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Can we invite aliens to dinner?

  Before answering this question, we must first clarify two questions: What is food? Why can we eat food?
Eating is building blocks with parts

  There are foods that people can eat in almost all biological categories: from single-celled bacteria (such as bifidobacteria in yogurt and lactic acid bacteria in pickles), fungi (such as yeast, mushrooms, and fungus) to plants in all categories, including Lower algae (such as kelp, laver), ferns (such as hair fern, cabbage fern, water fern), gymnosperms (such as pine nuts, ginkgo), angiosperms (including various vegetables, fruits, seeds, rhizomes, etc.) Even flowers). And almost all kinds of animals, including coelenterates (such as jellyfish), mollusks (such as snails, mussels), crustaceans (such as shrimp, crabs), insects (such as silkworm pupae, locusts, scorpions, ants); and then to vertebrates , Including fish, amphibians (such as frogs), reptiles (such as snakes), mammals (such as cattle, sheep, pigs), and birds (such as chickens, ducks, and geese).
  It can be said that there is no category of creatures that humans cannot eat.
  The reason is that all living things on the earth nowadays evolved from the same ancestor, so they are close or distant relatives of each other. At the beginning of the birth of life on the earth, there may have been life with different mechanisms, but later life with only one mechanism prevailed in the competition and finally became the common ancestor of all living things on the earth. Therefore, although life on earth looks very different, the basic life mechanism is exactly the same. In a molecular sense, life on earth is very monotonous.
  Since all life on earth came from a common ancestor, they all have the same basic structural unit. For example, all cellular organisms use the same four nucleotides to compose genetic material (DNA, or deoxyribonucleic acid), all of which are composed of 20 kinds of amino acids that play a key role in life activities, and all have similar cell membrane composition. (Phospholipids), both use carbohydrates (glucose, starch, etc.) and fats as energy and energy storage substances.

  In other words, from the simplest creatures to the most complex creatures, the “building materials” used are the same, or the basic “parts” are the same, so they can be used everywhere. Just as a limited number of building blocks can be used to build an infinite variety of structures, a limited variety of “biological building blocks” can also be assembled into tens of millions of creatures on the earth. The so-called eating is to “dismantle” the “parts” or “building blocks” of other creatures to build our own bodies. This “demolition” refers to the process by which food is digested in our stomach and intestines. For example, protein is broken down into amino acids, starch is hydrolyzed into glucose, and fat is digested into glycerin and fatty acids. These amino acids, glucose, and fatty acids are the three basic “building blocks” or “parts.” Together with other small molecules such as glycerin, they are absorbed by the digestive system and used to build our own bodies.
  In this sense, any living thing on the earth can “eat” any other living thing on this planet in principle, as long as it can get rid of the toxic substances in the food. Therefore, animals can eat plants as well as other animals; plants can also eat plants and even animals; bacteria, fungi and molds “eat” dead animals and plants, as well as living organisms. For example, athlete’s foot and ringworm are fungi that eat “living people”; tuberculosis, infected purulent and septicemia are bacteria that eat “living people”. Even viruses use the “parts” in our body cells to build new viruses.
Depends on whether it is a carbon-based organism

All cellular organisms use the same four nucleotides to form genetic material (DNA, or deoxyribonucleic acid).

  The identity of life on the earth has a deeper meaning-the biological macromolecules on the earth are all based on carbon. Among the 100 or so elements currently discovered in this universe, only carbon atoms are the most able to connect to each other to form long chains and ring structures. This is the basis of biological macromolecules.
  But carbon-based life does not necessarily have to adopt the current model of life on Earth. For example, genetic material is not necessarily DNA or RNA, and molecules (enzymes) that perform catalytic functions are not necessarily proteins. Such creatures and the creatures on the earth are different in “parts”, so they can’t be used in common with each other, and they can’t be food for each other. Even if their “parts” are organic matter, burning can produce heat, but there are no enzymes in the earth’s organisms that they can use. To have a party with such creatures, both parties have to bring their own food.
  The element similar to the carbon atom is silicon, which belongs to the same group as the carbon element in the periodic table. It can form a three-dimensional four-bond structure like carbon atoms, and can be connected to each other to form long and branched chains, and combined with hydrogen atoms to form silane . It can also combine other elements to form functional groups, such as hydroxyl (a group formed by connecting an oxygen atom and a hydrogen atom) to form a hydroxysilane. So some people speculate that some alien life may be based on silicon, that is, silicon-based organisms. In this way, the constituent materials of these alien life are completely different from those on Earth-not only the “parts” are not interchangeable, but also because silicon-hydrogen compounds are easily hydrolyzed in water, water cannot be used as a medium for chemical reactions. Water may be poisonous to these creatures. For these silicon-based alien guests, don’t say please eat, even drink water!