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. Here are some definitions of life:
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 (Eric Chaisson 2001).
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 (Margulis & Sagan 2002).
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 (Daniel Koshland).
Life depends on the availability of free energy. Without an input of free energy or 'negative entropy' (see below), all processes would tend to take a system towards a state of entropic death. Intake of food keeps an organism alive by providing negative entropy. The complex molecules constituting food are full of free energy or negative entropy, which is derived ultimately from the Sun.
In 1943-1944 Schrödinger wrote a little book What is Life. 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 the 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, but highlighted an important property of life by invoking the idea of NEGATIVE ENTROPY. He characterized living matter as that which stays alive (‘evades the decay to equilibrium’) by feeding on negative entropy or negentropy.
How can entropy be negative? How does living matter feed on negentropy, and what is the source of this negentropy?
The answer to the latter question is that the Sun has been bombarding our ecosphere with low-entropy or ‘high-grade’ energy. Why ‘low-entropy’? Recall that differential entropy is defined as dS = dQ/T (cf. Part 6), and the average value of temperature T for the Sun is huge compared to that of the Earth. On entering our ecosphere, the energy of the photons coming from the Sun gets degraded to a large extent through the processes of ‘thermalization,’ namely dissipation into a state of much lower average temperature (and therefore a correspondingly high value for the entropy).
A small fraction of this energy, however, gets trapped as free energy. It is stored in our ecosphere in the form of simple or complex molecules. Some of the energy-rich simple molecules in which the free energy from the Sun gets stored are: H2S, FeS, H2, phosphate esters, HCN, pyrophosphates, and thioesters.
When food is consumed by a living organism, its processing by the organism builds up a high information content (negative entropy) for the organism. Boltzmann-Gibbs entropy S is the same as missing information I. This means that –S (negentropy) is the same as available information (cf. Part 22).
Until 1944 most scientists were of the view that genetic information was carried by the proteins of the chromosome. Schrödinger’s book, 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 important, 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.