Serious questions for atheists with evolution knowledge?

I'm an English major but I'm pretty interested in evolution. My understanding is that everything on the earth is descended from an ancestral population of single-celled organisms. If this is confusing to you consider the possibility that the first cell had, say, 10 offspring which were identical. If each of these 10 offspring went into different parts of the world with different environments, and each had 10 of their own, and in one environment it was advantageous to be slightly faster, or slightly more efficient at photosynthesis--OR SOMETHING--eventually these changes add up over time and after billions of years you get completely different organisms. In the same way you get the different languages of French, Spanish and Italian descended from the same ancestral language--Latin. Hope this clarifies things for you. Imagine it, the ancestors of Spanish-speakers, Italian-speakers and French-speakers were all at one time speaking one language!