You are being asked to write what is called a compare and contrast essay. There are a few ways that you can frame your assignment. The one I would choose is to find two primary sources, one from the late fifteenth or sixteenth century and one from the late nineteenth century. Read both carefully, noting what seems to you the most striking difference in their views of the American landscapes and peoples. Note two to three salient and related differences.
For a thesis, try to developed a causal account for these changes, i.e. something on the order of "European beliefs about X, changed because of several factors, including A, B, and C". Next, research primary sources from the intervening centuries to elucidate the nature and causes of the changes you have identified.