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It may help to remove the label of "humans" when thinking about the non-essentialist definition of a species.  Ancestors will invariably pass on some of their attributes to their offspring, but there will still be variation in other attributes.  As I understand it, the non-essentialist view comes from the idea that no matter how an offspring is different from its parents, it is still of the same species as its parent was.  Speciation takes place over a long period of time and is a gradual process; it is not as if speciation will occur with the birth of just one child with a certain variation.  It is only when this variation spreads throughout a population and is unable to produce viable offspring when crossed with a different species that a speciation event has occured.

Actually, I am not certain of what a speciation event means, exactly; and because of that, I think it is a rather restricting term.  It implies that there is a single moment when a new species emerged, and I am not sure that this is what actually happens. Like I said before, it seems to me that a new species emerges over time, and not in any single moment (or generation).  Of course I could be wrong about this (and correct me if I'm wrong!), as I haven't taken biology since my sophomore year in high school. 

I did notice, however that Darwin also seems to feel some uncertainty on where the line can be drawn between designating a species and acknowledging a variation.  He spends a good portion of chapter 8 discussing the creation, fertility, and occurence of hybrids, and uses his findings to explore the definitions of a speicies and a variety.  He concludes on page 268 that "the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties."  So I do not  think that Darwin himself believed that certain attributes were necessarily a marker of a species.  It seems to me that he also held this non-essentialist view, that species cannot be defined by attributes but by it's descent.