Producing food baring plants results in greenery. Eating food from food baring plants tends to involve eating seeds, which when pooped naturally, will result in more plants and more greenery. Without this basic principle, there would not be mammals.
What evidence is there that cows exist because of humans? As far as I am aware they are descendents of wild Ox and:
A cow and an ox are members of the Bovinae subfamily. In terms of physiology, cows and oxen do not have significant differences. But humans differentiate cows and oxen according to their specific use in the farm. So here are some unique differences between a cow and an ox.
Read more: Difference Between Ox and Cow | Difference Between http://www.differencebetween.net/science/nature/difference-between-ox-and-cow/#ixzz5UVke3ZGH
They are many 1000s of years old at least.
Without humans there would be less cows, yes - and less suffering as a result.