This assumes that that evolution is a factor when it comes to intelligent creatures, which isn't the case in DnD worlds. There are various gods and they make various races to worship them and spread their beliefs. Thus humans are highly sociable, ordered, and prone to commerce, banking, agriculture, and urban build up because Erathis, goddess of civilization, decided she wanted a race that upheld her ideals. Orcs live in the harsh wilderness and frequently raid and enslave everyone around them because the savage god Gruumsh wanted a race that embodied his belief that strength comes from surviving and thriving through constant, bare knuckles conflict. There are lots of different gods, many of whom want their own race, not to mention magical mishaps, corruption from the Far Realms, interbreeding, celestial and demonic meddling, and the influence of various other planes of existence. Now, that might be too high fantasy for some, but it’d fairly well explained. A constructed language. It’s okay, I know hardly anyone will use it, but I’ll start work on one anyway. If nothing else, it might make it into a few words or phrases.