I don't know when the life of a human being starts.
But, I do have some thoughts on the topic and I believe I know for myself when life does not begin. Most scientists are afraid to talk publicly about this question and probably for good reason given its controversial nature.
It would seem there are supposed to be three main authorities on when life begins.
First, those with moral authority. They base their perspective on their own personal moral and ethical beliefs, often based on their own religion.
Second, there are doctors. Many people think that because of their medical training that doctors have a unique, rigorous perspective on when life begins for human beings.
Third, there are scientists. A lot of people turn to scientists hoping for some kind of scientific proof of when life begins.
I would argue that none of these three types of authorities have the answer because there is no "the answer" that applies to everyone. Regrettably, there is no experiment to determine when life begins although one scientist argued there was and I rebutted her claims in this blog post.
There is no equation like E=MC2 for defining the moment when a human life begins.
To me personally, there are six mains possibilities when the life of a human being might start: (1), before conception (yes, you read that right), (2) at conception, (3) at implantation, (4) when distinctively human, organized brain activity begins, (5) when the fetus can survive outside the womb, and (6) at birth.
Let's talk about these in order of timing, but first I wanted to note that in a poll on my lab's blog the most popular answer by far was (5). You can see the poll results and vote here.
First, we have the possibility that a human life starts before conception. This may seem counterintuitive at first, but let me explain. Most of the time for a human embryo to start growing, a sperm has to fertilize the egg forming a zygote, which then begins dividing and producing the embryo. However, sometimes, an egg spontaneously starts the process of development on its own in an event termed parthenogenesis.
Parthenogenesis occurs widely throughout life forms on Earth, but as far as I know it has not been proven to have naturally occurred in humans. Even so, given the number of people on Earth, I think it is likely that it has happened even if it has not been documented. It can also be stimulated in a lab and there is no reason why parthenogenesis could not theoretically produce a normal human female if the embryo was implanted in a surrogate mother. So in theory, a human egg has the potential to create a living human being. Thus, one might argue that a human egg is a living human being because of this potential. To me, this seems like a stretch, but keep it in mind as we go forward with our discussion.
Second, there is the idea that the life of a human being begins at conception. The case for conception is fairly straightforward, which is appealing in a way. It is undeniable that the first step that must be taken to end up with a living, breathing human being is conception. However, the case against conception is that a fertilized human egg is not by any stretch of the imagination an actual human being, at least not in my opinion. The fertilized egg, also known as a zygote, has the potential to make a human being, but often it does not. From my perspective, which of course may be wrong, a fertilized egg is very much like a seed. I really like this analogy. Let's run with it.
OK, picture this in your mind--a small Sequoia seed that just fell out of a cone onto the forest floor in Yosemite. See it? That seed is not the same thing as a 2,000 year old Sequoia Tree, right?
Even if that seed has the potential to become that tree over a period of thousands of years by growing trillions of times in mass and developing leaves and other specialized structures, it does not mean that that seed is a tree.
In fact the odds are very much against that seed ever becoming a living tree. First it has to implant in the ground, send out roots, etc.
Key idea: Potential does not mean equality. A seed can become a tree, but a seed is not a tree. They are different. A fertilized human egg can become a human being, but that potential does not equate the fertilized egg with a human being. It has to survive, implant, grow trillions of times in mass, etc.So for me personally, conception is not when human life begins, but again that is not some universal "right" answer that I expect others to believe.
Third, the life of a human being might start at implantation. In human and other mammalian development, implantation is a key event. This happens in humans around 2 weeks after conception. I'm not sure if life begins at implantation, but I can see the appeal of this idea. It's very much like a seed sprouting into the ground. No embryo can survive without becoming attached to the uterus and forming deep connections.
Fourth, is the idea that human life begins when the fetal brain (the most distinctive of human organs) begins functioning like a human brain. There is some debate about when this happens, but it is at the earliest in the 2nd trimester. Earlier than that a human brain can fire off random neuron activity, but it is from all evidence I've seen, nothing like the activity related to thoughts. It's like chaotic electrical activity. Even a dish of human neurons in the lab can produce such activity. What we are talking about here, as a defining point of the start of a human being, is brain activity more similar to human thought. To me this definition is interesting, but very unclear.
Fifth, some people argue that human life begins when a fetus can survive outside the womb. With advances in medical technology, the date at which a human fetus can survive has gotten earlier. With rare exceptions, the earliest is 23 weeks of age, which is quite remarkable. Again, as mentioned earlier, this notion that independent survival ability is the definition of the start of human life, was the most common answer in the poll I did (which admittedly was far from scientific). I can see the appeal of this definition, but what makes it less than ideal for me is that this point of independent survival will be different for every fetus. It's nebulous.
Sixth, many people believe that a human life begins when a baby is born. The appeal of this idea is that birth is such a well-defined event and is indisputably the time when a mother and the fetus/baby become separated. To me personally, this definition seems too late. I think the life of a human being starts earlier, but again I am not telling you what to think.
So overall, there is no one answer about when a human life begins, but I believe that it is a question that people should give serious thought to given its importance, particularly if you are taking a stand on embryonic stem cell research one way or another. Note that embryonic stem cells are made 100% in a lab from blastocyst embryos produced during in vitro fertilization procedures. They have nothing to do with natural pregnancies. The embryonic stem cells are made from blastocysts that are embryos that are only a few days old, that have no organs of any kind, and have only a few dozen cells. The fate of these embryos if they are not used in potentially life-saving embryonic stem cell research is most often to be discarded. Alternatively rarely they might stay in liquid nitrogen for years.
Cultures around the world have many different answers about the start of life. For example in Islam and Judaism, life begins after 40 or so days. In other cultures it is earlier and in others it is later.
I personally think that everyone has to decide this question, one of the most important we can ponder, for themselves.
Please let me know what you think!