I recently took a course in cosmology in the hope of trying to fill some voids in my knowledge. We were spared all the mathematics that are needed for a comprehensive understanding, but the excellent tutor certainly put across key points on the age of the universe, its size, its changes through time, and the features of some of its components. My knowledge remains superficial, but it is enough for me to realise that it is impossible to get any real sense of the distances and times involved and humans will never be able to do this as the numbers are so large that we cannot compare them with with anything familiar to us. Another impression that the cosmology course left with me was that some explanations of events and structures are close to being science fiction.
The topic that created most enthusiasm among members of the class was discussion of the origin of life and whether living organisms exist elsewhere. The desire to find organisms has extended to looking for chemicals that may be involved in their make-up and we also make projections about possible habitats; a popular one being to equate finding water on other planets, or in other parts of the Universe, as being tantamount to finding living organisms. We are interested especially in the possibility of intelligent life, with which we can communicate, providing us with a feeling that we are not alone.
It was clear that most of us in the cosmology class had varying views on what we meant by life, with most thinking that it was only possible in a distinct organism, perhaps consisting of one cell, perhaps of several cells. So, is there an agreed definition? As a starting point, I read Schrödinger’s What is Life?, based on a series of lectures he gave at Trinity College Dublin in 1943 . Schrödinger’s discourse detailed the way in which cells, and thus multicellular organisms, were controlled by the physics and chemistry of genes (although the mechanism of control was not known until the structure of DNA was discovered 10 years later). I would like to quote two sections :
What is the characteristic feature of life? When is a piece of matter said to be alive? When it goes on ‘doing something’, moving, exchanging material with its environment, and so forth, and that for a much longer period than we would expect of an inanimate piece of matter to ‘keep going’ under similar circumstances.
The unfolding of events in the life cycle of an organism exhibits an admirable regularity and orderliness, unrivalled by anything we meet in inanimate matter. We find it controlled by a supremely well-ordered group of atoms, which represent only a very small fraction of the sum total in every cell. Moreover, from the view we have formed of the mechanism of mutation we conclude that the dislocation of just a few atoms from within the group of ‘governing atoms’ of the germ cell suffices to bring about a well-defined change in the large-scale hereditary characteristics of the organism.
I am comfortable with Schrödinger’s view that life is a feature of organisms and that the biology of organisms is under the control of what we now know to be DNA. However, we still lack a definition of life and, for this, I turned to Pincock and Frary's The Origins of the Universe for Dummies. Having reviewed complexity, metabolism, development, autonomy and reproduction, the authors write :
Taking all the elements we describe in the previous sections, and all their shortcomings, into account, we can come up with a simple, one-sentence definition of life. (Of course, scientists tell you that this description isn’t perfect, but it serves as a rough definition, at least for now.)
Here’s the working definition, which is sometimes known as the NASA definition of life: Life is a self-sustaining chemical system capable of Darwinian evolution [my emboldening].
There is no mention of organisms here and it is a definition that is difficult for all to accept, as Pincock and Frary anticipated.
It has been pointed out that there is unlikely ever to be agreement on a definition of life. Carol Cleland and Christopher Chyba write :
The philosophical question of the definition of ‘life’ has increasing practical importance. As science makes progress towards understanding the origin of life on Earth, as laboratory experiments approach the synthesis of life (as measured by the criteria of some definitions), and as greater attention is focused on astrobiology and the search for life on Mars and Jupiter’s moon Europa, the utility of a general definition grows. In particular, definitions of ‘life’ are explicit or implicit in any remote in situ search for extraterrestrial life.
Is science making progress towards understanding the origin of life on Earth and do laboratory experiments approach the synthesis of life? I question whether this is so, but, as Cleland and Chyba point out, it depends on the definition used. Does anyone consider DNA to be alive? Isn’t there an elusive essence to life that involves more than the interaction of molecules? Isn’t that what Schrödinger implied?
Cleland and Chyba’s Abstract is worth quoting :
There is no broadly accepted definition of ‘life’ Suggested definitions face problems, often in the form of robust counter-examples. Here we use insights from philosophical investigations into language to argue that defining ‘life’ currently poses a dilemma analogous to that faced by those hoping to define ‘water’ before the existence of molecular theory. In the absence of an analogous theory of the nature of living systems, interminable controversy over the definition of life is inescapable.
How true. One reason that I left a University astrobiology research group was because we all talked about life, but there was no consensus on what we meant. I couldn’t sit back and listen to colleagues talking about the origin of life when they really meant the first appearance of RNA and DNA. I had similar problems when listening to them, and others, saying that life probably originated in hydrothermal vents; that extremophile microorganisms are likely to be found in extraterrestrial habitats with similar harsh conditions to those where these extraordinary organisms live on Earth; etc. While it would be wonderful to find living cells, or conclusive evidence of living cells having been present, on moons or planets elsewhere in the solar system (or elsewhere in the Universe), I’m not expecting such discoveries to be made, as I remain convinced that the first living cell formed on Earth in a once and once only event. Isn't that the best working hypothesis until we can prove otherwise?
 Erwin Schrödinger (1944) What is Life? https://docs.google.com/file/d/0B9bX852JMJ__NDdlZGIxNTctOWUyMS00MjE1LTg4YWYtZWQ5NzY4YzA5NzNh/edit?ddrp=1&pli=1&hl=en
 Stephen Pincock and Mark Frary (2007) The Origins of the Universe for Dummies. Chichester, John Wiley & Sons.
 Carol E. Cleland and Christopher F. Chyba (2002) Defining ‘Life’. Origins of Life and Evolution of the Biosphere 32: 387-393.