Many of Earth's ecosystems got their start some 7 million years ago, as global temperatures began to plummet.

According to new research by scientists at Brown University, a prolonged period of global cooling at the end of the Miocene epoch ushered in the expansion of grasslands across Africa and Asia, as well as North and South America.

During the Miocene, global temperatures were significantly warmer than modern temperatures. But a new record of ocean temperatures amassed by Brown scientists suggests temperatures dropped between 7 million to 5.4 million years ago.

As ocean temperatures dropped to near modern levels, subtropical regions shrank and modern ecosystems began to take shape. Rain forests receded across the globe, and the Sahara Desert formed in Africa.

"This is the first time the late Miocene has been put in a context of global sea surface temperatures, and we were surprised to see the amount of cooling we found," study leader Timothy Herbert, an Earth scientist at Brown, said in a news release. "In light of this temperature change, the paleobiological observations from this period start to make a lot more sense."

Herbert and his colleagues amassed the new record of ocean temperatures by sampling and analyzing ancient sediment from all over the world.

Their analysis suggests cooling was most dramatic near the poles and more moderate toward the equator. Though their study didn't measure CO2 levels, their findings -- detailed in the journal Nature Geoscience -- support the theory that a reduction in carbon dioxide triggered the global temperature decline.

In addition to cooling the planet, a reduction in atmospheric CO2 likely also encouraged the expansion of grasses and shrubs.

Researchers are now working to determine what precipitated the likely decline in CO2. An answer may be some time off, but what is clear is that the climatic shift at the end of Miocene was more significant than researchers previously thought.

Scientists believe the expansion of grasslands across Africa set the stage for the evolution of humans.

"The prevailing view was that this wasn't a particularly exciting time in terms of global climate," Herbert said. "It turns out to be more interesting than people thought."