A number of pollen diagrams from the Llanos Orientales of Colombia and the Rupununi Savannas of Guyana show that the actual open savannas were preceded by a closed savanna woodland or dry forest with Byrsonima as principal pollen producer. Human influence during the last 3000 years was apparently an important factor in the extension of open savannas. Nevertheless, open savannas occurred also earlier in the Holocene. Even in approximately Late-Glacial time, open savanna sometimes extended temporarily at the expense of the dry forest or woodland. It seems that climatic and atmospheric influences (periods of very dry climate, lightning) were the cause of these extensions. The pollen diagrams suggest anyhow a very unstable equilibrium between the woody Byrsonima association and the herbaceous open savanna. In the North Rupununi Savannas dry forest dominated in most of what probably corresponds to Late-Glacial time. Open savanna was common or dominating there during a shorter interval of the above-mentioned „Late-Glacial” and during most of the Holocene.