Religious people even think 'life' is something you get at some stage in your development as an embryo, although they will argue ceaselessly about when this occurs - the moment of conception, at the first 'quickening', at birth. In this respect, as in so many others, the ideas religious people have are almost unchanged from the opinions held by people in the childhood of our species when the world was so poorly understood it must have seemed a magical place.
It has even been claimed that there is something qualitatively different about the chemistry of carbon and the molecules based on it - the so-called organic molecules - as though they obey a different set of physical laws which separates organic 'life' from inorganic 'non-life', so life is what carbon atoms, together with oxygen, hydrogen, nitrogen, sulphur and occasionally other atoms produce.
This muddle and confusion over what exactly life is would be amusing if only Creationists in particular didn't keep challenging people to explain what it is, or to create it, or to say what its purpose is. Challenging one of them to define it first is a sure-fire way to bring the conversation to a shuddering halt, often with an indignant flounce and an insult hurled over the shoulder together with a passive aggressive threat to back it up. But never any coherent definition of 'life' of course.
It is a fundamental law of nature that entropy or the amount of disorder in a system tends to increase with time. This can only be halted or reversed using energy. Energy is obtained from somewhere else so, in order to manage entropy, living things need a source of energy.
Life on Earth gets it's energy ultimately from the sun, which gets its energy from a nuclear fusion reaction which creates helium out of hydrogen, releasing vast amounts of energy in the process. Entropy is increased in the sun, releasing energy which is used by living things on Earth to reduce their local entropy.
Most of the sun's energy is radiated out into space of course, so Earth life only uses a tiny fraction of it. Never-the-less, this system shows how energy can be used locally to apparently go against the Second Law of Thermodynamics provided the Law is not violated in the system over all. The Second Law of Thermodynamics only applies in a closed system and the only truly closed system is the Universe itself. Local apparent violations of it are perfectly permissible provided the energy in the system overall remains constant.
What this means for 'life' is that life is a chemical and physical process. When that process ceases, life stops. We call this death. But where does life start?
Life started when the first replicators evolved efficient mechanisms for catalysing reactions in a controlled way. Any improvement in efficiency at replicating would have given them additional advantage and success - expressed as producing more copies of themselves, together with the improvements. There is no sense in which chemical reactions are turned on by some external 'spark'. Chemistry happens because the result is a lower energy state for the atoms and molecules involved. We still carry that early 'life' in that the chemical processes that make us alive are directly descended from those first reactions involving the earliest replicators. Life is a continuation of the chemical and physical processes which started some 13.5 billion years ago and which are continuing inside you right now. If you like the poetic metaphor of a 'spark of life' that's fine just so long as you remember that there was no initial spark to start it going and there is nothing magical or metaphysical keeping it going.
Life does not start afresh at every new generation or with every new individual. New individuals and new generations are produced out of living cells produced by living things.
Very basically, all higher organisms, and especially multi-cellular ones exist in two different phases, one of which isn't even multicellular (although in mosses, ferns and liverworts, and in some other creatures it is, or become multicellular). A sperm and an ovum are the single-celled phases of many animals including us mammals. What we see as the normal animal is simply the multicellular phase. In most animals the single-celled phase has one of each chromosome. Two of these come together to produce a cell with two of each. These phases are called haploid and diploid respectively.
How this works, and how this system evolved in multi-cellular organisms can be seen by looking at how it works in plants.
|In mosses, both phases are there but the haploid phase is what we see as a typical moss plant. At the right time, this phase produces special egg and sperm cells. The sperm cells swim across to the egg cells and fertilise them to produce the diploid phase. (This is why mosses need to live in damp conditions so there is some moisture for the sperms to swim in). The diploid phase then produces a multicellular stem with a little fruiting body on top in which haploid spores are produced. These get distributed and grow into new moss plants.|
|Ferns have taken this process one step further. What you see when you look at a fern is the diploid phase. At the right time, usually underneath the fronds, dust-like haploid spores are produced. A few of these will find the right conditions in which to grow into a flat little haploid phase or gametophyte in which develops egg cells and sperm cells. After fertilisation, the resulting diploid cell divides and grows into the adult diploid phase. Evolving this system meant ferns were able to live in conditions where the whole plant didn't need to be wet so long as the ground it grew on was damp.|
|In flowering plants, the normal plant, as with ferns, is diploid phase. The haploid phase is the single-celled pollen and ovules. When an ovule is fertilised it divides and grows to form an embryonic plant in the form of a seed, which is the distributive diploid phase which grows into a diploid plant. Using wind, insects and other creatures to distribute the pollen means that flowering plants don't need to be wet or even grown in especially damp conditions. For an outline of how this system evolved, especially it's co-evolution with insects, see Does God Hate Bees?|
This should illustrate how talking about life starting at conception (union of two haploid phase cells to form the initial cell of the diploid phase) makes no sense biologically, nor does talking about a new life at any stage in the reproductive process. Life is always present and has always been present as long as replicators have been replicating. We are all descended from those original replicators so we all share that 'life' in that we are all still managing entropy locally.