system
Given the following post, respond as if you're commenting on Reddit:
human
Why did modern man's ancestors migrate to cold climates from warm ones?

It is well established that modern human ancestors began in warm climates, and migrated over many centuries into colder climates. So, why did they leave warm, hospitable climates for colder, harsher climates in the first place?
gpt
Any area can only support a limited amount of population, after that its starvation or migration. Plus natural disasters make people move. Repeat. In most places it gets cold just very little over distance so the migration is very gradually.