"by boat" - This describes how the first human colonisation of America occurred. Ex. "I got to work by car." "He wrote a book by pen." - This is the same as "He used a pen to write a book" (他用笔写书) "along the Northwest Coast" - 'Along' is a preposition used to describe an area spread out in a straight line. Ex. "I walked along the beach." "during the very late Ice Age" - This describes when it happened. The Ice Age is 冰淇 in Chinese. "Very late" is used to describe when in the Ice Age the colonisation occurred (ie. near the end of the Ice Age). You could easily rearrange the clauses: "The first human colonization of America occurred by boat along the Northwest Coast during the very late Ice age." = "The first human colonization of America occurred along the Northwest Coast during the very late Ice age by boat." = "The first human colonization of America occurred during the very late Ice age along the Northwest Coast by boat." etc. It's better to stick "during the very late Ice Age" at the end so that it's next to "possibly as early as 14,000 years ago". You could put it in the middle of the sentence (ex. "The first human colonization of America occurred during the very late Ice age, possibly as early as 14,000 years ago, along the Northwest Coast by boat"), but then it'd split the sentence up. You could also remove any of the clauses. Ex. "The first human colonization of America occurred during the very late Ice age." "The first human colonization of America occurred along the Northwest Coast." "The first human colonization of America occurred by boat"

There are three prepositional phrases one after another.