We might not always notice it, but we live on (tectonic) plates.
The interior of the Earth is liquid and more semi-liquid towards the outside. The most outer layer is a hard shell, which consists of plates like plates of ice floating on the water in a pond during thaw. The plates move by the influence of underground currents by some millimetres per year. They float apart from one another or slide across each other or along- or against each other so the edges bend up. For example, the Alps and the Himalayas, were created by the latter.
Thus the American continents originally lay against Europe and Africa, and if you look closely at the world map you will see how beautifully they still fit against each other. The gaps between the plates are called troughs when they are under water, and rifts if such happens on mainland. Of the last, there are only a few examples in Iceland and California.
But 10 million years ago, there also emerged one in East Africa. Somalia tore itself away from the mainland and will become a new continent in the Indian Ocean in about 100 million years from now, because this movement happens only with 4 to 5 mm per year. This rift formation went hand in hand with the emergence of high mountain ridges along the edges. This created an area, between the mountain ridges, that was shielded from the wet winds from the Indian Ocean.
The Rainforest that was there changed into a savanna and that had big consequences for the monkeys who largely live in trees.