Life is a property of plants and animals, including humans. That is why they are called living beings. They also include bacteria and fungi. Inanimate things are called objects. These are stones, metals, and many other things.
The science of life is biology. But even the scientists, the biologists, find it difficult to say exactly what life is. The following things are required to speak of a living being: Living beings can sustain themselves. They have a metabolism, so they take in food and process it. living beings grow. So they are small at first and then get bigger or simply different.
Living things can reproduce. So they reproduce so they don’t die out. This also means that living beings can develop from one generation to the next. Living beings can move parts of their bodies themselves. But that doesn’t mean that they can move around independently, i.e. go somewhere. Plankton, for example, just happen to move with ocean currents. Living beings receive stimuli: They receive signals from their environments such as light, heat, or touch, and react to them. We, humans, do this with our sensory organs, which send signals to the brain.
Most living things can breathe, but not all. Humans and animals have an organ for breathing: the lungs or, in the case of fish and young amphibians, the gills. Plants breathe through their cells. But there are also very few creatures that cannot breathe. This includes some of the bacteria and some other tiny animals that usually live very deep down in the sea.
All living things are made up of individual cells. The cells store how the living being grows and what else it needs. There are living beings with only one cell, which are called “unicellular organisms”. These include most bacteria, individual fungi, and others. But they are not related to each other. However, most living things are multicellular.
The life of all living things, as biologists see it, always ends in death. Some creatures live for a short time, others for a very long time. A mayfly only lives for one day. But there is also a giant sponge, a sea creature that can live up to 10,000 years. In many religions, one imagines that the soul of a living being can live on forever.
Life has existed on earth for more than 3.5 billion years. Life has been found almost everywhere on earth. This applies to the hottest desert as well as to the icy landscapes of the Arctic and Antarctica. Even at hot springs on the sea floor, there is life, namely certain primordial bacteria that are now called “archaea”. They live on methane gas that comes out of the ground there and don’t need sunlight. So far, people have only known life on earth. However, it is believed that extraterrestrial life can also exist on other planets.
How can you classify living beings?
Living beings are divided into three domains. We know best the eukaryotes. All living beings in this domain have a cell nucleus in their cells. Eukaryotes are divided into animal, plant, and fungal kingdoms.
Bacteria form the second domain. They used to be called “bacilli”. They don’t have a nucleus.
The archaea form the third domain. They also have no cell nucleus. They usually live in extreme places: for example, it is very hot there, or the environment is very salty, or there is a lot of pressure, for example deep down in the sea.
It becomes difficult with viruses because they have no cell nucleus. If you assume that all life has a cell nucleus, viruses are not included. Most scientists see viruses as just stuff with a program, sort of like a part of a computer or a smartphone.