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When does life begin? is a question that seems simple and straightforward, but really isn't.
Defining life to begin with is hardly straightforward, but for the purposes of this question we mean "human life," and more specifically we mean the life of an individual human being, as "life" in general only began once, and has existed continuously since the dawn of the first cellular organisms billions of years ago. Hence, this is more a question of "when does personhood begin?" for a particular person, rather than any definition regarding abiogenesis. This question is crucial to a number of complex ethical debates regarding abortion, premature births and, at the other end of the spectrum, brain-dead patients. It is almost as complicated as defining life itself. Ultimately, it is not a question of science, but of morality, politics, and ethics.
 Scientific categories
Current scientific thinking regarding when "life" is considered to have started falls into five categories which are outlined below. This doesn't necessarily mean that there are five possible "points" to choose from and you just pick your favorite. The reality is complex and these aren't so much five different points as five different criteria leading to five different areas of change that could be defined as "life beginning." The complexities are best demonstrated in the first category, where life doesn't really "begin" at all.
One of the main viewpoints, and the one that possibly best reflects the reality of the situation, is that there is no one point where life begins. Instead, the beginning of life is a continuous process. It may have a start where there is "no life" and an end where there "is life," but there isn't a clearly defined boundary. This can be a problem for people who want their world to be black and white and their morals to be absolute, and it is certainly a problem from a legal perspective, where as far as possible things need to be clear-cut and even. Bear in mind that the simple act of fertilization itself takes up to twenty hours to complete — there really is no "magic spark" that some people may like to think that happens instantaneously.
From a metabolic perspective, i.e., cellular activity such as respiration, life is fairly easy to define. A cell is either functioning or it isn't (ignoring "dormant" cells and exotic organised chemical processes for now). This has profound consequences for the definition of "life" because taking this view there is, in a very real sense, no one point when life can be said to begin. Both the sperm cells and the egg cells are alive in the same sense as any other single or multicellular organism. Indeed, cellular life - and the metabolic processes performed by this - can continue to occur long after an organism can said to be dead. It's said that fresh (uncooked) sausages contain enough live cells on order to easily clone the pig it came from. Hence, from this cellular metabolic point of view, life begins when the gametes are formed from loose chemicals and ends when every cell in the body has ceased to be active.
This view states that a genetically unique person begins at conception - a fertilized egg now hosts a complete genome, making it distinct from the sex cells that came before it. This definition has the advantage of saying that a new individual has been created that can be distinct from its parents, but is still limited by the fact that this embryo is still in an early stage of development and far from viable as an individual.
This view also causes a funny paradox in the case of monozygotic (identical) twins: each twin does not exist as an individual when "its life begins" - that is, when it is conceived as the embryo doesn't split into two parts until later. This paradox could possibly be resolved by considering the pre-twinning embryo as a disparate entity from either of the resulting embryos. This is why viewing the formation of life as a continuous process rather than a single event is beneficial.
Scott Gilbert, in a recent paper which he has kindly given permission to quote posits four erroneous "stories" which support this as the beginning of life. Here is a very over simplified precis.
- Instructions for Development and Heredity are all in the Fertilised egg. The view that we are genetically determined by the combination of parental DNA has been shown to fall far short of the complete story. How the DNA is interpreted can vary greatly affected by things such as the maternal diet. Similarly some development requires certain bacteria to be present. Thirdly, and most surprisingly, the level of maternal care can determine which areas of DNA are 'methylated' which radically alters how they are interpreted. As such the view that we are 'complete but unformed' at conception is far from accurate.
- The Embryo is Safe Within the Womb. Modern research shows that 30% or fewer fertilised eggs will go on to become foetuses. Many of these early miscarriages are because of abnormal numbers of chromosomes. The view that every fertilised egg is a potential human being is wrong in around 70% of cases.
- There is a Moment of Fertilisation when the passive egg receives the active sperm. Again recent research has shown that the previous commonly held view that the fastest sperm races towards the egg and, bingo, we're up and running is wrong on many levels. Fertilisation is a process taking up to four days. As such there is no magic moment, rather there is a process.
- There is consensus amongst scientists that life begins at conception. There isn't even consensus amongst scientists as to whether there's consensus. However, Scott Gilbert's paper lists embryologists who support each of the major view points belying the common and oft repeated assertion that there is consensus amongst embryologists, let alone scientists.
