(Note: All previous parts of Dr. Wadhawan’s series on complexity can be accessed through the Related Posts list at the bottom of this article.)
We live only to discover beauty. All else is a form of waiting. Khalil Gibran was happy describing life like this, but scientists have a lot of trouble defining it succinctly and comprehensively. It is not easy to give a crisp definition of life, just as it is not easy to define complexity in a context-independent and unique way. Perhaps there is no clear dividing line between life and nonlife. Nevertheless, the emergence of what many of us intuitively understand to be life marked a major milestone in the evolution of complexity in our world. I survey some of the scientific attempts at defining life, as a prelude to discussing the likely mechanisms for the origin of life in a future article in this series.
10.1 What is Life?
Here are a couple of descriptions of life. Eric Chaisson (2001) first:
But what is life? Like time, life is obvious to discern yet elusive to define. Although most biologists generally skirt the issue, we suggest that our very essence can be defined as follows: Life is an open, coherent, spacetime structure maintained far from thermodynamic equilibrium by a flow of energy through it – a carbon-based system operating in a water-based medium, with higher forms metabolizing oxygen.
Margulis and Sagan (2002) next:
Life does not exist in a vacuum but dwells in the very real difference between 5800 Kelvin incoming solar radiation and 2.7 Kelvin temperature of outer space. It is the gradient upon which life’s complexity feeds.
The origin of life (as also consciousness) is the most dramatic of all emergent phenomena in nonlinear open systems. But it has not been easy to define life the way we define so many other things in science. For every characteristic believed to define life, people have come up with an example from the world of the nonliving which also possesses that characteristic. In fact, as we make further progress in the development of sophisticated ‘artificial’ smart structures, including truly smart or intelligent robots, the distinction between the living and the nonliving will get more and more blurred. I have discussed these things in my book on smart structures, and also in an article on robots of the future.
Daniel Koshland is an ex-Editor-in-Chief of the prestigious magazine Science. He attended a conference focused on the vexing question of defining life. He recounts that, after considerable discussion, somebody formulated the essential characteristic of life as ‘the ability to reproduce.’ There seemed to be general consensus on this, till somebody said: ‘Then one rabbit is dead. Two rabbits – a male and female – are alive but either one alone is dead.’ Similarly, a mule must be a dead entity by this definition. Nevertheless, many of us still have, at least intuitively, an idea of what is living and what is nonliving. Koshland has come up, reluctantly, with the following short definition of life: A living organism is an organized unit, which can carry out metabolic reactions, defend itself against injury, respond to stimuli, and has the capacity to be at least a partner in reproduction. But he is happier giving a large set of criteria for deciding what constitutes life. He calls them the seven pillars of life, like the pillars of a Greek temple.
10.2 The Seven Pillars of Life
I asked the rose how long
was its life,
The bud heard and
Mir Taqi Mir
Koshland’s (2002) seven ‘pillars’ of life are the essential thermodynamic and kinetic principles which enable a living system to operate and propagate. The mechanisms listed by him are with reference to life as we know it on our Earth. The same seven principles may involve other mechanisms for other forms of life, or for life elsewhere. The acronym PICERAS introduced by Koshland has seven letters, corresponding to the seven principles or pillars of life: program; improvisation; compartmentalization; energy; regeneration; adaptability; seclusion.
The first pillar of life is a program that describes the ingredients themselves, as well as the kinetics of the interactions among the ingredients. For life on Earth, this program resides in the DNA molecules that encode the genes.
Improvisation is the second pillar of life, and is to be distinguished from another pillar, namely adaptability, discussed below. Both involve a response to change. The difference is in time scales. Adaptability is about direct response to quick changes, and does not entail a change of the genetic program of the organism. Improvisation is about gradual change (evolution) in response to long-term changes in the environment.
A living organism depends on the reaction kinetics of its ingredients. The kinetics requires certain concentrations of the chemicals involved, and that requires confinement in a ‘container’. Therefore the third pillar of life is compartmentalization, namely the presence of a membrane or skin that confines the organism to a certain volume. In fact, large organisms have several compartments (organs), because the concentration requirements of different organs are different.
Energy is the fourth pillar of life on Earth. Without a steady input of energy, a living system would soon approach a state of equilibrium and death.
Regeneration (including reproduction) is the fifth pillar of life. There are always some thermodynamic losses and wear and tear as a living organism is sustained by the myriad chemical reactions going on inside it. Food and other intakes are one means of ensuring that the organism does not degenerate or degrade inexorably with time. Several organs of the human body (e.g. the heart) have a mechanism for tissue regeneration. However, over time, aging effects become too strong and the organism dies. To ensure that the species can still survive, Nature has evolved replication (cell division) and reproduction (which amounts to regeneration by starting all over again through the progeny) as an essential pillar of life.
