Life is the property or quality that distinguishes living organisms from dead organisms and other inanimate matter. But this simple description omits to say how to make the distinction. Some life forms seem inanimate at first sight, for example spores or slime. And continued argument about how to decide exactly when human death occurs, with such concepts as brain death replacing earlier concepts such as cessation of heartbeat or breathing, shows that the criteria for being alive are uncertain.
A common biological definition is that living organisms possess four properties:
* metabolism – using material and energy within the body to support continued functioning;
* reproduction – producing, from within the bodies of living parents, new separate organisms that become similar to their parents;
* growth – increasing in size from infant to adult;
* response or adaptation to the environment – taking action needed for metabolism, growth, reproduction and safety.
Other definitions specifically require also that living beings move, communicate, evolve, keep their internal environment stable, have feelings, are intelligent and/or are conscious. I think these additional characteristics might apply to all or most species of life on Earth, bur I would not regard them to be necessary. Some people would require living organisms to function using chemical reactions, so as to exclude manufactured devices. (Simulations of life, such as on electronic screens are not life: they have none of the basic requirements.)
A definition that invoked the science of thermodynamics first appeared in the book What is Life written by the famous quantum theorist Erwin Schrödinger. He said that living beings are “the class of phenomena that are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form.”
This very mechanistic and abstruse definition does not address growth or reproduction. However, the concept is used in the search for signs of life in other parts of the universe, where the by-products of life are the only signs that could be detectable from Earth. Scientists look at distant planets to see if there is evidence of chemicals that would normally react together. If at least one such chemical would normally have disappeared but is found to be still there, then it is assumed that its presence is the result of some living organism continuing to produce it.
Whatever the precise details of the definition may be, if life is to perform at least some of the four basic functions, then it must depend on a system that can continue performing these functions. That is, life depends on a sustainable arrangement of interworking parts. For every form of life, its parts and their interactions must be able to operate in the conditions of its environment, such as the temperature, the pressure, the winds and the presence or absence of particular chemicals. Otherwise the system will fail, and that piece of life will die.
This applies no matter how simple or complex this piece of life happens to be.
Some people think certain types of things are alive that other people think are not. Bacteria, fungi and plants are always included as life forms. Things, such as fire, machines, virtual life and viruses, all of which can have some lifelike characteristics, are commonly excluded.
Viruses have no metabolism: they neither take in nourishment nor grow. They do not reproduce, but may be replicated whenever their DNA or RNA gets into cells of living organisms. However, they are given identification by the use of taxonomic names in the same way as living organisms. They can be mutated and evolved into new strains, and hence they appear to act as if they are alive
Because of these characteristics, viruses are treated in a similar way to living organisms in the practice of medicine (taking due regard of the different characteristics of organisms and viruses). The term “live virus” is used to distinguish an intact virus from one that i unable to be virulent, for example for use in a vaccine. Viruses do not have a lifespan: they will continue indefinitely until some external agency damages them or until they deposit their genetic material into the cell of an organism.
Fire, machines and virtual life do not have a metabolism or a life cycle or take action to reproduce, find food or avoid danger. Virtual life may be programmed to appear to do these things but it doesn’t actually perform any of them. It is just a depiction of them.
So far, all the discussion about life has been physical characteristics and processes. It implies that life is nothing more than a physical process. Most scientists and many other people believe that is all it is. They would regard it to be purely the result of phenomena that started from arrangements of inanimate matter, and diversified through many stages of evolution to produce the diversity of life forms that now exists.
But there is no accepted scientific theory of how living organisms first came to exist: Darwin’s theory of evolution addresses only how there came to be many different life forms.
Biologists and other scientists have identified a lot of things that they think would need to happen for life to be able to emerge from inanimate matter. They have shown how some of the required chemicals, including strings of RNA, could have occurred “naturally”. They have shown how membranes could be formed that might wrap around the RNA and other items to create an organism. But for the RNA to be able to recreate itself plus all the other materials, and assemble them so they form a living organism would require a very unlikely series of coincidences.
But even this would not have been enough.
