The reality that the mere _category_ of a species designation is irrelevant to moral capacity is something we are going to have to confront sooner or later. Classifications are simplifications for ease of use: they are rarely either consistent across all usages and examples or reflective of detailed reality.Now, I agree to an extent with the mutability of species categories over time. As I wrote about a while back under Speciation and Nominalism, our most basic definitions of what makes a species (the primary one being that members of the same species can reproduce and bear fertile offspring) get tricky when you go back through time. Since a species is a population rather than an individual, its existence gradually becomes apparent as population groups drift apart.
If you aren't prepared to consider that the other modern apes, our closest living relatives, who are separated from us only by generations and the death of the intermediates, might deserve some moral consideration, where are we? What is morality even good for other than making humans feel important?
You love your mother. Your mother presumably loved your grandmother just as well. And so on. Are you suggesting that somewhere along that chain of familial love, there is a break where killing one of our ancestors for convenience was acceptable? Where does the chain break (it has to at some point, I don't deny that). Ultimately, the only way one can rationalize the sort of break that must come is by discussing functional capacity. At which point we're forced to admit that a brine shrimp has more concern for its own existence than a blastocyst.
However, I'm hesitant to say that this means we can't draw specific moral conclusions about individual organisms simply because they are members of the human species. As I tried to think about why I felt this way (because, as is often the case, the conclusion came first and I found myself searching for the reason I believed that conclusion to be correct) two things came to me which may or may not seem as important to others as they did to me at the time, so I'll toss them out and try to keep this from becoming one of my marathon posts.
Firstly, it seems to me that a lot of the "culture of death" issues that we find ourselves facing result of people addressing moral issues by category rather than by individual. I recall some time back (I forget in reference to what) Mel Gibson was quoted as saying in regards to stem cell research, "I myself was once a zygote. You too were once a zygote. In fact, if you can find me one person who was never a zygote, I will give you a cigar." The philosophical point being reached at here is that to make any sense of the world one must establish moral norms based upon the individual, not the category the individual falls into at a given moment. It's certainly true that human zygote exhibits far less development of any sort than a brine shrimp, and yet one knows that a human zygote will (unless it dies) develop into a fully formed human being capable of drivelling on for hours on blogspot (ah, that's me!) -- the brine shrimp, on the other hand, will never undergo such a change. Although a zygote and an amoeba may temporarily share many of the same characteristics, it seems it would be a categorical mistake (pun intended) to say that the zygote and the amoeba are the same (or even similar) in essence. Rather, we must grant individuals the dignity appropriate to them according to their natures. Thus, a human needs to receive the dignity appropriate to a human, a dog the dignity due to a dog, a protist... Well, I'm not sure protists inherently deserve any dignity at all. On identifying the individual, one must then apply the same moral weight to the individual throughout its existence, not act as if there are a series of individuals with different natures: first an individual who is a single celled life form, then another individual appears who is a "blob of cells", then another individual who has a nervous system and increasing sensory perception, then an individual who has a Sony Playstation and lacks a nervous system or sensory perception in regards to anything other than that. Regardless of the sad devolution of the individual in the preceding example, he remains the same individual throughout, and may yet put away the Playstation and exhibit signs of sentience yet.
This brought me to my second point. I was thinking about certain base assumptions that I tend to make when tackling a moral or philosophical question: such as that human beings all possess a common "human nature" that both sets them apart from the rest of animal creation (in a way that supersedes our differences in function and ability) and that certain apparently inborn moral strictures such as "don't take innocent human life" represent an inborn moral norm rather than merely an evolutionarily useful urge not to kill too many of the same species. Perhaps because of the "scientific" mentality which holds so much sway in the modern mind, people often seem to think that philosophical statements should be based only on observable or revealed phenomenon. Either you can say "based the fact that humans exhibit cognitive thought, they should not be killed, and assuming this we can deduce..." or you can say "if we accept as true the revelation of the ten commandments, and thus 'thou shalt not kill' one can then deduce..." However, Plato is willing to do something much more ambitious when looking at these questions. When confronting the question of how much weight to give innate understanding of a quality such as "Justice" that we have, Plato accepts that understanding as touching upon some deeper reality, and speculates that before birth human soul experiences the world of the forms. Thus, in Plato's system, our varying understandings of "Justice" or "Good" represent imperfect memories of the form itself. Plato has no hesitation in positing a supernatural solution to the question.
All of which leads me back to a question I haven't totally satisfied myself on in the past: Is it possible to make any sense of concepts like "human nature" and "morality" without assuming a supernatural element that sets humanity apart from the rest of nature? I tend to lean towards thinking that unless you accept that humans have souls, and are in some sense supernatural as well as natural beings, that a lot of things which we innately feel ought to make sense (such as morality) start to fall apart. Nor does this necessarily seem to me to be something that only comes from revelation. Most of the schools of ancient (pre-Christian) philosophy also assumed the existence of a soul of some sort. I think that the existence of the human soul falls into that category of knowledge that Vatican I discussed as knowable by human reason acting upon the created world without reference to revelation.