The question of human exceptionalism is not just one of science. Set aside for the moment, questions of natural and supernatural and empiricism. We want humans to treat humans differently than other animals. With very few exceptions (Peter Singer comes to mind) we take for granted the idea that humans warrant special treatment. Thus, we have the concept of human dignity.
Our categories shape our understanding and the category of human does more than empirical work. It does legal, moral, and theological work as well. It impacts our ethics. When we ask about human uniqueness, we are often asking about the rights or responsibilities attached to human life. Why treat humans well? Why give humans preference over other animals in terms of life, liberty, and flourishing? What freedoms do we expect for humans?
Note that these questions cannot be answered in a vacuum. Unlike questions about human faculties or essential character, the treatment question cannot be disentangled from issues of free will and culpability. We cannot claim that liberty is a human right without thinking humans choose effectively, that they have non-deterministic agency. The book Brave New World deals with this well in proposing humans engineered to enjoy their surroundings. There would be no point to liberty that could not be exercised.
[Opponents of free will may point out that we cannot comprehend the details of determined choices and, therefore, must act as though they humans were free even though we know they aren’t. We can be pragmatic libertarians. First, I’m not sure what the point is of believing in something and acting as though it were not true. It strains the definition of belief. Second, it opens the door, ethically, to anyone who claims a new technology for pacifying the masses, something that happens surprisingly often in human history.]
Neither can we speak of human dignity without asking what it is, operationally, we wish to preserve. Is it human life, human autonomy, human happiness and flourishing? Modern questions about the beginning and end of life – abortion, reproductive technologies, euthanasia – have everything to do with how we put bounds on the categories of things deserving of our moral care. Is a fetus human? Does it have dignity? Does it have rights independent of the mother? Do our rights to autonomy and happiness trump our right to be alive? Do they ever trump the right of another to be alive?
Within the last two centuries, we have seen a gradual shift in thinking to make the category of “human” bigger. There was considerable debate about whether sex and race might affect our rights to life, property, autonomy, and speech. This was not just a question of who gets to be involved in public life; it was a question of what we counted as a “who” in the first place. Moving the border between humans and non-humans has concretely changed politics, economics, and governance within the past hundred years
We still argue over whether our autonomy leads to our dignity or our dignity justifies our autonomy. In the first case, you get rights because you can choose, but if you lose the ability to choose (e.g., a coma) or act in a way that demonstrates unacceptable choices (i.e., crime or immorality, your dignity can be taken away. [This is why I will never be a preference utilitarian or even a Kantian.] In the second case, you are allowed to make choices because of some inherent or essential character of humans. It doesn’t work well with science, but it gets you much closer to what we think of as “human rights.”
There is more I could say (and more I have said here and here), but for now, I simply want to highlight the importance, even the necessity of distinguishing humans from other animals for ethical reasons.