What is life?
We all have some basic understanding of what life is. Plant, insect or person are alive, but a rock is not. But what is the characteristic that separates living from not living? From practical point of view, if we would send probe into outer space, land on some planet and we could do any measurement, how could we say that this planet has life?
Characteristics of living organisms
There is no one thing that separates living from not living. But we can find some key characteristics shared among all living organisms even tho lines are not exactly clear.
1. Reproduction
Organism should be able to reproduce itself - make an approximate copy of itself.
Contra-example: We might have a computer program, that is able to reproduce itself, but it doesn’t necessarily makes it alive.
2. Metabolism
Metabolism is a process of taking in one type of chemical and producing a different type of chemical.
Contra-example: We can take a bunch of chemicals and put them in a jar to make a system, that is able to take in for example sugar and produces protein. But it doesn’t necessarily means that this jar is alive.
3. Homeostasis
Homeostasis is ability of living organisms to maintain their ideal internal conditions, such as temperature, pH, …
4. Cells
At least life that we know on Earth (in a way biology understand life and define it) is organised in this units of structure called cells.
It might not be necessary for all life that could potentially exist to be packed in cells. But if we are talking about life that came from Earth’s conditions, we find it organises in cells.
Life as emergent property
We can see that none of characteristics by itself is enough, but we can try to combine them and look at them as a whole.
When we think about cells supported by homeostasis paired with metabolism, then life is creation of local environment with special ordering of chemical that protects this environment and has a mechanism for producing imperfect copies of this type of organisation.
Personal note:
I like to look at life as emergent behaviour. In philosophy, systems theory, science, and art, emergence occurs when an entity is observed to have properties its parts do not have on their own, properties or behaviours which emerge only when the parts interact. We can try to find simple characteristics of life, but it is not enough. Life emerges from interactions of those characteristics.
When we are talking about obviously living things (like bacteria, plants, etc.), we will find them to be organised in cells and have some core of hereditary information, which is encoded in nucleic acids (at least on Earth). Hereditary information is essentially a blueprint how cell operates and reproduces. Combining this concept of having hereditary information with process of reproduction (which is imperfect process) we get an emergent property of evolution.
Grey area of life
As we don’t have strict definition for life, there is a grey area between alive and not alive. We can find viruses in this grey area. They have some local organisation but not cells. Main issue with viruses is that they don’t have their own metabolism, homeostasis and they cannot reproduce by themselves. Another grey area is transposable elements. Transposable element is a small piece of information (coded in a chemicals) that jump around and embed itself into another parts genetic information. It kind of has a live on its own. But it doesn’t fit other characteristics of life. Live or not alive, they have some life-like characteristics and we can consider them to be a subject studied in biology.