In these pages we have begun to look at the organisation of the human body and its development before birth. With this knowledge, we should now feel more confident in making a distinction between what is alive and what is non-living. Certainly, if we are studying the human body with a view to practicing in one of the healthcare professions, we shall need from time to time to examine and clarify our beliefs about life, death, and the non-living.
As a biologist I have studied life for many years now, and although I have an intuitive grasp of what it is, as do most people, I must admit that I am still unable to provide an unambiguous and cogent definition of life that can clearly distinguish between living and non-living systems. As we gain information about living systems at an ever-increasing rate, we come across more and more examples of objects that challenge our ideas about what life may be. Added to that, we are now realising that our planetary system is just one amongst many - it now seems likely that most stars have planets orbiting them - so it seems most unlikely that life as we know it here on Earth is the only form of life in the universe. So this adds another requirement to our definition of life - will it be applicable to other worlds?
Most definitions of life are descriptive, suggesting for example that living things will demonstrate the following properties: cellular organisation (one or more cells), homeostasis (maintenance of a relatively stable internal environment), metabolism (chemical and energetic transformations), adaptation to the environment, response to stimuli, growth, and reproduction. These are the qualities of life you will often find listed in a textbook definition. To remind us of the way that words are given meaning through consensus, we can put it the other way round - if we come across something that exhibits these properties, than we shall apply the label 'life'.
Let's see if a definition like this works in every day experience. Here's an example you can try for yourself. A supermarket carrot taken from the fridge after being there for a week - is it alive or dead? We can put it to the test quite simply by slicing the top 1 cm from a carrot and placing it base down in a saucer of water. (I remember my mother showing me this when I was a toddler.) As time goes by, the carrot slice will develop leaves, roots, can be potted on, and eventually will flower after several weeks, decisively answering the question “is it alive?” affirmatively.
Another interesting example is a thundercloud. This has a fascinating ‘life history’ - solar energy warming the ground, a rising column of warm, moist air, condensation of the water vapour in the air, formation of ice particles which fall and generate a downdraft, electrical charge separation by the falling ice particles producing a potential difference between the base of the cloud and the ground, followed by lightning discharges and rain, and then gradual dissolution of the cloud and its powers. According to standard thermodynamics, things like this shouldn’t really happen - electrical energy should degrade to heat energy, and not the other way round, for example. Charge separation occurs because there is an existing electrical gradient in the atmosphere, a vestige of previous thunderstorm activities, so the thundercloud we are considering has interacted with information left by previous generations - a form of inheritance. Solitary thunderclouds sometimes congregate to form ‘supercells’ which have a much longer and more vigorous ‘lifespan’ than solitary clouds, thus showing something of a social capacity. I don’t think many people would consider a thundercloud to be alive, but it is interesting when you apply standard definitions of life to the cloud - it passes many of these tests.
Let's test the definitions with subtler examples. There can be ‘degrees of life’ even within a single organism. So for example, there are structures such as hair, nails, outer epidermal layers, bone matrix, and tooth enamel which in themselves are not ‘living’ but which form essential components of a living organism. A mature tree has an inner scaffold of now-dead wood encased in a thin skin of living tissue. Then, when we look at the cellular building blocks of an organism, there seem to be different levels of completeness and activity. For example, red blood cells have an outer cell membrane and are packed with haemoglobin molecules, but not much else. During their formation, the red cells have disposed of their genome and most of their metabolic systems to enable the packing-in of more haemoglobin. So a red cell in the circulation can do very few of the things that we normally expect of cells. Is it alive? I can’t answer that, but I believe an analysis of the remaining dynamical organisation might give a reasonable answer. Then there are platelets, fragments of megakaryocyte cytoplasm, circulating in the blood - are they alive? Frozen cells, frozen embryos - are they alive? They can be alive before and after freezing, but are they still alive when frozen?
Are viruses alive? This question has been discussed extensively, but it remains impossible to answer decisively. Viruses posses genetic information, and reproduce by commandeering the metabolic processes of a host cell. However, viruses of themselves do not metabolise.
Many definitions of life focus on the properties of individual organisms, but it is worth remembering that organisms form an interdependent web of life on this planet, and cannot survive in isolation. Together, living things help to maintain the necessary conditions for life on this planet (the Gaia hypothesis). The processes of reproduction and evolution help to sustain the balances between living and inorganic systems, so perhaps our definition of life should include these qualities too.