In addition to the things specified in the description of life, organisms need to be able to recognise the aspects of their environment that are relevant to their needs of metabolism, growth, reproduction and safety. They also need the ability to perform the appropriate actions for each of these. And they also need some compulsion to respond to their needs at the appropriate time and to the appropriate degree. For example, they must know what to eat, what not to eat, when to eat and how much. They may need to be compelled to look for food if no food is readily available. (However, the ability of some organisms to look for food is very limited, and organisms often make mistakes in their recognition of food or danger.)
All this amounts to a very complex set of requirements. So, could something else, something supernatural, have been needed to guide the assembly of the first life forms and make them come alive? Many people believe that a supernatural element is necessary for life. Until there is a complete and satisfactory explanation of how a physical process could produce some primordial living organism, I will have to regard this issue as not yet resolved.
However it may have arisen, primordial life would have emerged in an environment very different from our present environment, and it would have been extremely different from any of the forms of life that we are familiar with, including bacteria. So how did the more complex organisms, like us, occur?
Development of Life on Earth
One supernatural explanation of life, which gets around the issue of how life came to exist, is that the various life forms were each designed and created in their present shape during some act of specific intelligent creation.
I think the evidence for intelligent creation is, at best, extremely small. There is overwhelming evidence of an evolutionary process continually producing diversity of life through a process whereby offspring are slightly different from their parents, and some of these differences are passed on to the following generations. (Chapter 7 on my website explains this in some detail, and a version of Chapter 7 is one of the chapters of The Australian Book of Atheism.)
So once there was some primordial form of life, natural processes developed and diversified it into the multitudinous forms that we now recognise. There is abundant biological and geological evidence of this.
Geological evidence points to some form of life existing at least 2.7 billion and possibly 3.8 billion years ago. The first living entities on Earth would probably have consisted of something like individual microscopic films enclosing the RNA or DNA that would have controlled the organisms’ functioning and reproduction. Such organisms would have diversified into multitudinous forms of bacteria that proliferated and occupied many parts of the earth’s surface. Evidence also points to the appearance, after more than billion years, of eukaryotes, i.e., organisms with their genetic material enclosed in a nucleus. (The earlier organisms, bacteria and archaea, are known as prokaryotes) There are no multicellular prokaryotes, but there are signs of multicellular eukaryotes at about 700 million years ago. These developed into increasingly complex organisms – animals, plants, fungi and a variety of other groups of organisms.
In simpler types of single-cell organisms, reproduction consists in the cell growing and then splitting into two or more cells that are like smaller versions of the cell that split. This is referred to as cell division. Cells resulting from the division are occasionally slightly different from the parent cell and have slightly different genetic material, which can lead to the development of new species.
Cell division also occurs within most of the cells of multicellular organisms – in fact this is the main process in the growth of these organisms. During these organisms’ lives, cells are continually dying and being replaced by the division of other similar cells.
In the most common type of reproduction by division, each “daughter” cell is a half of the parent. (Some kinds of bacteria produce one or more daughter cells that are much smaller versions of the parent but are still fully functional.) Each succeeding generation is, in a sense, just a continuation of its predecessor. All individual organisms have arisen from having beein parts of previously existing organisms.
Unity of Life
There is a remarkable unity between all forms of life on earth. The processes of metabolism, reproduction, mutation, separation and combination of all known life forms on Earth are all controlled by DNA and/or RNA. Many types of microorganism readily exchange segments of their DNA, often incidentally giving recipients new abilities or features. In genetic engineering, segments of DNA are deliberately transferred from one species to another – often very remotely related – species to provide the recipient with some specific additional characteristic. There is evidence that cross-fertilisation continually occurs between eukaryotes (including us) and bacteria and viruses, with the progeny acquiring characteristics of both "parent” species, and passing them on to further generations. An interesting example is a species of snail that is green. It has acquired plant genes to produce chlorophyll. The snails turn red in autumn.
Almost all eukaryotes have other types of organisms living within their bodies, some providing essential processes for the hosts, and some parasitic. There are about ten times as many “non-human” organisms in a human body as cells of the body itself. (But the host cells are very much larger than the resident organisms, and comprise most of the total body mass.) Evidence suggests that eukaryotes arose as a result of single-cell bacteria becoming functionally incorporated into the bodies of single-cell archaea, with the nucleus and mitochondria of each subsequent eukaryotic cell being descendents of incorporated bacteria.
In contrast to reproduction by cell division, is sexual reproduction, the fusing of specific types of parts of two biologically compatible but distinguishable organisms, usually of the same species.
