Rebecca Roache on Ethics etc discusses arguments in favor and against that sort of bio-engeneering that she calls "human enhancement" ("that is, the use of medicine and technology to raise human capacities above what we might consider to be normal", as she writes) based on the notion of "human nature". I posted a reply there, saying that intuitively the strongest objection against the development or the making available of technologies for human enhancement has to do with their threatening human equality and diversity. (Equality, in so far as they will allows the richest and more powerful to develop their competitive advantage as against the poorest, and diversity in that the process is subject to market pressures and it will prize those capacities that are of current market value as opposed to those which are truly valuable or can be discovered to be valuable.)
This is the sort of reasons that I would give for my opposition against the development or availability of technologies of human enhancement. However, this argument relies on further undiscussed premises, namely that human equality and human diversity are valuable. These premises are far from self -evident (they are even contradictory, when diversity and equality are understood as referring to equality and diversity in the same respect.) I myself do not believe that equality as such is unconditionally valuable.
In this point, I shall try to explain why I think that the sort of equality and diversity that are threatened by human enhancement techniques are important. The funny thing is that my explanation for this connects back to the fundamental theme of that threat, namely whether human nature has direct normative implications.
In this post, I argued for a view of flourishing that was connected to the idea of human nature, where human nature is understood not so much in terms of empirical generalizations, but in terms of those features which are central for our self-image as humans, features such that if somebody were lacking them we would not recognize her anymore as a human being. This applies also at the species level: the philosophically interesting notion of human nature regards features such that we would not recognize as "human" a species that did not have such characteristics.
Now I think that human equality is a fundamental aspect of human nature in this sense. One of Aristotle's arguments in favor of "constitutional government" was the fact that no man on earth is superior to every other man and every other group of man in virtues and judgment. (Aristotle famously wrote - in the politics - that a group of somewhat average virtuous man can be wiser, collectively, than the most virtuous individual.) While Aristotle thought that this reasoning included only males and Greeks, I believe it applies to the human species in full generality. Similar remarks were made by Hobbes, who wrote in the Leviathan that no man is cleverer than others to a significant degree (he also wrote, relevantly, that no man was stronger than others to such a degree that a group of other people could not threaten his life).
Now such facts about humans are the sort of facts, I believe, that characterize the human species to such a degree that a species to which these facts did not apply would not be anymore recognizable as the "human" species anymore. Human enhancement, therefore, threatens a sort of equality which is deeply importance in that it characterizes ourselves as species in a way that is central for our self-image as humans.
This example is also useful to explain the sort of reasoning which leads, I believe, to determining something as an aspect of human nature in a normatively significant sense. The idea of human equality that I just spoke about is one of these truths which, although empirical or descriptive in a sense, is subject to moral learning of a sort. It became to be part of our self image as a species, historically speaking, in a process that is marked by significant moral change (some people would say "progress"). I believe that the statements that humans are equal in the sense I just described looks as a factual descriptive statement, on the face of it, but could cannot be justified in "purely" empirical terms, or better, to people who see the world through morally distorted lens and who are not cognitively impaired or irrational (in any other sense-if you like).
The process through which we acquired the image of ourselves as a species of individuals who can be different, but not so significantly different that any one man is distinctively superior to any other man (or group) was clearly marked by its dependence from the acquisition of a "moral" outlook. And I believe that still any possible justification for this claim cannot work if it is taken in isolation from a certain way of framing and weighing "empirical observations", which in turn is not independent from our moral outlook . And this is true, despite the idea that the business of descriptive propositions should be that of "mirroring" the world, according to the "naive realist view".
A similar reasoning can be made about the idea of diversity. The idea of diversity that I have in mind is one that is coherent with the sort of equality I have referred to above. Side by side with equality, a mark of the human species is the diversity in people's traits, characters, and faculties. This type of diversity is not in opposition with equality, on the contrary it makes it possible. For the reason why people can be said to be equal in some fundamental sense, is that there are so many different traits, characters, abilities, and so on, - in short - there are too many dimensions along which the value of a human being can be determined or appreciated. This makes it difficult if not impossible, to compare the "excellence" of two different people (as everybody who has more than one friend knows).