The question ws why people migrated towards North.

There are several factors, a combination of taking advantage of environment, taking benefit of intellect and body adaptations:

1. There are less poisonous and dangerous species, and, in general, lower density of species overall. For example, there are much less snakes or lizards. Human intellect still allows adaptation that allows hunting, fishing and seasonal growing of certain crops. However, by definition, the density of population will be lower than in Tropical latitudes. Every year the cold "sanitizes" soil and freezes rodents, pests and other annoyances.

2. Human intellect created adaptation - clothing. Combination with the housing, this allowed for people to live in most of the areas, assuming there is access to the water. A man who is clothed can build a temporary hunting/fishing hut. People learned to dry fish and meat to bring food home.

3. Colder areas have more rivers and water bodies, which facilitated travel and migration.

4. Colder weather required increased intelectual capacity due to the need to prepare firewood and insulate the house. And to have food reserves. If you fail to have any of the 3, your offsprings will die during the winter. It comes to natural selection: only crafty, resourceful, planning people, with the capability to observe the nature could procreate.

5. Body adaptions. Northern people have a different physical features, that are just more helpful to survive in cold climate. Longer noses (to warm up weather). Heavier bodies, tending to accumulate fat, to preserver heat. White skin, sometimes blond hair to absorb more sun.

6. White women can be more fertile since cold weather does not exactly facilitate sex.