Today the feature of DNA that defines the molecule is the fact that the two strands are entwined as a right-handed double helix. In common parlance, DNA is ‘the double helix’. While this double-helical character is not required by the base complementarity per se – a simple straight ladder structure would fulfill this function just as well – it does impart crucial physical and chemical properties to the polymer. It is these properties that play a major role in the biological function of DNA. The genetic functions of DNA can thus be understood as the synergism of two properties – a tape containing the information store encoding the sequences of proteins and RNA molecules and a polymer existing as double-helical string enabling the packaging, accessibility and replication of the information store. Crucially not only the coding of proteins and RNA molecules but also the physicochemical properties of the polymer are specified by the base sequence.

The evolution of biological systems as we understand them has been accompanied by a general increase in the complexity of their organisation. Comparing more complex organisms to simpler ones, this is apparent at many levels – a substantially increased amount of DNA, more polypeptides are required involved in the same task, a strong tendency to multicellularity, an increased sophistication of secondary information systems such as epigenetics and pheromones, mutual dependence and cooperation between disparate genomes, and of course, the evolution of a centralised information-processing device – the brain. Such progressions raise the issue as to whether the all increases in complexity are ultimately driven only by a DNA genome both containing more information and utilising this information more efficiently, or is such a perspective a reversal of the actual situation? Put simply, are there other, additional, sources of information that act as drivers of biological evolution and complexity? In other words, is it primarily an increase in total effective information content rather than just that in DNA that is responsible for the complex biological world in which we exist? If so, then a dominant role of DNA in specifying a system on which natural selection acts comes into question.

A central issue in modern biology is the question of the mechanism of evolution of social groups. Such groups are considered to be integral components of biological complexity (Mayr, 1963), and the discussion of this point has been one of the most animated – and often the most sterile – debates in biological thinking. It is often framed as a distinction between kin selection, driven by altruistic behaviour by members of a family group sharing a panoply of genes, and group selection where the cohesion of the group, and not necessarily genetic relatedness, is the driving factor.

At the heart of the central issue of the origin of life and genetic information lies Schrödinger’s simple question, ‘What is life?’ or more precisely, ‘What is the physical nature of life?’ In essence life is a highly complex carbon-based chemical system that is maintained far from chemical equilibrium by a constant influx of energy. The rates of the chemical reactions maintaining the flux of metabolites in a cell are enhanced, according to the Law of Mass Action, by the utilisation of high local concentrations. These can take the form of absolute high concentrations, the precise localisation of a reaction substrate at the reaction centre of an enzyme and the catalysis of successive reactions in a pathway by an array of catalytic centres, ordered on a physical platform – effectively a module acting as a factory. But which of these characteristics can reasonably be regarded as fossils reflecting the primitive origins of life chemistry and which simply conserve subsequent crucial innovations in life’s evolution? Evolution is of course a largely continuous and incremental process but is punctuated by ‘emergent’ events that may provoke a step change in evolutionary trajectory.

Many discussions of the biological world use the terms complex or complexity. But what is the real sense of these words? Conceptually the usage of complexity is slippery and is often used synonymously with diversity. In any system consisting of many components, its diversity, in the sense used in this book, is a measure of the number of different types of entity present and does not, on its own, necessarily imply any interactions between them. The components of a system could be entirely independent. However, complexity is a different beast and is admirably exemplified by biological systems. In a complex system the different components interact with, and influence the behaviour of, other components of the system. In this sense the complexity of a system is a measure of a functioning whole.

Almost by definition, increases in biological complexity, whether at the molecular or cellular levels, or even at the level of species diversity, imply an increase in the effective information content of DNA in the system as a whole. An important question, which goes to the heart of the concept of biological complexity, is to ask to what extent any diversity-associated increases in information content are essentially discrete or are part of a continuum in which the evolution of complex biological systems is linked to incremental accretions of DNA information.

Schrödinger’s solution to the question, ‘What is life?’ was to postulate that ‘information’ in the genetic material, characterised as the ‘codescript’, was essential for the thermodynamic minimisation of entropy (Box 1.3) accompanying the increasing organisation of biological systems. This critical insight paved the path for understanding the mechanisms of biological evolution. But how would a ‘codescript’ potentially lead to an increase in organisation? A simple, although not exact, analogy would be a small person given a pile of (appropriate) Lego bricks and asked to build a model of a space rocket. With no knowledge of how the rocket is put together, this would be an essentially impossible, or at the very least highly improbable, task, akin to the thousand monkey scenario (Chapter 1). However, given a book of instructions, such small people would – and, by observation, do – make short shrift of the problem. These instructions thus facilitate the construction of a specific complex structure and in so doing dramatically increase the probability of success. Similarly, without appropriate instructions ‘all the king’s horses and all the king’s men’ failed to put the smashed Humpty Dumpty together again. Again they lacked the information to transform an essentially irreversible event – with its statistically highly improbable reversal – into an organised structure (for an excellent discussion on this point see Penrose [2010]).

The perennial question – what is life? The simple answer is that life, either considered in the totality of all its incredible diversity or even in the context of an individual organism, is a highly complex chemical system with a capacity for self-reproduction. But what fuels this system, and what drives the evolution of such extreme apparent complexity? The principle underlying the answer to the first question was initially propounded by Ludwig Boltzmann, the nineteenth-century physicist and natural philosopher. Boltzmann had a tremendous admiration for Darwin and suggested, ‘Available energy is the main object at stake in the struggle for existence and the evolution of the world’.

In the latter part of the eighteenth century William Bateson (1894) asserted as a central tenet that ‘Variation, in fact, is evolution’. By conflating the then novel science of genetics with established studies of biological variation – notably those of John Stevens Henslow (the first director of Cambridge University Botanic Garden and likely ‘the father of variation’) and his better-known student, Charles Darwin, as well as those of Alfred Wallace – Bateson argued that variation, whatever its source, is a central driver of evolution and for biological evolution genetics – and hence DNA – plays a major role.