Full Version: When does "life" begin?
QUOTE(turnea @ Jun 16 2005, 09:21 AM)
QUOTE(logophage @ Jun 16 2005, 11:12 AM)
Again, we're doing the language dance or, more accurately, semantics dance 
. To speak of a zygote as something independent of the mother (without technological intervention) is ...well... illogical. Both are required for the developmental stages to progress. If we can't agree on this point, then the debate will go no further.
. To speak of a zygote as something independent of the mother (without technological intervention) is ...well... illogical. Both are required for the developmental stages to progress. If we can't agree on this point, then the debate will go no further.You will notice I did not say independent. A zygote is certainly dependant.
I said it was separate, because it is. The zygote is not part of the mother. It needs the mother most desperately but they are two different humans.
I suppose this is precisely where we differ. At the zygote stage, to talk of the zygote as a separate entity is analogous to talking of the spleen as a separate entity. Just because there is genetic material that is different from the mother's material is an insufficient criterion. Note that mitochondria and its DNA do not take part in the phenotype of an animal (i.e. its DNA is not part of the animal's DNA), yet we don't suppose it to be separate (as in other). Also note that mitochondria are passed directly from mother to zygote. It would be impossible for an animal cell (including zygote) to survive sans mitochondria.
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QUOTE(logophage)
If homo sapiens were marsupials instead of "placentals", then we would still be talking about zygotes and embryos, however the debate would likely involve the ethics of ex vitro embryos; perhaps, in a parallel universe this is occurring . The law does define an in vitro stage of development after which voluntary miscarriage is illegal.
. The law does define an in vitro stage of development after which voluntary miscarriage is illegal.Were are not talking about the law. In biology the birthed infant kangaroo is (as noted in the earlier Wikipedia article) roughly analogous to an embryo.
I suppose one could argue that a birthed placental mammal is analogous to a birthed marsupial mammal. Umm.... where is this debate going then?
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QUOTE(logophage)
No, I continue to argue that it is the context that is important when using terms. The term "human" is overloaded. It is used in many different domains -- morphology, genetics and ethics -- to name a few. Each domain says something different about human. I would either like to use different words or get commitment that a word in one domain will not then be used to transfer into another domain (under the covers, if you will).
I don't see where this has occurred at all. We are speaking of the pre-natal development a human. That is a biological concept by definition.
I thought we already agreed to stick to the sphere of biology.
Well, let's stick to a particular field of biology then. Let's talk of genetics which is different from morphology. Morphology deals more with characteristics/body shape/etc., that is, phenotype, whereas genetics deals with genotype. So, for discussions under the domain for genotype, I would agree that if a human were strictly defined to be "the DNA necessary to express all various developmental stages of an individual of the species homo sapiens", then I'd be fine with using the term "a human". Of course, this means that the DNA in skin cells would also qualify.
QUOTE(logophage @ Jun 16 2005, 12:49 PM)
I suppose this is precisely where we differ. At the zygote stage, to talk of the zygote as a separate entity is analogous to talking of the spleen as a separate entity. Just because there is genetic material that is different from the mother's material is an insufficient criterion.
First according to biologists it is indeed. That is why a mother's immune system would attack an embryo as "foreign tissue" were there not a mechanism to prevent it.
Second it is not the only thing to consider. A zygote does not even attach to the mother until is is in the embryonic stage. At conception a zygote is in the uterus but is not implanted.
As spleen on the other hand is mass of differentiated tissue that form one organ of an organ system of a individual human.
It is a part of a human, a zygote is not.
Again, this is not a matter of opinions to differ on. It's biological fact.
QUOTE(logophage)
Note that mitochondria and its DNA do not take part in the phenotype of an animal (i.e. its DNA is not part of the animal's DNA), yet we don't suppose it to be separate (as in other). Also note that mitochondria are passed directly from mother to zygote. It would be impossible for an animal cell (including zygote) to survive sans mitochondria.
Very well, but in a zygote the nuclear DNA is different from the mother. That does change the zygote's genotype (and affect its phenotype).
QUOTE(logophage)
Well, let's stick to a particular field of biology then. Let's talk of genetics which is different from morphology. Morphology deals more with characteristics/body shape/etc., that is, phenotype, whereas genetics deals with genotype. So, for discussions under the domain for genotype, I would agree that if a human were strictly defined to be "the DNA necessary to express all various developmental stages of an individual of the species homo sapiens", then I'd be fine with using the term "a human". Of course, this means that the DNA in skin cells would also qualify.
Morphology merely follows from genetics and is not a separate context.
A skin cell cannot (under natural development) develop into a human because it is a differentiated cell. The genes that cause it to adopt the structure of a skin cell have been turned on and it has no potential to be anything else.
Potential is not merely a loose concept in biology. The totipotent zygote is that way because it is fundamentally different from a skin cell.
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