However, the concept of species, in fact the concept of classifying all life into domains, kingdoms and so on down to genera and species, while very useful, does not mean that life is rigidly divided into these categories. There are many cases where the distinction between species is not clearly definable.
Most biologists think that all existing life on Earth must have descended from the same common ancestor. So it seems that all life on Earth has been one biological unity since then. From this point of view, there are new arrangements of living matter but no new beginnings of life.
Notwithstanding biological unity, there is a distinction between life and a life. Organisms can be seen to be spatially separate from each other, distinctly recognisable from each other, independently capable, and differently intelligent and emotional. They are individual lives that come into being and die.
Parts taken from the bodies of some plants and animals can be grown to become mature organisms or body parts. New plants can be grown from cuttings or from the culture of individual cells. Small parts from some animals, for example planarian worms and some jellyfish, can regenerate into complete organisms provided that they contain some of each type of the animal’s tissues. Organs can be transplanted between compatible complex animals of the same species. These are examples of both unity and difference suggest that the individual parts, including cells, of multicellular species are separately alive.
Multicellular organisms can be thought of as associations of cells whose interactions make the unit a new living entity. But there are degrees of coherence in associations. Slime moulds are types of single-cell organisms that normally hunt as a herd, feeding on other microorganisms. Under stressful conditions the individual slime microbes form coherent associations looking and acting like slugs or fungi, with differentiated body parts. When these associations look like slugs, they also move and behave a bit like slugs. Under dire conditions some individual members may become spores that will awaken and produce new colonies when conditions improve, while many of the other members die. These and other types of associations – such as corals, for example, which are polyps symbiotic with algae – are loosely held together, and the members are still separate organisms.
Some loose associations may cease to exist without any of the members dying, as when a stressed slime mould structure disassembles upon conditions becoming favourable. In more complex associations, the members form differentiated groups, i.e., organs, that depend on each other for survival. Failure of an organ such as the heart or liver can cause the whole association, i.e., the body, to die. The more strongly they depend on each other, the greater the likelihood that all cells will die if the association ceases to exist. Very complex (multicellular) organisms, including humans, are strongly interdependent associations, and usually all cells die from lack of support when the organism dies. But as mentioned earlier, it is possible for individual cells to be artificially kept alive, to divide and grow. Also, the association itself can survive if it loses larger parts that are useful but not essential, in animals for example, a leg or an eye.
From now on I will concentrate on multicellular organisms, and in particular mammals.
Sperm and eggs do not have the functionality that allows them to exist as organisms, and the composite newly-fertilised egg cell also lacks these functions. New bodies develop by single-cell eggs continually dividing into more and more cells which remain coherently associated but become differentiated into specialised organs. Their functioning as organised units develops incrementally. At some stage the association becomes the life of the new multicellular organism, superseding the separate lives of its members.
So, while the life of a new (single cell) prokaryote may be said to begin with the division of its parent cell, different criteria are needed to identify the beginnings of mammalian lives.
The philosopher Peter Singer has suggested it would be humane to euthanise babies whose disabilities would make their lives so miserable as to greatly outweigh any pleasure that they might experience. In his view their life as persons should be considered to begin when they were 30 days old. After that age, euthanising them would be regarded as murder. This choice of 30 days seems to be arbitrary. Some people would put the beginning at some time before birth. How could any such date be justified?
In other words, when does a life begin?
What Identifies the Beginning of an Individual Mammalian Life?
From a biological point of view, some possible criteria for identifying the beginning of a multicellular mammalian life are:
* stage of development.
It would be consistent for these criteria to be applied equally to all species of organism.
In this context, separation means that the new life is physically separated from, or is not a part of, any other organism. The obvious example is a baby at or soon after birth. This may suggest that a baby that is removed from the womb by caesarean section before its full term begins its life earlier than it would have if it had been allowed to proceed to natural birth. (Its official birth date implies this.)
There could be argument about how separate an unborn foetus or baby is. What is the significance of the umbilical cord? Does the baby become separate only when the cord is cut after it has been born?
And conjoint twins are not separate but are separately alive and have individual lives.
But being separate does not necessarily mean that something is, or is yet, a new life. A kidney removed for transplanting is not a life, even though it is alive. Its only function is that of filtration, and it is able to do that only as part of a living body. So, while being separate might usually be a requirement for identifying individuality, it does not seem to be sufficient.
