I’m not expert enough to give you a perfect answer but here’s a shot:
- All evolution, all the way back down to single-celled organisms, affected us directly or indirectly (by helping set the path we were on).
- During this extremely long process genes were selected in relation to whatever factors affected the survivability of their carriers in the carriers’ environment (heat, cold, high pH, low pH, predators, viruses, etc.).
- Mammals developed the ability to deal with more variable temperatures (why exactly I’m not sure actually).
- Moving on to early humans we started in Africa, and migrated out. At some point in the relatively recent past ( within the last 100,000 years) there was an ecological crisis in Africa and we almost died out. Then especially extreme living conditions were present for a significant time, and one of the reasons our species’ survived was the early development of tools.
- Besides ecological crises, movement was motivated by the pursuit of more food elsewhere.
- During these movements humans already had some technology and ability to make clothes and tools and housing, so winter became more survivable (but remember, even in Africa it’s not warm all the time everywhere and the desert for example can get very cold at night).
- They also had time to gradually adjust to conditions as they could only move at a walking, riding or sailing pace.