In the early twentieth century, migration within the united states moved towards:

early human migrations are the earliest migrations and expansions of archaic and modern humans across continents and are believed to have begun approximately 2 million years ago with the out of africa migration of homo erectus. this initial migration was followed by other archaic humans including h. heidelbergensis, which lived around 500,000 years ago and was the likely ancestor of both denisovans and neanderthals. early hominids were said to have "crossed land bridges that were eventually covered in water" .

within africa, homo sapiens dispersed around the time of its speciation, roughly 300,000 years ago.the "recent african origin" paradigm suggests that the anatomically modern humans outside of africa descend from a population of homo sapiens migrating from east africa roughly 70,000 years ago and spreading along the southern coast of asia and to oceania before 50,000 years ago. modern humans spread across europe about 40,000 years ago.

the migrating modern human populations are known to have interbred with local varieties of archaic humans, so that contemporary human populations are descended in small part (below 10% contribution) from regional varieties of archaic humans.

after the last glacial maximum, north eurasian populations migrated to the americas about 20,000 years ago. northern eurasia was peopled after 12,000 years ago, in the beginning holocene. arctic canada and greenland were reached by the paleo-eskimo expansion around 4,000 years ago. finally, polynesia was peopled after 2,000 years ago, by the austronesian expansion.

the answers is b.   environments.