The evolution of language is a complicated issue and lots of people research into it. The language of different groups of people changes over time. If two groups that started by speaking the same language don’t have much contact with each other over a long period of time then their languages gradually evolve into different languages. But I can’t give you any more details than this.
That is a good question. I think it is probably because people originally lived in isolated populations with little or no contact with other groups of people. They then developed their own languages to communcate with their particular group of people.
Good question. I am not a specialist on this but latest I have heard is that language didn’t evolve all that often in humans, so that there is only one ancestor – an “Ur-language” if you will. But that was so long ago that people living in different parts of the world changed ended up changing the words and the grammar a lot. Like, I am used to English speakers in London but if I listen to someone Scottish or from the Souther US, although they are technically the same language I just don’t understand them. These dialects only separated a few hundred years ago, so imagine what tens of thousands of years can do to the exact same language!
Language changes in a similar way to how species change, but on a smaller timescale. Just as different populations become isolated, then change to sub-species and eventually form new species, in the same way languages mutate into accents, then dialects, then whole new languages.