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:

Wikipedia: Dingo

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
  • $\begingroup$ This seems like it is just a simply history question. And a broad one at that. In what ways have humans changed the world? A lot. And this is wrong site to ask those kind of questions. Then again, this doesn't even have a question specifically stated in it. It could just as well be closed as unclear what you're asking. $\endgroup$ – Xandar The Zenon Mar 27 '16 at 19:29
  • $\begingroup$ I can see how you tried to constrain the question, however it's still incredibly broad. o.m.'s post is probably the closest you're going to get to an answer, I'm afraid. $\endgroup$ – AndreiROM Mar 27 '16 at 19:34
  • $\begingroup$ I agree this question might be too broad. This is why I asked for general processes instead of detailed scenarios, but maybe I wasn't clear enough about it. $\endgroup$ – Markus von Broady Mar 27 '16 at 19:48
  • $\begingroup$ I think you should ask a similar question in history.se, if you really want to know $\endgroup$ – Xandar The Zenon Mar 27 '16 at 20:00
  • $\begingroup$ Writing is only about 5000 years old. I'm not sure how Humanity could have changed a whole lot in that period. Humans from now in a developmental respect are pretty much the same as those from 5000 years ago. $\endgroup$ – Nzall Mar 27 '16 at 21:57

I was watching a BBC history documentary about the Romans in Britain earlier today and words along the lines of

History is not like science, when you change one variable in history the others don't stay the same. It's all linked, change anything and there's such a massive cascading effect that it's impossible to say what may have happened.

Here are a couple of the key aspects:

Extermination of the mega-fauna. This is still uncertain but it's considered likely that human hunters destabilised the populations and effectively exterminated a lot of these animals.

Deforestation. This is a bigger issue than previously mentioned. Not just Central Europe and the Near East but from Britain, which was solid forest, all the way across Russia.

With just these two differences the world would be a place unrecognisable to us.

but let's consider the way the things we affect, affect the world in their turn.

Elephants knock over trees, fewer elephants, more trees.
Wild herbivores eat young shoots, also keeping down new trees, fewer predators, fewer trees, more erosion.
Beavers build dams, meaning more shallow lakes, lower speed flow on the rivers, less erosion, but we killed all the beavers and cut down the trees that would bind the banks, so even the rivers don't follow the courses they would without us.

A few other things we've done.
Draining the wetlands, this was even done by the Romans.
Extermination of predators, the Iberian Lynx is almost gone, the Eurasian Lion has been gone a long time. The Sabertooth Cat? Not known if this was humans. Marsupial Lion, believed to be humans.

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  • $\begingroup$ The statement in the quote (paraphrase) denies the purpose of whole Worldbuilding site. $\endgroup$ – Markus von Broady Mar 29 '16 at 8:10
  • $\begingroup$ @MarkusvonBroady, only alternate history questions, butterfly effect applies. $\endgroup$ – Separatrix Mar 29 '16 at 8:13
  • $\begingroup$ @MarkusvonBroady, you've said it yourself in the question, but I think you've underestimated quite how much it applies. $\endgroup$ – Separatrix Mar 29 '16 at 8:20
  • $\begingroup$ Obviously the smallest change 5000 years ago could completely turn our world upside down. The point was not to deal with the butterfly effect but to gather information on the processes in which humans change the world. In the end, a writer doesn't need to present an accurately simulated world, he just needs to create a setting that is interesting for readers, and knowledge about such processes help. As for alternate history - there is such tag on this site. Also creating an alternative world is quite similar to creating an alternative history on the same world. $\endgroup$ – Markus von Broady Mar 30 '16 at 11:21
  • 1
    $\begingroup$ @MarkusvonBroady, on top of the way humans affect the world directly, there are the ways in which the things we affect, affect the world. Elephants knock over trees, fewer elephants, more trees. Wild herbivores eat young shoots, also keeping down new trees, fewer predators, fewer trees. Beavers build dams, meaning more shallow lakes, lower speed flow on the rivers, less erosion, but we killed all the beavers and cut down the trees that would bind the banks, so even the rivers don't follow the courses they would without us. $\endgroup$ – Separatrix Mar 30 '16 at 11:30

Deforestation is a big issue. Not just in the Mediterranean but also in central Europe.

Monoculture is another issue.

Unnatural selection through animal husbandry.

Removal of other apex predators.

While a civilization might try to be careful and environmentally conscious, humans would leave their mark -- killing other predators, growing more efficient food.

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Not just deforestation, but also de-deforestation. Before humans got the ability to fight wildfires (not all of which are started by humans), they burned out of control. Now we can put them out or create firebreaks.

The other major change to the landscape, courtesy of humanity, are dams and other hydrological projects.

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  • $\begingroup$ +1 I was missing the point about constructions and the influence on rivers like regulating river deltas. $\endgroup$ – Markus von Broady Mar 27 '16 at 21:40

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