Is there a fallacy for this?

It depends what you mean by "->". If "->" means "becomes," then you simply have a non sequitur because "becomes" isn't the same as "is." It doesn't follow that A=D.

A=B=C=D, therefore A=D is mathematically correct, however.

In the case of your examples, this is more a definitional question. When we use the word "aquatic," we generally don't mean "have ancestors that used to live in the water;" we mean something more like "currently living in water," so it simply is inaccurate to say that humans are aquatic.

In your second example, in a way we all are African, but through common usage, we generally say someone is "African" if they themselves are from Africa.

I can also see hints of the argument of the beard here if this idea is used in an argument. Allow me to change your "African" to "African American," for those of us in the States. Of course, any child of two Black parents is considered "African American," and so is any child of one white and one Black parent. Even 1/4 Black counts. At what point is the person no longer "African American?" Logically and biologically, there is no clear demarcation so we have to draw a line somewhere and all we can really say is some people are clearly African American and some people clearly are not, and there are many in between.