answer lies in why cows must spend all day chewing the cud. The process of extracting energy from grass is incredibly slow and labor-intensive. Ultimately, humans and cows chose different survival strategies. Cows opted for the ubiquitous grass, while humans chose easily digestible, high-energy foods. What decisive impact did this choice have on human evolution?

2. Evolutionary Choice: Trading Digestive Efficiency for a Brain

Why did humanity abandon cellulose, such a sweet energy source? The answer lies in cost-effectiveness. Grass is everywhere, but its nutritional value is low. To live on grass like a cow, one would have to spend more than half the day chewing and digesting. Human ancestors took a different path: high-efficiency fuels like protein and fat.

This choice completely transformed the human physique. The need for a massive stomach and long intestines to digest grass disappeared. As a result, the body’s energy could be redirected from the digestive system to the brain. The brain is the most energy-consuming organ in our bodies. If we were still wasting energy digesting grass, we might never have developed the complex thinking and intelligence we possess today.

An intriguing fact is that the discovery of fire widened this gap further. Cooking food drastically increased digestive efficiency, allowing us to extract even the tiny amounts of nutrients trapped behind cellulose walls. Instead of the ability to break down tough fibers, humans gained the intelligence to create tools and devise strategies.

However, a question arises. What if we faced a food crisis like the one today? If humans could digest cellulose again, perhaps hunger would vanish from history. If we could derive energy from weeds or tree bark, how would the Earth’s landscape change?

3. If Humans Could Digest Grass: The Magic of Solving Food Shortages

What if all the weeds and leaves scattered across the planet became energy equivalent to white rice? The moment humans can digest cellulose, the word food shortage might disappear from our dictionaries. Currently, the crops humans consume represent only a tiny fraction of Earth’s plant resources. But if we could decompose cellulose, everything would change. Discarded rice straw, corn stalks, and even paper could become a hearty meal.

But does this magical situation only mean a rosy future? There is an intriguing twist hidden here. If humans were to digest grass directly, our body structure would have to change entirely. Grass has a very low nutrient density. To obtain the necessary energy, we might have to spend every waking hour chewing. If we had to eat dozens of kilograms of grass like a cow, we would have to give up our current sleek physiques.

Our bellies would bulge due to the massive stomach required for fermentation, and we would have to deal with the enormous amount of gas produced during digestion. Furthermore, the global ecological order would be overturned. If humans began to view even weeds as food, the food supply for wild animals would vanish instantly. While the food crisis might be solved, it would be difficult for the Earth to remain green.

Why then do we remain unable to eat grass directly? Perhaps it is because humans chose to build massive factories outside our bodies instead of fermentation chambers inside them. Even if we cannot digest grass ourselves, modern science is preparing to utilize this tough cellulose in other ways. What is the identity of this incredible latest technology?

4. Modern Science’s Challenge: Technology Turning Waste into Food

Instead of digesting grass directly, humanity has invented a powerful external organ: technology. It is a magical process that transforms tough plant matter we cannot eat into useful energy. At its core is White Biotechnology. Scientists can now extract glucose from wood scraps discarded in forests or residues left after farming.

The most representative method is borrowing the power of microorganisms. Microbes that decompose cellulose already exist in nature. Scientists manipulate the genes of these microbes to maximize their decomposition efficiency. As microbes diligently break down cellulose walls inside industrial tanks, pure sugar pours out. This sugar then becomes food for growing alternative proteins or is transformed into biofuel.

Recently, research into artificial digestive systems has also been active. This involves creating bioreactors that mimic a cow’s stomach. Humans don’t need to suffer from bulging bellies by eating grass. Large machines digest the grass instead, and we simply consume the essential nutrients produced. If this is realized, food crises will no longer threaten humanity.

Why are scientists so obsessed with cellulose? Because it is the most common and abundant resource on Earth. The moment discarded waste becomes food, the way humanity survives will be completely transformed. Instead of expanding cultivated land by destroying nature, we are moving toward an era of efficiently using the common plant resources around us.