How do natural features such as rivers, lakes, and mountains help unite people, or keep them isolated from each other?

Natural features like lakes, rivers, and mountains have always played a large part in our history and divided different countries. Our world is divided into seven continents, which makes four large, separate landmasses. Each one of these landmasses were not connected until the 1700's. During the era of exploration three new continents were founded: North America, South America, and Australia. Each one of these continents had been isolated from the lands everybody knew because of oceans and seas. The ocean had kept North and South America completely isolated from Europe, Africa and Asia. On a smaller note rivers and lakes can also divide people even if they are not that large. Before the Egyptians could sail across the Nile they were separated from the people on the other side. The Himalayas in Asia could have also kept the people who lived there isolated from the rest of the people in Asia. Natural features could also keep people united because they might have to keep together to overcome some obstacles; like the people in Egypt.