You might want to examine in particular the way in which Conrad chooses to describe and present the African landscape that Marlow sails through in order to answer this question. The African landscape seems specifically designed to show both us and Marlow how puny and insignificant man is faced with the might of nature. Consider the following quote, whilst Marlow is still sailing round the continent of Africa:
Here and there greyish-whitish specks showed up clusted inside the white surf, with a flag flying above them perhaps. Settlements some centuries old, and still no bigger than pinheads on the untouched expanse of their background.
Notice how this quote presents the utter insignificance of man in the face of the might of Africa. Even though the settlements that are being described are "some centuries old," they are nothing more than "specks" or "pinheads," not having left any tangible sign of their presence on the landscape surrounding them. Nature in this novel then is a way of reminding man just how insignificant he is.