Those searching for the "golden moment" point to the block on polyspermy. A recent study (2012) completed by the Mio Fertility Clinic in Japan, has shown that egg activation (i.e. the mechanism that blocks polyspermy) occurs in as little as ten seconds after the first sperm has penetrated the egg.  Because this change is so dramatic and rapid, and since it happens at the precise moment that the fertilisation process begins then, if you follow the genetic argument, this is the moment at which life begins.
This places the start of life at gastrulation, about fourteen days after fertilisation. After this point much of the uncertainty about the state of the zygote becomes fixed - twinning for example. For many scientists, this determines the start of an 'individual'. The likelihood that the embryo will continue to be viable is now much higher than before. Until about 6 weeks, the embryo is in a proto-female state, i.e., its sex isn't actually formed yet (even if the genetics are there) and is still reliant on the proper hormones causing normal development, hence why males still have nipples and undeveloped mammary glands even though they're not needed. Thus at this point there is still a lot unknown about what the embryo will develop into. Using this as a 'start point' has been crucial in the debate about embryonic stem cell research as a lot more of what constitutes a full individual life (rather than an arbitrary collection of cells and genes) hangs on this stage.
Just as death is usually defined by the cessation of brain activity, so the start of life can be defined as the start of a recognisable Electroencephalography[wp] (EEG) pattern from the foetus. This is usually twenty four to twenty seven weeks after conception.
The point of using neurological factors rather than other signs such as a heartbeat is that this is a much more useful indicator from the point of view of science. A heart beats using mostly involuntary muscle movements so is really little different from any other spontaneous motion or metabolic processes. A heartbeat means relatively little in real terms, although it is more dramatic from an emotive point of view.
Here the start of life is defined as when the fetus can sustain itself outside the mother's womb. Until this point, a fetus is very much dependent on its mother's womb to the degree that it can't operate as an independent unit. This is roughly at twenty five weeks, approximately two thirds into a pregnancy. Even at this stage when it can technically survive, however, a birth would cause major problems for a baby. The odds of it growing up with brain damage are much higher and it certainly would need specialist medical care to ensure that it would survive for an appreciable length of time. However, this point can be hard to pin down precisely, as it is less a point and more of a continuum of the probability of survival, and negative consequences of premature birth. In addition, this continuum can vary depending on available medical technology.
Further past this point, a baby is born at the natural time. However, there is still one hurdle to jump in defining unique "life" and that is the nature of sentience, or self-awareness. Experiments on very young children show that they are certainly not as self-aware as adult humans - indeed in some cases other primates can beat them on the tests administered. The fact is that all humans are born somewhat prematurely, while the young of other animals can drop out of the womb or hatch from an egg and be up and running in minutes, human infants need far longer care. This is due to a developed human cranium being too large to be held by the mother and be given birth to safely, this problem essentially forces the mother to give birth at nine months when in an ideal universe it should be longer. So defining life based on self awareness, you're not really alive until sometime after your first birthday.
When discussing the philosophical and/or ethical issues surrounding the start of life, the desire for science to provide a clear cut human/non human boundary is very understandable. We need to be able to define this because it is important in our laws and our understandings. However, even from the brief descriptions given above, it is clear that there is no simple answer that science can give. It may well be that reality doesn't have an answer for us, and that "when does life begin?" is, in fact, a meaningless question.
Scott Gilbert concludes based on these premises that:
Indeed, the potential for human life can begin very early, but it is personhood that is the sticking point. The question is very much whether the two are equal and therefore happen at the same point. Leaving the answer in the hands of philosophy and opinion however makes the distinction between "life" and "non-life" purely subjective and the answer will be different for everyone. This is the most important fact to bear in mind, particularly when discussing legalities.
 Religious categories
 Christianity and Judaism
Traditionally for Christians, "a human becomes a person at ensoulment." In both ancient Hebrew tradition and early Christian tradition, ensoulment happened when the child took his or her first breath — the soul being directly equated with the breath of life. However, for these same groups, the quickening (the first movement felt by the mother) some time between 40 days and 4 months depending on the pregnancy was the moment that the pregnancy becomes a potential human. This distinction is important because the law allowed for women who wanted to pursue abortion to do so before the fetus was considered human. Given their understanding of biology, when a woman naturally miscarries, there is no obvious baby, just lumps of tissue before the 3rd or 4th month, so it's a reasonable assumption that there is something critically different about a moving fetus in a womb, and a pregnant woman.
