As it is nowadays commonly accepted, based on astrophysics data (in particular the Kepler space mission data), our astrophysics colleagues arrived at the conclusion that there are several hundred exo-planets (see for instance Figure 2), where life in principle may be possible. Difficult, and till now impossible, to prove experimentally. But let us assume that life may exist in some of these planets – and that somedays our astronauts are going there. How should they demonstrate that life, in a form which may not be familiar to us, exists?

They should have a “definition of life” to start with, and use it, to discriminate life from non-life. There is in fact what in the literature is known as “the NASA definition of life”. Originally, this was simply an operational perspective by the Exobiology Program within the National Aeronautics and Space Administration – a general working definition – apparently originally proposed by Horowitz and Miller in the far 1962. However, people working in the field of the origin of life began to use this definition as a definition of life, the life to search for:

Life is a self-sustained chemical system capable of undergoing Darwinian evolution.

This definition is the most popular, even today, in the field of the origin of life, and probably is so popular because it is simply a tautology. If NASA astronauts are going to find in a distant planet a colony of bacteria that behave just like the terrestrial ones, they are going to hastily communicate back that yes, they have found life!

The term “Darwinian” is particular arguable. The term refers to a population, has no meaning for a single item in isolation. If our NASA astronauts meet with a single carnivore monster, they may finish up devoured before they are able to find a corresponding population to study their evolution mechanisms. Victims of a wrong definition of life. In fact, the Darwinian mechanism is a consequence of life, not its necessary condition. As already mentioned, the popularity of the NASA definition lies in an obvious tautology – life is our cellular life, based on nucleic acids and their replication mechanisms.

To understand better this kind of challenge, let us address this question in the reverse way Let us assume, namely, that some alien being, let us call him the Green Man, comes on our planet from a distant world, and wishes to understand what life on Earth is. He has received from his scientists a list of terrestrial things about which they are in doubt as to whether these things are alive or not. Our Green Man is supposed to descend on Earth and clarify the situation. Let us see what happens. He encounters an intelligent but scientifically naive farmer to whom he asks the question, showing the long list. The farmer very rapidly divides the items of the Green Man into two lists (Figure 1) a list of living and one of non-living things. The Green Man is surprised by the rapidity by which the farmer has done such a discrimination and asks him how he did it: he wishes to know the quality which characterizes all living things (left-hand side), and which is not present in items of the right-hand side.

When the farmer, pointing at the mule, says “movement” and “growth,” the Green Man nods reservedly, as the tree or the coral in the same living list do not move about, nor show any appreciable sign of growth in a reasonably long observation time; conversely, a small piece of paper moves in the wind and the moon moves and grows periodically. When the farmer then gives “reaction to stimuli” as an alternative criterion, the Green Man again nods unconvinced, as the mushroom and the tree seem insensitive to a needle; and on the other hand, the computer and the radio easily become ineffective upon interference with a stick.

“Living things” – adds the farmer who begins to get irritated – “are up taking food and by doing so are able to perform their functions by consequent production of energy. Energy is transformed into action.”

But the Green Man indicates the car and the robot, which are able to move about by doing precisely that – converting energy into action. “Reproduction!” cries the farmer. “All items in this list are able to reproduce themselves!”. “It seems to me that reproduction is a property of life, not its cause. Isn't so, that in order to reproduce, you have first to be alive? Furthermore, reproduction is not possible for the mule, which is unable to reproduce,” scorns the Green Man. “Nor is for all your babies, or old people.”

The farmer gets more and more angry, but then he arrives at a kind of enlightenment. He looks at a tree and realizes that it loses leaves and fruits in winter, but generates them again in the spring – from the inside. He looks at the sheep which he just shaved to get wool, and sees, that new wool is growing again on the animal – from inside its body. The same, actually, with his own beard: he cuts it and it grows back again – and it comes due to an activity from inside the body! Again, this growth comes from inside his body! He concludes – and tells the Green Man that in all elements of the “living” list there are internal processes that continuously rebuild from the inside the structure itself. Living organisms are then characterized by an activity that maintain and regenerates themselves!

The Green Man, this time, nods positively. The farmer has finally articulated the quality that discriminates the living from the non-living! The robot, computer, radio, moon, and so on, are not able to regenerate themselves from the inside. If a part of the radio breaks, the radio itself is not going to build it again. However, all items on the left-hand side of the table do have this quality: they utilize external energy to maintain their own structure, and have the ability to regenerate it from within the structure itself. This seems to be the property of life that one is looking for.

The Green Man now draws a figure on the ground (Figure 4). In this figure, which represents something that is open to the medium outside, S represents a component of the living system, which is being transformed into a product P. But the system is able to regenerate S by transforming the entering food A into S again. Actually, the Green Man is rather happy about all this. So, accordingly, he and the farmer make up the following “operational” definition of life: a system can be said to be living if it is able to transform external matter/energy into an internal process of self-maintenance and production of its own components (see Figure 3).

Notice that the two have arrived at a “definition” of life by using macroscopic, common-sense observations. Such a simple definition might have been derived by laymen of a couple of centuries ago; you do not need molecular biology or cellular biology for that; however, as it is easy to see, it is also valid for the description of cellular life.

Introducing autopoiesis

Thus, the Green Man and the farmer arrived at an agreement about what distinguishes the living from the non-living. The farmer was ignorant of biology, otherwise he would have answered from the very beginning that all living things are made up of cells – that this is the most discriminating factor. However, the Green Man would have then asked: “What is a cell and why do you call a cell living?”

In fact, the life of a cell is the starting point for the development of the ideas of autopoiesis (from the Greek auto, or self, and poiesis, producing) developed by Maturana and Varela (see references). Autopoiesis deals with the question “what is life?” and attempts to isolate, above and beyond the diversity of all living organisms, a common denominator that allows for discrimination between the living and the non-living. Autopoiesis is not directly concerned with the question of the origin of life; rather, it is an analysis of the living as it is – here and now.

Bacterial cells of Figure 5 (Panel A) are organisms much smaller than one millimeter – something apparently small and simple, but the reality of a cell, when seen with the zoom of a biochemist, is something with terrific complexity: see Figure 5 (Panel B), which represents only part of the metabolic network of the bacterium E. coli, which dwells in our guts. Each point represents a chemical compound, each line a chemical reaction, namely a transformation which is catalyzed by a specific enzyme – a large protein. Where is, in this maize, life localized?

Well, life is not localized, not in that bacterium, not in the elephant: life is the entire network of reaction, life is a global property (see Capra and Luisi, 2014; Luisi, 2016).

Probably, our Green Man understand this fully. But he may ask: where is the environment in all this? Can life on Earth exist without the environment, which furnishes nutrients and energy? Of course, the answer is that each living system is a thermodynamically open system, capable of in taking specific compounds as nutrients, and capable of expelling waste material – as illustrated in the Figure 6.

This environment also includes, necessarily, all other living organisms. Our farmer is linked to his family, and they have planted the seed of the tree, and this is part of the entire forest, in turn alimented by the sun, the water, the clouds, the entire atmosphere, which means by the all life of Gaia, which is in turn part of the cosmos. So that the life of a single organism is linked and part of the life of the whole planet and the whole world. This is the message that the Green Man can take home.

F. Varela, Principles of Biological Autonomy, North Holland, Elsevier, 1979. H. Maturana and F. Varela, The Tree of Knowledge, revised, Shambala 1998.
F. Capra, P. L. Luisi, The Systems View of Life, Cambridge Univ. Press, 2014.
P. L. Luisi, The Emergence of Life, 2nd Edn., Cambridge Univ. Press, 2016.