“The same language” is sort of oversimplifying. Languages are smooth things that vary over time and between places, there’s not one “definitive” form of a language that is more valid or precise than the others. We only learn what sound an ‘a’ or an ‘e’ makes by listening to other speakers. We don’t all hear the same speakers, and we don’t copy them perfectly, and that lets languages drift over time.
By “Same Language” I assume you mean “closely similar language syntax” – English in one village, being the same as the English in another village. Differences of these are differences of dialects.
However, even today, there are different dialects and most dialectics have an accepted formal dialect that is universal among them because now most humans can read and write.
The reason is because of geography and trade. Not too long ago, it wasn’t uncommon for someone to be born in a village or town and never leave. You are born in an area, you speak with the same couple hundred of people, and you would never really communicate with anyone else except for trade or perhaps war, and then you die. With that said, if you picked up a rock and you called it “raak” someone else on the opposite side of the world might pick up the same item and call it “chattaan” – the variable here being that you are able to read what I’m writing here instead of just hearing how I’m saying it and never seeing it written down. It gets more complicated too, because two different people within two different social classes could very rarely communicate with each other at young ages and two different dialects would blend together even less, even if these two hypothetical people lived in the same geographical area.
Its hard not give a historical example, so I’m going to try.
So, you have the Australian Accent which is an English dialect. To oversimplify the founding of Australia outside of their pre-existing native population, Britain colonized Australia with a minimal amount of Britain’s upper class that spoke a slightly different accent than the majority of middle class and lower class (prisoners). While the ships from Great Britain were filled with people from very different regions with different dialects, the educated were primarily from Portsmouth. Here is a Youtuber speaking an older Portsmouth based dialect ( [https://www.youtube.com/watch?v=GLBQH8_D0Jc](https://www.youtube.com/watch?v=GLBQH8_D0Jc) ) – and notice in the comments how people are saying he sounds like someone from Australia. In the same language you can see how the dialects twist and evolves. Then factor in how someone from Australia was probably more stationary and didn’t travel much during the 1800s, which doesn’t change the dialect as much as someone from Portsmouth might have during 1800s.
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