Modern "pro life" Christian views of the ensoulment tend to equate first conception with the new soul. This does have some problems, however. If ensoulment happens at conception what happens if the zygote produces twins? Do identical twins have identical souls? Secondly, if ensoulment happens at conception, and science has shown that roughly 90% of all products of conception will either not implant or will abort within the first month of the pregnancy, isn't that a lot of wasted souls?
However, it is now widely recognized that the modern "pro life" view of ensoulment is more indebted to Platonism than it is to official church teaching. This point is made most vociferously by the Catholic Church. In the teaching of philosopher Thomas Aquinas, whose metaphysics were bequeathed to the Catholic Church, the human being is not "a soul with a body," but a soul and body composite. Contra Platonism and modern fascination with the latter, the soul, then, is none other than the form of the body, rather than a separately existing substance in and of itself. Additionally, according to Thomistic thought, human life by necessity starts at conception, since it is at that point that the zygote, if both left to its own natural devices and successful, will necessarily develop into an adult human being. In this respect, then, the zygote, no less than the infant or adolescent, is a human being not yet fully developed. Something thus begins to be human when it has the potential to develop into an adult human being in and of itself, per se. The objection adduced above, namely that "90% of all products of conception will either not implant or will abort," is thus undercut: first, if the zygote produces identical twins, so much the worse for "pro choice" positions--abortion is a termination of two lives rather than one. Moreover, the fact that 90% of zygotes are terminated naturally does nothing to show that they do not contain the potential for developing into human life--it merely shows an unfortunate aspect of reality. Further, according to this view, it cannot be said to be "a lot of wasted souls" any more than it is a waste of human bodies, since the soul is the form of the body and both exist simultaneously. There is then no longer an issue of "soul wasting" but rather the problem shifts to that of theodicy: how could a God, if existent, allow such a tragedy?
Hinduism, like Christianity, has gone through phases of what it means to have a fetus ensouled. In the oldest texts  ensoulment happens in the 7th month of life. Like the logic behind "quickening," there is a biological rationality to 7th month ensoulments. Most pregnancies do not take when a woman is malnourished or ill, which describes ancient people well. Consequently, to prevent the need to mourn over every miscarriage, you only consider the child alive after a point when it has a good chance of survival even if the woman gives birth prematurely.
Around the 1st century CE, Hinduism adopted a view that the child was ensouled in the 7th week using other parts of the Ṛgveda to justify the change. In the most recent Hindu writings, ensoulment happens at conception.
Buddhism is a highly diverse religion, with different schools often having conflicting views. However Buddhists generally consider that a new human being is created at conception. This is based on an Iron Age view of Human biology as represented in the earliest Buddhist texts.
“”when there is a union of the mother & father, the mother is in her season, and a gandhabba is present, then with this union of three things the descent of the embryo occurs. 'Mahātaṇhasāṅkhaya Sutta.' Majjhima Nikāya 
Arguments exist over what the gandabba represents, but a loose consensus exists which treats it as the karmic residue of a previously lived life. As Thanissaro puts it in the notes to his translation, "it means the being, driven by kamma, who will take birth on that occasion." Buddhists, however, are at pains to deny that this being is the same being who previously lived (contra Hinduism) or entirely different from that being (which would block the moral force of karma).
 Social, cultural and governmental categories
The date of birth is used in legal documents, people celebrate the anniversaries of their birth and use it as a starting point when calculating their age. In terms of biology there's little change heading through the birth canal and this point is really most meaningful to the mother. The major change at this point is that the infant does start breathing air and feeding rather than relying on nutrition through the umbilical cord.
Although they make no reference to souls, most professionals in bioethics consider experimentation on embryos after the primitive streak stage (14th day) to be unethical. 
 See also
- ↑ Possible mechanism of polyspermy block in human oocytes observed by time-lapse cinematography National Center for Biotechnology Information
- ↑ When Does Consciousness Arise in Human Babies? Scientific American
- ↑ There are references in the Talmud to suggest that ancient Hebrews from 2000-4,000 years ago felt the first 3-4 months was the woman's body preparing for pregnancy, and that you could stop that process, and when she felt movement, the child had taken root in the body.
- ↑ Both the Ṛgveda and the Upanishads make reference to 7th month ensoulment,
- ↑ http://www.eubios.info/EJ144/ej144j.htm
- ↑ Damien Keown, Science and Theology News, April 2004
- ↑ Thanissaro (trans.) Access to Insight. http://www.accesstoinsight.org/tipitaka/mn/mn.038.than.html
- ↑ The President's Council on Bioethics, Human Cloning and Human Dignity: An Ethical Inquiry. July 2002.