Some people (including some I know — you know who you are) believe that there are no objective natures, since all species are continually evolving. Human beings are not the same today as they were 2,000 years ago, and won’t be the same in another 2,000 years as they are today. This belief has profound implications for morality.
To begin with, if there is no objective universal human nature, there can be no definition of “man” which applies to all “men”. That being the case, how can there be a general prohibition against, for example, murder? “Murder” is “the deliberate killing of an innocent human being”. But if we don’t know what a “human being” is, how can we say it’s wrong to kill one? Or if it’s wrong to kill one, maybe it’s not wrong to kill another whose nature may differ from that of the first.
“Human being” or “man” traditionally has been defined as “rational animal”. But that really means “animal which is rational by nature“. Thus it includes human beings who are not currently capable of rational thought, such as infants or adults who may be in comas. By virtue of possessing human nature these still qualify as “rational animals”.
But if you remove “by nature” from the definition, then it only refers to people who are capable of rational thought at the present moment. Therefore babies in the womb, or out of it, as well as Alzheimer’s patients, are fair game; the former because they haven’t yet reached rationality, the latter because they’ve lost it.
Further, if people are continually evolving, then people in different places must evolve differently. The way evolution works in practice (as I understand it) is that creatures evolve in response to specific local conditions such as weather, the presence of predators, availability of food sources, etc. Therefore human beings would not evolve uniformly worldwide. People living in different places and circumstances would evolve at different rates and in different ways.
How then can we say that “all men are equal”? There are two problems with this: First, as noted, if there is no objective universal human nature then it’s unclear who is included in the phrase “all men”. Second, if we evolve according to local circumstances, then it’s not necessary to believe that people from different places are equal. Some might have physical abilities, or for that matter rational ones, that others do not. (This doesn’t mean the more rational ones are evolutionarily more “advanced”, since if evolution is unguided then for all we know people might evolve to become less rational over time.)
Morality based on natural law also becomes unworkable since in natural law, the question of what men may and may not do is based on what is good according to the natures of things. If there is no universal human nature, then my nature is different from yours. Therefore, the good of my nature differs from the good of yours.
It seems, then, that if there is no objective universal human nature, morality becomes a subjective, ad hoc affair, to be determined on the spot. Based on what criteria I couldn’t say.
[This post was inspired, in part, by a recorded lecture by Fr. Brian Mullady, O.P. The parts I get wrong, of course, are my fault.]