The fact that the global ecosystems form broad latitudinal belts is mostly because of climatic reasons. It works very simply, the shape of the Earth doesn't allow the sun to heat up the surface equally in every place, so on the equator and around it the sun heats up the Earth the most, as the latitude changes the sun rays become weaker and weaker because they fall at a smaller angle and are dispersing much more, this forms the different climatic regions on Earth thus creating different ecosystems with it, ecosystems that have their borders mostly on the lines of change in climate from one region to another, so we mostly have latitudinal belts of ecosystems.
When the Aryans ruled over the region
Due to two factors..
The first factor is the placement of Europe in relation to latitude lines.
Up north is bound to be more cold than most places down south near the Mediterranean sea. (Which is closer to the equator)
The second factor is the physical geography of the region. Ranging from mountain ranges like the Alps and the Urals to the flat planes of much of mid Europe. The weather interacts with these regions and determines the climates making up much of the continent. (Only takes a rudimentary understanding of basic earth science and geography to understand that. If you have more questions pertaining to the subject, pm me.)
