Expert: Dana Krempels, Ph.D. - 8/22/2007

Question
Dear Dr. Krempels:  You were recently quoted as saying that the human zygote was not a member of the species Homo sapiens.  If you really "believe" that, how does the gestating human "change into" a member of his/her own species, at what point in gestation does this occur, and of what species are we before that?

Answer
Followup to Mr. Chadwick's contention that my answer (appended below) was evasive:

My answer was not evasive.  It was quite precise.  The fact that you did not fully understand it does not make it evasive.  

You obviously have a political/religious agenda here, and will not read my answers with a fair and open mind.  Unless you start to do that, please do not expect more well-reasoned answers from me.  They are a waste of time, given to an opaque mind.

--DMK

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Dear Doyle,

If you are referring to a previous AllExperts answer I gave, then the precise quote is:  "Even a zygote, which may have the *potential* to become a Homo sapiens, but is not an organism by any stretch of the imagination, is not considered an individual Homo sapiens by any members of the scientific community that I know."  

That is not the same as saying that "the human zygote is not a member of the species Homo sapiens," which is how you quoted me.  I did not say that, as you can see.

In any case, the argument over how a zygote should be classified is not a scientific one, as it cannot be subjected to scientific testing.  A zygote has a potential to become whatever species its DNA dictates.  But is it a member of its species?  Is a sea urchin zygote a sea urchin?  Or just a potential sea urchin?  This is a philosophical question, not a scientific one.

To the truly rational, dispassionate biologist, there should be no difference--in terms of this particular question--between a sea urchin and a human. But the question still remains philosophical--and moot--not scientific.

Systematics is the science of studying evolutionary relationships among related groups of organisms.  Taxonomy is the (related) science of naming and placing organisms into classification hierarchies that not only reflect evolutionary relationships, but also offer the investigator a way to categorize organisms for identification and other scientific study.

At this time in our history, classifications and naming of organisms are based on the morphological descriptions of *adult* organisms.  Systematists study comparative ontogeny (embryo development) to help establish evolutionary links among related taxa, but whether they classify a zygote as a member of its species is not particularly relevant to most scientific studies.  DNA and protein sequencing are now becoming useful tools for very fine-resolution systematics, but these techniques still cannot answer the question of when a Canis familiaris zygote becomes a dog.  

Biological systematics and classification are tools used to facilitate science and the study of evolution. They are not belief systems used to shore up religious or socio-political arguments about when life begins.  The rules and design of systematics and taxonomy are not even compatible with the latter pursuit, and so your question truly does not have a scientific answer.

The point at which an embryo becomes a member of its species is moot.  Some might say that it's at the moment of fertilization, others might say it's at the point when the developing individual is able to be viable outside its incubator--be that an egg or a uterus--and still others might say it's somewhere in between.  It's an emotionally charged argument in part because there is no scientific way to address or answer the question.

If you were to present a systematist with a chicken embryo, that systematist might agree that the embryo could become a member of species Gallus domesticus.  But if it is not yet viable outside the egg, is it a chicken, or just a potential chicken?

I would pose the same question for a human.  And in my personal opinion, the zygote is not a human.  It is a cell that has the potential to divide and develop into a human. When one decides to call it a member of Homo sapiens depends on the motives of the namer, but it doesn't really matter, scientifically.

Since your questions are outside the realm of scientific testability, they must remain in the realm of philosophy.  And that means that their answers will be different, depending on whom you ask.

Hope that helps.

Dana