Johannesburg – The arrival of medium and large antelope on African soil coincides with the evolution of thorn trees in the African savanna, according to research by the University of the Witwatersrand.Scientists found that trees like African acacias evolved thorns as a defence mechanism at exactly the same time that antelope arrived in Africa, a remarkable example of co-evolution, the university said in a statement on Tuesday.They compared the evolution of thorns on 2 000 woody tree species in Southern Africa with the arrival of antelopes. The research was conducted by a group of South African scientists, including Dr Gareth Hempson of the School of Animal Plants and Environmental Studies at Wits. Hempson said thorns appeared to be effective against medium and large browsers like impala and kudu.He developed a herbivore biomass map of Africa while a post-doctoral fellow at Wits and the University of Cape Town.When he compared his map with a map of spiny tree abundance which UCT researcher Dr Tristan Charles-Dominique had drawn up, they noticed how they overlapped.“The match with medium and large browsing species was remarkable,” he said.The study was published in the journal, Proceedings of the National Academy of Sciences, on Monday. The study used DNA data from African trees collected by Prof Michelle van der Bank and her team at the University of Johannesburg, to reconstruct the history of spiny plant evolution.In collaboration with Prof Jonathan Davies, an expert in phylogenetic analysis from McGill University, the team was able to date the evolutionary origins of spines.“We were shocked to discover that spiny plants only appeared about 15 million years ago, 40 million years after mammals replaced dinosaurs” said Van der Bank.For most of this time, Africa was an island continent dominated by now-extinct ancestors of browsing elephants and hyrax.Antelope were latecomers to Africa. They appeared only after the continent collided with Eurasia. They browsed in novel ways and were highly efficient herbivores.