When we talk about "Man" as mankind, we use "l’homme". "Un homme" would be a particular, single man. Every sentence referring to man as mankind will always be "l’homme". This holds true for animals among other things.

L’ours est un animal solitaire

Or simply like "the dog" in the beginning of the sentence.

It makes sense to say "un" for a country, because it’s true for each and every country, individually. The difference is subtle, you can even say "du /de l’" instead of "d’un" sometimes :

Les quatre côtés du carré ont une longueur égale

Les quatre côtés d’un carré ont une longueur égale

Both are correct. Remember that for a statement with man as in mankind, always use "l’homme".