A living organism may face a variety of sudden hazards. Adaptability is therefore the sixth pillar of life. For example, a living system must quickly move away from an environment that is too hot for its well-being and survival.
The seventh pillar of life listed by Koshland is what he calls seclusion. In a living cell there exist a large number of chemical reaction pathways, all simultaneously active. Natural evolutionary processes have ensured that the enzymes catalyzing the various reactions have specific shapes and reactivities, so that the different reactions do not interfere with one another. The specificity of an enzyme provides a high degree of seclusion to the relevant chemical reaction occurring in the living cell. Various feedback and feedforward mechanisms also ensure that the specificity is not completely unchangeable, but changes only under certain special signals.
The existence of many of Koshland’s pillars of life can be ultimately traced back to the DNA molecule, portions of which constitute the genes. DNA is a large molecule with very high information content. Life is information. Therefore there is a direct link between life and free energy (cf. Part 4). Without an input of free energy or negative entropy, all processes would tend to take a system towards a state of entropic death (cf. Part 6). Intake of food keeps an organism alive by providing negative entropy. As Szent-Györgyi (1957) said, ‘We need energy to fight against entropy’. The complex molecules constituting food are full of free energy or negative entropy, which is derived ultimately from the Sun.
10.3 Schrödinger and Life
Life is a partial, continuous, progressive and conditionally interactive self-realization of the potentialities of atomic electron states.
J. D. Bernal
The Nobel laureate Erwin Schrödinger made a profound discovery in 1927 by showing that the discrete energy states of matter are determined by wave equations. He became one of the founders of modern science, best known for the famous wave equation in quantum mechanics, named after him:
Here is an equivalent formulation of the Schrödinger equation:
Here Ĥ is the so-called Hamiltonian operator. The Hamiltonian H is defined as the sum of the kinetic and potential energies of the system. This equation determines all phenomena in our world, subject to the constraints of the first and the second laws of thermodynamics. How did Schrödinger arrive at this very basic equation? Here is Richard Feynman’s answer: ‘Where did we get that [Schrödinger’s equation] from? It’s not possible to derive it from anything you know. It came out of the mind of Schrödinger.’
In 1943-1944 Schrödinger wrote a little book What is Life: The Physical Aspect of the Living Cell. This is how Roger Penrose described this book (in 1991): ‘… which, as I now realize, must surely rank among the most influential of scientific writings in this century. It represents a powerful attempt to comprehend some of the genuine mysteries of life, made by a physicist whose own deep insights had done so much to change the way in which we understand what the world is made of. … Indeed, many scientists who have made fundamental contributions in biology, such as J. B. S. Haldane and Francis Crick, have admitted to being strongly influenced by (although not always in complete agreement with) the broad-ranging ideas put forward here by this highly original and profoundly thoughtful physicist.’
It is important to realize that when Schrödinger wrote his book, the atomic structure of DNA was not known (it was determined later by Watson and Crick). I quote Freeman Dyson (1985): ‘Schrödinger’s book was seminal because he knew how to ask the right questions. The basic questions which Schrödinger asked were the following: What is the physical structure of the molecules which are duplicated when chromosomes divide? How is the process of duplication to be understood? How do these molecules retain their individuality from generation to generation? How do they succeed in controlling the metabolism of cells? How do they create the organization that is visible in the structure and function of higher organisms? He did not answer these questions, but by asking them he set biology moving along the path which led to the epoch-making discoveries of the subsequent forty years: to the discovery of the double helix and the triplet code, to the precise analysis and wholesale synthesis of genes, and to the quantitative measurement of the evolutionary divergence of species.’
How did Schrödinger define life? He avoided giving a direct definition of life, but highlighted an important property of it by invoking the idea of negative entropy, which I have outlined in Part 4 of this series of articles (also see Part 3 for a fuller description). He characterized living matter as that which stays alive (‘evades the decay to equilibrium’) by feeding on negative entropy or negentropy. Karl Popper did not agree: ‘Now admittedly organisms do all this. But I denied, and I still deny, Schrödinger’s thesis that it is this which is characteristic of life, or of organisms; for it holds for every steam engine. In fact, every oil-fired boiler and every self-winding watch may be said to be “continually sucking orderliness from its environment”. Thus Schrödinger’s answer to his question cannot be right.’
