15,000 years ago should be the answer. The Cro-Magnon man (which is basically us) went into the North American continent around 45 to 12 thousand years BC.
The North survived on factory work mostly. The textile (clothes & fabric) industry hit big in the North. However, the North didn't seem to realize what the clothes came from, which was cotton. Cotton was grown in the South, which was largely of an agricultural place. You know, farms and stuff...