In part two of our series exploring the origins of Africa’s wildlife, William Gray explains how the mammals made their move after the demise of the dinosaurs, 65 million years ago.
They had already been around for some time – secretive, nocturnal little critters lurking in the undergrowth and snacking on clutches of dinosaur eggs if given half a chance. Primitive rodent-like mammals evolved long before the afrovenator, or any other reptilian heavyweight, ruled Africa. Once the mass extinction of the dinosaurs was complete, mammals were poised to steal the limelight – by way of a remarkable process of diversification that would ultimately lay the foundation for today’s safari tick list.
Curiously, however, most of Africa’s familiar species – from zebras, giraffes and antelopes to lions, rhinos and wild dogs – began evolving not in Africa, but on the continents of Eurasia and North America.
Picture the scene 50 million years ago. With temperatures averaging 14ºC warmer than today, tropical forests smothered every scrap of land, from pole to pole. In this humid, tangled stew, African mammals were already sprouting evolutionary branches that would lead to elephants, primates, hyraxes, tenrecs and elephant shrews. Fast forward a few million years. By now the earth was beginning to cool and the North American and Eurasian forests were thinning, giving rise to patchy plains – a new land of opportunity to be exploited by the ancestors of horses, camels, rhinos, buffalos, deer, cats and dogs.
Africa’s mammals were also diversifying, even though the continent remained largely cloaked in jungle. Recent fossil discoveries at Chilga in Ethiopia have reaped several species of archaic elephants (including the palaeomastodons and more advanced deinotheres and gomphotheres), as well as a veritable ark of other species. Some of these early African mammals were huge and, it has to be said, spectacularly ugly. Take the arsinoitherium (pictured), for example. Standing 2m at the shoulder, it sported a pair of massive horns protruding from either side of its snout – a kind of early attempt at a rhino.
Modern day rhinos are the descendants of a species of European origin. So why, you may ask, does Africa’s treasured Big Five contain this European import rather than Africa’s homespun version? Well, scientists are still pondering the demise of the poor old arsinoitherium, but it undoubtedly had something to do with the fact that 24 million years ago ‘island Africa’ collided with Eurasia (which has long been connected by a land bridge to North America). Mammals from all continents began to mix and compete for the first time and the evolutionary shakeout inevitably had winners and losers. While more primitive species became extinct, the Eurasian influx modernised Africa’s fauna. Some mammals cradled in Africa, such as hyraxes, elephants, aardvarks, monkeys, spring hares and golden moles, held their own (and some, like the elephants, invaded new territory), while Eurasian herds began to roam Africa.
By the onset of the Miocene epoch, 13 million years ago, Africa’s newly formed grasslands were witnessing an influx of antelope and buffalo. Short-necked giraffes, ancestral hippos and sabre-toothed cats spread from Eurasia, while dogs and bear-dogs arrived from North America. Africa’s primates showed the first signs of venturing from their ancestral forest home to forage across the spreading savannahs.
This momentous ‘meeting of mammals’ reached its climax in the Pleistocene, about two million years ago, when Africa was home to an unrivalled range of mammal families. While the Ice Age reaped its toll of great mammals elsewhere in the world, Africa’s megafauna got off lightly.
Ironically, it was only with the arrival of humans – the continent’s most recent mammalian progeny – that Africa’s empire of mammals began to crumble.