February 12th, 2025
What is "Life"?
What exactly is "life"?
Earth is a very special planet. It’s the only place teeming with life. Surely we can all recognise life when we see it, right? A rabbit is definitely a living being. It hops across the meadow, munches on its grass, reproduces… It’s alive! Fits so far. An oak tree stands firmly in the forest – that’s an important difference to a rabbit. But the oak absorbs water and nutrients through its roots and releases oxygen into the air. The oak is certainly also a living being. But what about the moving flame of a fire that can consume whole areas of land? Is fire also life? How exactly can we recognise life? What are the characteristics of living beings?
There is actually less agreement among scientists on this seemingly simple question than one might initially think. In the relevant books you will find between five and eight, sometimes even up to 48 or more definitions and characteristics.
We will focus here on the eight characteristics on which there is agreement among scientists and use them to check whether a rabbit, an oak tree and a flame are living beings. To be recognised as life, our candidates must fulfil all eight characteristics.
Figure 1: Life and non-life are very diverse phenomena. How can we tell them apart?
Movement
A very important characteristic of living things is movement. With living things, something is always moving. In the case of rabbits, it is obvious: the rabbit perks up its ears, hops across the meadow and does many other things involving movement.
But it’s a different story with an oak tree. The oak is firmly anchored in the ground. Only the wind sometimes hits the crown and shakes it. The oak cannot move itself. Or can it? If we look at the green leaf of an oak tree under a microscope, we can see a lot of movement. We can see many currents in the cells of the leaf. The green chloroplasts change their position depending on whether we illuminate the leaf strongly or weakly. In strong light, the chloroplasts arrange themselves very close to the cell wall in order to receive as little light as possible. Conversely, in low light, the chloroplasts spread out as far as possible in the cell in order to absorb as much light as possible. And if we film the branch of an oak tree in fast motion in spring, we can see the unfolding of the new leaves. The oak thus also fulfils this characteristic of life.
A flame moves in a current of air and will take every opportunity to spread and move from one place to another. Here, too, the criterion for life is fulfilled.
Rabbit:
Oak tree:
Flame:
Metabolism
The second important characteristic is metabolism. This means that substances from the environment are absorbed, somehow processed and other substances are released again. This is easy to see in rabbits. The rabbit eats green leaves and releases dark-coloured dung – this can be observed in any large meadow.
In oak trees, the metabolism is not quite so obvious, but can still be observed with the appropriate precautions. The oak absorbs water and various salts from the soil with its roots. It evaporates the water on its leaves, where it also absorbs carbon dioxide from the air and releases oxygen. Very good: According to this criterion, the oak is also a living being.
The flame, when it burns on a match, for example, absorbs organic substances from the wood and oxygen from the air and releases soot, water vapour and carbon dioxide. According to this criterion, the flame is also a living being.
Rabbit:
Oak tree:
Flame:
Homeostasis
The criterion of homeostasis is closely linked to metabolism. This refers to the maintenance of very specific conditions inside the living organism that differ from the conditions in the outside world. For example, the acidity of the rabbit’s blood is pH 7.6 and the rabbit must keep it very close to this value because even slight deviations can have serious, sometimes fatal, consequences. The rabbit’s body temperature is largely constant at 37°C, regardless of whether it is winter or summer. The oak contains various sugars in its leaves and roots that are not found on the forest floor. And the flame also contains a hot gas cloud of organic substances and oxygen in its core, within which the combustion processes that maintain the flame take place.
Here too: The rabbit, the oak and the flame are living beings.
Rabbit:
Oak tree:
Flame:
Growth
Another criterion for life is growth. A rabbit begins its life as a small baby rabbit and grows into an adult rabbit through food intake and metabolism. The oak germinates from the acorn and, with a bit of luck, after centuries it becomes a strong and stable tree. The flame begins as a small spark and can grow into a gigantic fire storm under favourable conditions.
Here too: All three are living beings.
Rabbit:
Oak tree:
Flame:
Reproduction
Reproduction means that an original generation produces a successor generation. In rabbits, males and females mate and produce a litter with several young. In oak trees, pollen is transferred from the male flowers to the female flowers and the acorn produces a seed from which a new tree can grow. The flame of a fire can produce further flames through division and flying sparks.
According to this criterion, the rabbit, the oak and the flame are also living beings.
Rabbit:
Oak tree:
Flame:
Irritability
Irritability means that a living being reacts in some way to an external stimulus. A rabbit, for example, seeks out a cooler place on a hot day to prevent its body from overheating. An oak tree orients its newly formed shoots and leaves so that they can catch the sunlight in the best possible way. And a flame? I see no example of irritability in a flame.
According to this criterion, the rabbit and the oak are living beings, but the flame is not.
Rabbit:
Oak tree:
Flame:
Cells
All living things are made up of cells. These are the smallest delimited units in which the chemical reactions necessary for the maintenance of life take place. The rabbit’s body is made up of cells, which have a nucleus, a cytoplasm, a membrane and other components. Oak cells also have chloroplasts and cell walls. And in the flame? There are no cells here. The flame disappoints us again.
According to this criterion, the rabbit and the oak are living beings, but the flame is not.
Rabbit:
Oak tree:
Flame:
Evolution
All living things are subject to evolution. This means that descendants will always differ slightly in their characteristics from their ancestors. Over many generations, this can result in improved or worsened adaptations to the respective living conditions. In the case of rabbits and oak trees, evolution from one generation to the next is not immediately obvious. However, we can detect minute changes in the genetic material of both species, the consequences of spontaneous mutations and minute copying errors. Whether these changes affect the physical form or the functioning of the body must be considered on a case-by-case basis. However, these minor changes fulfil the criterion of evolution.
And the flame? The flame does not contain any kind of memory that determines its appearance or function. The flame will always look the same under identical initial conditions. The flame therefore does not fulfil the criterion of evolution.
Thus, the rabbit and the oak tree are also living beings in this respect, but the flame is not.
Rabbit:
Oak tree:
Flame:
Conclusion
The rabbit and the oak tree have passed all the tests for the existence of life. This is reassuring, because animals and plants are generally regarded as living beings. This means that the criteria we use to define life make perfect sense.
The flame fulfils some of the criteria for life, but not all. The flame is therefore an example of non-life.
Was that comprehensible? What do you think? Are these characteristics already suitable for distinguishing life from non-life? Do you know of any examples that cannot be clearly categorised? Let me know in the comments.
Further reading
Erwin Schrödinger: Was ist Leben? Piper Verlag, München 2001, ISBN 3-492-21134-8. (Überarbeitung der 2. Auflage der deutschsprachigen Ausgabe von 1951)
Manfred Eigen: Stufen zum Leben. Die frühe Evolution im Visier der Molekularbiologie. 3. Auflage. Piper Verlag, 1993, ISBN 3-492-10765-6. (Erstauflage 1987)
Christian De Duve: Blueprint for a Cell. The Nature and Origin of Life. Neil Patterson, Burlington, NC 1991. (mit Bibliografie)
Daniel E. Koshland: Special essay. The seven pillars of life. In: Science. Band 295, Nummer 5563, März 2002, ISSN 1095-9203, S. 2215–2216, doi:10.1126/science.1068489. PMID 11910092.
Noam Lahav: Biogenetics – Theories of Life’s Origin. New York 1999
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