IaJ:
The reading of 'serpent' depends on your textual comprehension: it is metaphorically given this human trait in the setting described, at all times subject to it being set in a realm other than the physical one.
Thank you for telling us Genesis is not a scientific document. You say it employs metaphors that deal with spiritual rather than physical phenomena.
It follows that Genesis cannot set the parameters of what we may logically conclude from the results of valid scientific research. The subject of surreal images in ancient literature is off topic here.
Thank you for playing, Mr Joseph.
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Archer
All species are transitional.