The animals on Earth are extremely diverse. From fleas to elephants, from mosquitoes to goldfish, from earthworms to peacocks, from spiders to pythons, they seem to have nothing in common with each other and share no similarities. In fact, they share many commonalities, and one of them is that all animals need to eat to survive.
To stay alive, many animals have to search for food every day. This can be said to be the most important instinct of animals. Chicks will eat as soon as they hatch from eggs, and piglets will start nursing as soon as they are born. Once food is unavailable, all animals will face death. Why is that?
We know that all life activities require energy consumption. Breathing, heartbeat, and movement are all achieved through muscle contraction. Nerve cells send electrical impulses, digestive organs secrete digestive juices, and cells synthesize large molecules in the body, all of which require energy. Just like power plants need to burn coal or natural gas to supply electricity, animals also need to "burn fuel" to obtain energy.
For animals, this "fuel" is food, and one of the important functions of food is to provide energy. In fact, the principle of animals using food is the same as that of power plants burning fuel to generate electricity. Power plants burn coal and turn it into carbon dioxide and water. The energy released during combustion is converted into electricity. Animals also "burn" food, which is also converted into carbon dioxide and water. The released energy is not used for generating electricity, but for synthesizing ATP (adenosine triphosphate), an "high-energy compound", which supplies energy for various life activities.
In addition to providing energy, the second important function of food is to provide "parts" needed to replace those in the body. Amino acids that make up proteins, fatty acids and glycerol that make up fats, and vitamins (as "additional components" for enzymes, proteins that catalyze chemical reactions) needed to maintain normal life activities must be "extracted" from food.
Metabolism accompanies the life of all organisms, and the bodies of organisms need to be constantly renewed. Non-living objects do not renew themselves. A piece of rock can only undergo minimal changes over billions of years. Its age can be calculated from the proportion of isotopes it contains; the composition of deep-sea ice in the Arctic has also remained almost unchanged for a long time, and the composition of air in its bubbles can reveal the composition of the air hundreds of thousands of years ago. However, organisms are highly complex and fragile, and the "components" in their bodies need to be continuously repaired and replaced to ensure the continuation of life.
Many animals can live for dozens of years to several decades, but most cells in their bodies cannot survive for so long. Human intestinal wall cells are replaced every two or three days, epithelial cells every 28 days or so, and liver cells every year or so. Even the bones that people think do not grow or change after adulthood are also replaced every 10 years or so.
Just like cells, the "lifespan" of molecules in cells is even shorter. Some protein molecules only have a "lifespan" of a few minutes. Some "long-lived" cells, such as nerve cells, also have their molecules constantly replaced. After 10 years, especially for adults, although it may not seem like there have been any earth-shattering changes compared with 10 years ago, almost all cells and molecules in the body have been replaced during this period.
The continuous renewal of animal bodies, the repair and replacement of "components" in animals' bodies, and the replacement of molecules in cells all require food to provide energy.