No form of life is completely independent: all are dependent on their environment for food and energy, and often for cooperative assistance. But there is a difference between passive dependence where all the action to sustain life is taken by the environment, as with a new foetus which is completely supported by its mother, and “active dependence” where the organism takes action to provide for its own food and safety. We usually regard such active dependence as independence. But there are different stages of how much action a developing body takes, and how much the environment, including other organisms, actively gives. So if independence is to be a criterion for the beginning of a life, the decision of when it is sufficiently independent must be arbitrary. It might, for example, be claimed that the beginning of its life is when it has independently developed certain capabilities, such as an animal when it has been weaned and can forage. So this could be some time after birth.
(We are never independent of the billions of microorganisms that reside within our bodies, but I do not think this is relevant to this particular criterion.)
Activity comprises self-generated motion of all or part of the organism or its interior processing. Seeds and spores are quite inactive and presumably have not yet begun their lives before they begin to germinate. But apparent inactivity does not necessarily mean absence of life. A few years ago a bacterium that had been locked beneath a Greenland ice sheet for more than 120,000 years was revived by scientists at Pennsylvania State University. After being placed in a nutritious environment it began producing fresh colonies. When discovered it had been under three kilometres of ice. So life can be in a condition of inactivity for a long time. This particular case has little to do with the beginning of mammalian life. But it might suggest that if life begins at the moment of fertilisation, as some people think, it could be inactive for a very long time but still be a life.
During pregnancy there comes a time when a foetus starts to move in the womb, initially without its movements being detectable by its mother. Perhaps its life could be considered to begin when it first moves parts of its body or limbs. It would be difficult to determine when this occurred for each individual foetus, and the earlier movements would be involuntary. Although some arbitrary date could be chosen, based on average times after conception, this does not seem to be a very useful criterion.
Stage of Development
Stage of development could refer to the creation of processes such as heartbeat or the appearance of a brain. Or it could refer to the progressive acquisition of new faculties and capabilities such as mobility, intelligence and consciousness.
But what is the significance of a pulsating heart? A heart, which is essentially a pump, can continue beating when it is removed from the body, and an artificial heart can produce a constant flow of blood instead of a pulsating one. But a heart does not itself signify an individual life. Also, the existence of a rudimentary brain does not in itself confirm the beginning of a new life. But some stage in the development of its functions might.
To survive, all organisms must be able to distinguish between what is food and what is not, and whether an environment is safe and favourable or not. Survival also requires knowing how to respond in each type of event that it encounters. Organisms we would regard as extremely simple or primitive have a range of faculties in a form sufficient for their needs. For example, the gut bacterium Escherichia coli (which we all have living inside us) has more than thirty different systems for sensing such things as oxygen, light, pressure, kinds of food, and what other species of organisms are nearby, and the ability to act accordingly. And it has short-term and long-term memory. Having such faculties, at any degree of complexity or sophistication, amounts to intelligence. Some arbitrary degree of intelligence might be chosen as the beginning of life.
Could the beginning of consciousness be the deciding stage? Consciousness can be distinguished from Intelligence, with things like computers being intelligent but not conscious. Consciousness often but not necessarily includes the ability to feel pain and emotions. And, just as there can be grades of intelligence, there seem to be grades and types of consciousness. And this relates to the issue of when a mind begins. But a large part of the intelligence of a brain relates to things other than consciousness. There is no way of detecting when a developing brain begins to produce consciousness as distinct from having unconscious reactions.
Alternatively, could a new human life be considered to be established when it is equivalent to the intelligence or capability of an adult member of some other species, or of a particular computer?
If a stage of development is to be a criterion for deciding when a life begins, it implies a gradation between being alive and having a life. If so, then a particular, arbitrary, stage of development is necessary for the foetus to have become new individual life. But how do we make the choice?
So now I have discussed what life is, and how an individual life is more than just being alive. Also, four different criteria have been examined to see where the dividing line is between being alive and having an individual life. But none of the criteria produce a definitive dividing line.
Perhaps there are other criteria, and they might not be biological. I leave it to you to make your own decision.
But how you make the decision will depend on why you want to. Perhaps to calm your conscience? Or to blame someone?
I had better not say any more.