I'm rewording the original question as suggested, in an attempt to make it more specific.
We all know we humans affect our environment by building structures, leveling ground etc. but these changes are usually pretty obvious and visible.
As I want to create an alternative Earth world, I thought I could simply use a real map to describe the world; however, having some basic historic knowledge of my hometown, which is a coastal city, I know that it's not simply built around a river, but the river delta has been regulated during the city's development. It made me thinking, how close our current geography is to what it could be without us, and similarly, how whole nature would look if not being influenced by humanity.
I'm not asking to describe all the changes humanity has done, specifically, but for general processes that will help me to investigate specific regions and do a better job at trying to create an alternative history for them. Answers already given are good examples of what I expect and I wonder if there's more ways in which humanity changed the Earth.
An advice has been made by XandarTheZenon, that perhaps this question should be asked on History site. I agree and I might do that, but not before using all my options to make this question valid, since I already asked it here. I stand by this question, as I think understanding how humans affected their environment is the key to create rich, deep worlds and that could be a reason to allow this question to be more vague than others (although I have a feeling there are many questions on this site which introduce such world characteristics that lead to many more possible answers). The additional benefit to asking here is that it allows for more freedom in giving examples (for example, on history site it's less likely to introduce aliens, elves or magical beings in argumentation) as well as is more forgiving for factual inaccuracy and soft-science.
Here's two examples:
- global warming - people and domesticated animals produced greenhouse gases, which warmed the planet. Without humanity the Earth would be cooler [and with another civilization it could as well be hotter, but let's consider other civilizations outside of the scope of this question]
- domesticated animals - people changed the way animals evolved: sometimes indirectly [this could be considered the butterfly effect], and sometimes directly through intended manipulation. So without humanity not only wouldn't we see domesticated animals like dogs, but it's also possible we wouldn't see some "wild" animals like dingoes:
The dingo (Canis lupus dingo) is a wild dog found in Australia. Its exact ancestry is debated, but dingoes are generally believed to be descended from semi- domesticated dogs