Six million years ago, grass-eating mammals became more diverse in South America. Shifts in tropical atmospheric circulation, known as Hadley circulation, led to changes in climate and vegetation, especially grasses, which expanded the mammals' range, according to National Science Foundation-funded geoscientists. The scientists used a computer model to compare predictions of past climate with the record of rainfall and vegetation in ancient soils. South America's climate became drier, subtropical grasslands expanded and the number of mammal species that depended on grasses increased. The results offer a new understanding of a time when ecosystems much like today's evolved. The study, which took place in and around South America's Andes Mountains, showed that the mountains did not play a role in the ancient climate change and vegetation shift. The results offer a different perspective from the prevailing view that high mountains are often responsible for altering atmospheric circulation.
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