[RightWhale]

however would a largely nomadic society just decide, hey, lets head south non stop?

Of course. No question. Not the whole society. There are individual explorers in society. Even the animals have such. One lone person or polar bear takes off cross-country just to see what there is too see. Or maybe a single family unit.

Dad - We're going to the Amazon.

One day passes . . .

Kids - Are we there yet?

[HamiltonJay]

One person may have, but that one person would not have left much of an archialogical trail.. remember we're talking archeology here... when they say spread to the 4 corners it means society and civilization spread to the 4 corners, not just a "lone gunman".