JOHANNESBURG, South Africa, Sept. 6 (UPI) -- Several million years ago, planet Earth featured a much smaller percentage of grasslands. Africa was covered in thick forest.
Today, savanna makes up some 20 percent of the planet's land surface. What changed?
New research suggests the arrival of medium- and large-sized leaf-eaters like antelopes precipitated the transition from forest to grassland in Africa -- not climate change.
By looking at the arrival of thorned tree species and antelopes in Africa, a team of scientists in South Africa were able to show African acacias evolved sharp spikes as a defense mechanism against hungry, four-legged leaf-eaters.
"It all makes perfect sense," Gareth Hempson, a researcher at the University of Witwatersrand, said in a news release. "Spines really appear to be most effective against medium- and large-sized browsers like impala and kudu, and spiny trees are most common in the places where these animals are most abundant."
When researchers explored the link further, they found the abundance and diversity of spiny plants were greatest among arid savannas with large populations of mammal browsers like antelope.
The new data allowed researchers to retrace the arrival and evolution of spiny plants -- and the leaf-eaters that precipitated their diversification -- across Africa.
"We were shocked to discover that spiny plants only appeared about 15 million years ago, 40 million years after mammals replaced dinosaurs," said Michelle van der Bank, a researcher at the University of Johannesburg. "For most of this time, Africa was an island continent dominated by now-extinct ancestors of browsing elephants and hyrax."
Apparently, spines weren't an effective defense against Africa's original grazing mammals.
It was the antelope, researchers argue in a new PNAS paper, that began decimating young trees, allowing grasses to encroach on land once dominated by forest.