That’s my question from the very beginning, why does man have a Right to Life in the first place? and where does this right come from. Whether a society recognize it or not is irrelevant.
Your first question seems to be looking for some string of syllogisms or a “derivation” of the right to life. This is not a proper way of looking at this, and a misuse of logic. It is an inductive argument and if you want the proper way to go about it from the axioms, that is the whole of philosophy. Metaphysics -> epistemology -> Ethics -> Politics
Rights are a political concept. That is, they answer the question “How do rational men live together?”. Life, however shows up earlier than that, in ethics, and more specifically as the basis of meta-ethics. That is, the requirements of life as man (qua man), define what is the “good.” This is derived from man’s volitional nature, his requirement to use reason as his means of survival, etc. To that extent, it is a “natural” law as Zip puts it. So now that we have that, the question becomes how does man’s life form the basis of any political system. Since political systems must be congruent with the preceing ethical system, it would stand to reason that “life” is going to appear don’t you think? That is, the concept of rights and more specficially the right to your own life is how one keeps the ethical good preserved in a social setting.
To start studying Objectivist meta-ethics, I’d suggest Tara Smith’s Viable Values. That’ will show you how life, and what kind of life serves as the basis of ethics.
PS: going down the DNA-natural selection-survival track is the wrong approach as well. One woudl expect ethical concepts to be somewhat congruent with natural selection and you might think of them as “memes”, but they do not follow from the principles of survival of the fittest.
I am unaware of a single species that routinely kills other members of its own species for any reason.
Sidelight. I think there are numerous examples of consumption of mates after mating, and of eating ones young as well.
Let’s just say I’d hate to be a male praying mantis.