When does life begin?

Neither of your options is correct. Life is a long chain reaching back to its origin. Consider: a zygote is formed from gametes, gametes are living cells that in turn arise from the living cells of an organism, all the living cells in that organism have a lineage that can be traced back to a zygote, a zygote is formed from gametes....

If you are trying to ask the broader question of when does a developing human become a conscious person (i.e. attain "personhood") then depending on how you wanted to define person the answer could range from 5 weeks after conception to years after birth for healthy individuals.

 

Ambiguous question huh? As written I would have answered what, 3.5 billion years ago and even that can't be pinpointed to anything near a definite time. I think rather you mean when does the individual come into existence? In that case, I would argue at the moment of fertilization; as soon as the chromosomes meet, you're born.

 

It is a human cell and it is alive but that is also true of the gametes that make it.

As I stated all life can trace an unbroken lineage back to abiogenic beginnings.

That's not possible because the definition of life, human and personhood are not objective concepts. At best we can construct a sliding scale (e.g. animals are very alive, viruses are a little alive, chemical reactions in mycells are barely alive, a rock is not alive) but this is all to do with our subjective (albiet pragmatic) definitions.

The problem IMO is that statements like this are ripe for equivication fallacies. Sure the formation of the zygote is the point in which a genetically distinct organism is formed but is that really an "individual" in the social sense of the word? If so then all cells are individuals, if not we're back to the concept of personhood and when it comes into place, and IMO personhood is not something that rapidly arrives in an human but slowly develops with the span of that development crossing birth.

It's also not possible to seperate the legal and social connotations because a lot of our rhetoric is biased by these things. Classic example (and what is happening now) the idea that there is some sort of objective classification.

 

Definitely chicken. Or dinosaur. It's definitely one of those two species.

 

Maybe not "social" sense, but just rather a genetially distinct organism. Also, I don't think you can equate the fertilized zygote to other cells in the body. For example, a liver cell can't (normally) change into a heart cell. The fertalized zygote is I think distinctly different than all the other cells in the body.

 

Agreed however I'd like to reiterate that as scientists we should be careful in using language that will mislead laymen by setting up an easy equivocation fallacy e.g. "scientists determine that fertilised eggs are individuals!" or to use another recent example "scientists find God particle (vatican yet to comment)"

True, but to explore the issue further would that not make IPSCs individuals (ignoring totipotency for a moment)?

 

Here is the only scientific answer to your question:

- Human life didn't begin at a specific point but homo sapiens emerged as a seperate species about 200,000 years ago. Any human alive now can trace life back to that point (and beyond)

- Genetically distinct homo sapiens form at conception

- The term "human" can mean homo sapien or personhood. Biology can comment on the former and can weakly contribute to the discussion/debate on the latter but as it isn't an objective quality there is no way science can point and say "here it starts"

Lastly when a person says a fetus isn't human they aren't using the term in a taxonomy sense (i.e. they aren't denying it is biologically homo sapien) they are using it in a personhood sense.

 

I don't think they can invoke the entire ontogenisis of an organism but rather are capable of expressing only a subset of the process. Something is missing in IPSCs that is present in the feterlized egg.

 

I agree, to an extent, but for the sake of argument: Is an acorn an oak tree? Is a maggot a fly? Is a tadpole a frog? I assume this is a disguised discussion on abortion, but I'll avoid expressing any political opinions in the biology forum except to say that the question of whether or not it is alive is irrelevant, since the cell is the fundamental unit of life, and thus every one of our cells is alive, including the gametes that went into making the zygote. There is no "beginning", only a fuzzy demarcation between stages.

 

Yes totipotency. Pluripotent cells can differentiate into any cell type with the exception of extraembryotic tissues. So for the sake of argument again, let's say we made a ITSC (inducible totipotent stem cell), would that be an individual?

 

I don't see that genetic distinctiveness has much to do with anything. That would imply that the offspring of some species which reproduce via parthenogenesis, or for the sake of argument a human clone, is somehow less alive than organisms with two genetic parents. In the former case, that implication is clearly absurd, and in the latter, it is at best moral quicksand.

One can certainly make a biologically-based argument as to when something is part of a parent organism and when it becomes a separate organism which the parent is merely incubating, but the relevant kind of separation in this respect is something other than genetic separation.

It seems to me, though, that a teleologically-based argument might be better suited to this question. What makes a fertilized egg special is similar to what makes the keystone of an arch special - not any physical difference, but the role is plays.

 

I'm not sure. Maybe I was wrong to suggest a fertalized egg was an individual. Not sure though when it becomes one otherwise. Surely a newly born baby is, so is a fully matured fetus just prior to being born. Maybe I can resort to the "ontogeny recaputulates phylogeny" argument and suggest when the embryology reaches hominid form, human individual is realized.

 

Necessarily, yes, because there's certainly no way to objectively define it. People, I'm convinced, don't really know what they mean when they ask "where does life begin?". They certainly never find the true answer satisfying (i.e. that the line, if it exists at all, is fuzzy because because they came from a fetus came from an embryo came from a zygote that was itself constructed from living components that were synthesized inside of a living creature which came from a fetus...).

Humans are members of the kingdom Animalia; we are, by definition, animals.

 

I don't think you're ever going to get a satisfactory answer when you keep asking loaded questions...

 

.....Seriously? Are you about to argue that humans did not evolve from prior non-human ancestors and that we share these ancestors with other extant species?

If that is the case, a science forum probably isn't the place for you.