That doesn't answer the question because all roses belong to the genus "Rosa". So there is a common ancestor of all roses and every plant descendent of that ancestor is a rose by definition. I ask you for a definition of Roman that exists independent of the word and you have me an example that does have a definition that exists independent of people using the word. Romans, on the other hand are biologically indistinguishable from other humans. They look, smell and taste the same (allegedly).

The difference is that Roman is an identity marker and therefore a social construct. That doesn't mean it's not real. It's like asking for a home no one ever lived in. Before construction, there is no house, just stone or wood or what ever. You could point to a cave and say "that could be a home" but it's not a home unless someone lives there. A home doesn't exist outside social reality. It might have different names but it needs humans it identity it as such.

For identity markers, the name is much more tied to the construct. The name can change tho. The first Christians didn't identify as such. In fact, the word started as an insult and was later reclaimed. But you can trace back a social group that later grew to identity as Christians.

It's not that a social group came to decide they need a city and than built Rome and live there happily ever after. I'm not sure but I guess it was a settlement that grew or sever settlements that merged or something but at no point was there a social group that meaningfully preexisted the city.

Same with modern nationalities. There is nothing biological that differentiates Germans from its neighbors. Just a nation state that invented German identity and everything distinctly German came later.