10.4 Artificial Life
Life and artificial? Is that a contradiction in terms? Not at all. Christopher Langton is the originator of this subject. The term artificial life (AL) was coined by him around 1970. AL is ‘. . an inclusive paradigm that attempts to realize lifelike behaviour by imitating the processes that occur in the development or mechanics of life.’
In the field of AL, one uses computers to model the basic biological mechanisms of evolution and life. In abstracting the basic life processes, the AL approach emphasizes the fact that life is not a property of matter per se, but the organization of that matter. The laws of life must be laws of dynamical form, independent of the details of a particular carbon-based chemistry that happened to arise here on Earth. It attempts to explore other possible biologies in new media, namely computers and robots. The idea is to view life-as-we-know-it in the context of life-as-it-could-be.
In conventional biology one tries to understand life phenomena by a process of analysis: We take a living community or organism, and try to make sense of it by subdividing it into its building blocks. By contrast, AL takes the synthesis or bottom-up route. We start with an assembly of very simple interacting units, and see how they evolve under a given set of conditions, and how they change when the environmental conditions are changed. One of the most striking characteristics of a living organism is the distinction between its genotype and phenotype. The genotype can be thought of as a collection of little computer programs, running in parallel, one program per gene. When activated, each of these programs enters into the logical fray by competing and/or cooperating with the other active programs. And collectively, these interacting programs carry out an overall computation that is the phenotype. The system evolves towards the best solution of a posed problem. By analogy, the term GTYPE is introduced in the field of AL to refer to any collection of low-level rules. Similarly, PTYPE means the structure and/or behaviour that results (emerges) when these rules are activated in a specific environment.
What makes life and brain and mind possible is a certain kind of balance between the forces of order and the forces of disorder. In other words, there should be an edge-of-chaos existence. Only such systems are both stable enough to store information, and yet evanescent enough to transmit it. I shall return to this theme in the article on the origins of life.
Life is not just like a computation, in the sense of being a property of the organization rather than the molecules: Life literally is computation. And once we have made a link between life and computation, an immense amount of theory can be brought in. For example, the question ‘Why is life full of surprises?’ is answered in terms of the undecidability theorem of computer science, according to which, unless a computer program is utterly trivial, the fastest way to find out what it would do (does it have bugs or not) is to run it and see. This explains why, although a biochemical machine or an AL machine is completely under the control of a program (the GTYPE), it still has surprising, spontaneous behaviour in the PTYPE. It never reaches equilibrium.
The computational aspect of the AL approach invokes the theory of complex dynamical systems. Such systems can be described at various levels of complexity, the global properties at one level emerging from the interactions among a large number of simple elements at the next lower level of complexity. The exact nature of the emergence is, of course, unpredictable because of the nonlinearities involved.
Here are some websites devoted to artificial life and virtual worlds:
With the advent of artificial life, we may be the first creatures to create our own successors. . . If we fail in our task as creators, they may indeed be cold and malevolent. However, if we succeed, they may be glorious, enlightened creatures that far surpass us in their intelligence and wisdom. It is quite possible that, when conscious beings of the future look back on this era, we will be most noteworthy not in and of ourselves but rather for what we gave rise to. Artificial life is potentially the most beautiful creation of humanity.
Doyne Farmer and Alletta Belin
10.5 Concluding Remarks
Definition of life is problematic, particularly at the level of bacteria. This is partly linked to the question of what exactly we mean when we use the word ‘species’ for a bacterium.
Fresh challenges to what we understand by the term ‘life’ will also arise when the fields of artificial life and super-intelligent robots come of age. I shall discuss artificial evolution in a separate article.
Until 1944 most scientists were of the view that genetic information was carried by the proteins of the chromosome. Schrödinger’s 1944 book What is Life?, apart from invoking negative entropy for the sustenance of life, introduced new concepts for the genetic code. It inspired Watson and Crick to investigate the gene, which led to their discovery of the double-helix structure of DNA. In their 1953 paper they wrote: ‘It has not escaped our notice that the specific pairing [of the two strands of DNA] we have postulated suggests a possible copying mechanism for genetic material.’ This sudden blaze of understanding laid bare the inside story of heredity, and of present-day life itself.
What is even more relevant for the stated objective of the present series of articles, Schrödinger argued that life is not a mysterious or inexplicable phenomenon, as some people believe, but a scientifically comprehensible process like any other, ultimately explainable by the laws of physics and chemistry.
The good life is one inspired by love and guided by knowledge.