When does human life begin?

When does human life begin?  As a biologist, TPP gets asked about this every so often, and while this is not exactly his expertise, there are certain basic concepts that are straight forward enough.  From a biological perspective the human species began at some point in the past and it will continue for some time into the future, but most people don't mean the life of the human species although that is what actually matters in biology.  And no matter what the state of Kansas legislates, there is no simple easy answer and people of good conscience may disagree. First, understand that life is a continuous process, but people tend to focus on the individual, and doesn't the life of each individual begin at fertilization, at conception?  Well, not always.  Sometimes at a rate of about one out of every 800 conceptions, the zygote or more commonly an early embryonic stage divides and each product develops as an individual resulting in identical twins (or more rarely triplets, quads, or quints).  This can happen up to 12 days after conception. So twins do not begin their lives at conception. Sometimes people argue that a unique genetic combination begins life at conception, but doesn't this argue that twins are not unique individuals?  You can't murder one and escape punishment because an identical genetic copy still exists.  And what of genetic uniqueness?  One pair of parents can produce over 8 million genetic combinations of which only one to a few will become incorporated into an individual.  Medical research also has found that something on the order of 70% of all conceptions spontaneously abort early in development, so early most are not recognized as pregnancies, so it would seem nature puts very little value on conception, much less than the Kansas legislature.  Everyone realizes that the cost and commitment of parenting at this stage is virtually non-existent.  Another interesting point is that there is relatively little debate about when you are dead, when the individual's existence ceases, and that's when the brain stops functioning even if the body remains alive. The person's uniqueness was in their personality, memories, and thoughts, not in their genes. This is what makes organ donation ethical. But a zygote or an embryo has none of those things, just genetic instructions, a potential, but an unknown potential that cannot be predicted from the genes.  Human brain waves that define death do not appear until the beginning of the third trimester of development, and even then the fetus is not able to survive on its own.  So what does this mean?  It means that the idea of life beginning at conception is more about religion grabbing onto one particularly convenient biological event in a continuous process and embuing it with special significance, and declaring that this is when the soul appears. But now you can start the cycle over again with twins.  Do they share a soul?  Where does the soul reside?  In the chromosomes, and what happens to it when they divide?  What happened to the soul obtained by the original zygote at conception?  Do twins have hemi-souls? Such questions matter not, but show that it takes a particular type of simplistic thinking to try to legislate such matters, to try an use biology to bolster a belief system and make everyone adhere to it, something that is surely unconstitutional. This takes remarkable dumbth.  Biology doesn't provide a single, straight forward answer to this question.