Life is not a discrete entity, it needs to be defined. Whilst on initial inspection we can say that a dog is alive whilst a rock is not, when getting to the level of viruses, fungi, bacteria and other single cell organisms the distinction becomes blurry. Accepted definitions of life require the following attributes: organisation, […]
Life is not a discrete entity, it needs to be defined. Whilst on initial inspection we can say that a dog is alive whilst a rock is not, when getting to the level of viruses, fungi, bacteria and other single cell organisms the distinction becomes blurry. Accepted definitions of life require the following attributes: organisation, metabolism, growth, adaptation, response to stimuli, and reproduction.
Organisation is where an organism is made up of one or more cells. A cell is the fundamental building block to life. Cells are self-contained replicating units, from single cell bacteria to multi-cellular humans. All cells contain the ability of reproduction through cell division, the ability to divide into a near exact copy.
Metabolism, whether at a cellular or multi-cellular level, is the ability for an organism to take matter (photons in the form of photosynthesis, nutrients from other organisms, minerals from the environment) and convert it into organic matter (cells) that are useful to them (synthesis) and converting there own organic matter to inorganic matter through waste (catalysis).
Growth is where the rate of synthesis exceeds the rate of catalysis, so an organism become progressively larger.
Adaptation is where an organism is able to change (adapt), either by it’s own volition through modifying behaviour, or more commonly through heredity.
Response to stimuli is perhaps the most telling factor of whether an organism is alive. A rock rolling down a hill is not reacting to gravity, it is under the force of it. Plants rotate their leaves to follow the sun, animals experience physiological change when seeing prospective food, feeling the cold, mating etc.
Reproduction is the final common attribute. Living organisms must have an ability to reproduce themselves, whether it is from a single parent such as cell division (asexual) or two different parent organisms (sexual).
It is important to note that whilst these definitions in combination are a good way of describing life, there are many exceptions to the rules. When a horse and a donkey are cross-bred the offspring, called a mule, is infertile and incapable of bearing their own offspring. No one would doubt that a mule is alive. Conversely, most biologists actually regard viruses as not-alive as they do not move, metabolise or undergo waste (catalysis). Viruses require living organisms to survive, infecting their cells and using their hosts cellular makeup to propagate themselves.
Computer viruses which can be programmed to replicate and adapt to surroundings would not be classified as alive, yet raises a pertinent question, in line with the theme of artificial intelligence in the scientific novel ‘Do androids dream of electric sheep?’ by Philip K. Dick, as to when machines will be classified as alive.
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