So what are the different influences that led to humans in Europe developing different languages as opposed to different dialects of a universal European one, or just the same language?
Dialects evolve into different languages over time. Groups of people who live apart will develop different accents, and if they meet something new they won't name it the same (probably), some words or forms of address might fall out of use due to negative or positive connotations (the word dictator comes to mind, in modern usage it's pretty negative, wasn't always that way though). Before you know it they've got their own dialect, then *poof* a new language is born.
As for different influences... conquest is usually a big one. Let's go with English, England's been conquered by Celts, Normans, Angles, you name it. Loads of words from many different origins: alchemy is Arab, pork is French, dyke is Dutch. It also works in reverse, colonise India and suddenly you need names for all of their foods, which means you end up copying words like curry.
A couple thousand years of conquest and trade tends to add loads of foreign influences to a language
