User blog comment:Thedoucheinthenorth/Skyrim alignment charts-verdict?/@comment-4586417-20120804150651/@comment-5135631-20120805114322

Think of it this way. A Breton climbs over the mountains into the Reach, where a Nord maiden is picking flowers. The adventuring Breton and the Nord make a kid. That kid grows up to have traits of both races.

Simply put, the Reachmen couldn't have been in the Reach before the Nords because the Reachmen were born in the Reach to a Nord and a Breton, therefore meaning the Reachmen were not the first humans in the Reach.