The most important aspect of the monroe doctrine (1823) was that it

  showed the US backed the independence of Latin America.

Explanation:

The United States was still young at the time the Monroe Doctrine was declared, and did not have a powerful navy to be patrolling the South American coast at that time.  But the US did want to keep European powers from encroaching into the Western Hemisphere, and wanted to put Europe on notice to that effect.  

President James Monroe asserted the doctrine in his annual address to Congress in 1823.  The doctrine was that the US would not interfere in European affairs, but also would view any attempts by European powers to take control of any nation in the Western Hemisphere as a hostile act against the United States.

As reported by the US Office of the Historian, there were some additional motives in mind in the US position, in addition to backing the independence of Latin American nations.  "Monroe’s administration forewarned the imperial European powers against interfering in the affairs of the newly independent Latin American states or potential United States territories. While Americans generally objected to European colonies in the New World, they also desired to increase United States influence and trading ties throughout the region to their south."

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.