When does one life begin?
At what point during the growth and development of an organism do we consider it to be autonomously directed? That is, what level of complexity or developmental inertia must an organism achieve before we should consider it a distinct, independent living entity? This can be separated into two questions.
- What is an organism’s inherent potential for autonomous re-generation as a function of its genetic architecture?
- How is the proportion of this potential realized as a function of (developmental) time?
Generally one can consider an object to be alive, when it displays a marked or notable independence in its ability to organize and complicate itself by means of local (small-scale) displacements of disorder. Hence, objects such as viruses and non-autonomous retrotransposons (SINES), which depend on the molecular machinery of host objects, are not generally considered to be alive. (In addition, the object’s ability to self-organize must be a self-sustainable feature, lest the object quickly fall into a state of decay as lesser objects do.)
Consequently and perhaps surprisingly, under this definition one cannot consider pre-zygotic embryos to be alive before the first wave of zygotic transcriptional activation. This would upset the current practical definitions of the sanctity of life. In humans zygotic genome activation occurs between the 4 and 8 cell stage, and at the 2 cell stage in mice. In absolute time this corresponds to about 4-5 days post-‘conception’ in humans1.
These thoughts suggest a theory of the onset of livelihood, where the precise point in the decoding process when an object becomes alive (i.e., passes the threshold from order into complexity), is that of the first zygotic wave of transcription, initiated often through the help of maternal mRNA and other parental factors.
Generally however, this theory suggests a continuum of life from generation to generation that is rather difficult to discretize unambiguously. The problem it poses for conscious agents as ourselves is whether we should act as caretakers of all life and trim its branches as we see fit or acknowledge our hubris and let nature, as the abstract mother of all life, maintain each individual life. I think something can be said for each position.
1. This of course ignores the requirement for implantation of the embryo in the mother’s uterus, which some might argue is a necessary event for embryonic development. However my point is to argue for the definition of life as the marginal (read: independent of the environment) ability for a given genome to survive.
This point is about the timing of an inevitable event, and is analogous to stating that a newborn is not alive until it can suckle milk without help from the mother. This in turn requires me to clarify the above statement about viruses by saying while humans inter alia will eventually become environmentally independent of the mother ‘host’, the environment of a virus is solely the host. Note my inclusion of self-sustainable in the second paragraph.