In human daily diet, salt is essential. Not only humans, but also various animals need to eat salt. Domestic animals eat salt and wild herbivores lick salty soil. Carnivores don't have to lick salty soil because the blood of the animals they prey on contains salt. Why is salt so important? A theory dating back to the Late Cambrian holds that metazoa in the ocean formed a closed liquid circulation system for the first time 500 million years ago. Seawater may have become a chemical model of all animal body fluids. Generally, the surrounding environment is ever-changing, and the activities of cells are always based on it. Therefore, the serum of species far away from the ocean today is still very similar, and the salt content of the liquid around their blood cells is no different. When ancient marine animals went to land and eventually landed, salt was always the key component to maintain their internal environment. All kinds of animals have different halophilic mechanisms, but they can successfully meet the needs of reproduction, temperature change and resource competition. In the process of reproduction, not only the tissues in the fetus need salt, but also the milk can not be produced without salt. Another factor that affects the salt balance of animals is population density. High density often leads to invasive behavior, which first stimulates the pituitary gland and adrenal gland, and then inhibits the production of aldosterone (a hormone that maintains salt), thus accelerating salt loss. In other words, in places with high population density, individuals' demand for salt increases, but in salt-rich environments, the situation is the opposite. All animals have the same ability to keep salt balance in their bodies. Scientists have done experiments on sheep. Secondly, a large amount of salt-rich saliva is extracted from sheep, which destroys the salt balance in its body and then feeds it with salt water. It can drink salt water, just to make up for the lost salt in 2-5 minutes. This ability to stop drinking water is still a mystery, because the excess physiological saline stops drinking water before it enters the blood and human tissues. People's demand for salt is not so precise, and it is generally adjusted according to physiological needs. In cooking, salt is the most important condiment. Without salt, even the best dishes will have no taste. Salt is indispensable to human beings, but diseases such as hypertension are related to salt and have attracted people's attention. It is now known that lack of salt will make it difficult for the human body to maintain an appropriate plasma volume, while too much salt will increase the plasma volume and lead to an increase in arterial blood pressure. Animal experiments show that whether an individual will suffer from hypertension mainly depends on the ability of the kidney to eliminate excess salt. Genetic differences in renal function can explain why some people suffer from hypertension, while many people are safe despite taking too much salt. Of course, the cause of hypertension is very complicated, which is only one of the important factors. The human demand for salt is 1 ~ 2 grams per day, but the intake of modern people is often 5 ~ 10 times this amount! Since excessive salt is potentially dangerous, why do people still like to eat salt? How can this harmful eating custom be preserved after a long process of natural selection? It turns out that this is related to the composition of human food. Early humans were born vegetarian, and recently meat has dominated. When people live on a vegetarian diet, excessive salt will not cause problems.