Tuesday, January 22, 2013

The Albertine Rift: A Schism in Great Ape Solidarity

The Virunga volcano range, as seen from the Space shuttle Endeavour
Space, the final frontier…” So proclaimed Captain Kirk at the start of each episode of Star Trek. But what was the first frontier?

While there’s no fossil evidence to support the theory, it is believed that at the end of the Miocene epoch, seven million years ago, the common ancestor to gorillas, chimpanzees, bonobos, and humans was forced by climatic changes in the Congo Basin to migrate eastward. She may have been bipedal and even more humanoid than gorillas: something of a ‘Humarilla’.

Eventually the jungle came to an abrupt end and the land dropped away into a vast inland sea. She had reached the western escarpment of the Albertine Rift Valley, and the ancient Lake Obweruka, the first of the great lakes to be fed by the Katonga and Kagera rivers. 

As the Humarilla made her night nest, while the shadow of the escarpment moved slowly eastward across Obweruka’s waters then into a faraway forested land that faded into darkness, the Rift must have seemed as impassable as outer space. 

Lake Edward, as seen from the western escarpment of the Albertine Rift Valley

Two million years later, brachiating and palm-walking across the Congo basin jungle with their promiscuous females in tow, our homo-chimp ancestors were much more predisposed to conquer Africa’s geological obstacles. Whether they passed north or south of the 550 kilometre-long Lake Obweruka is a matter of speculation, as they certainly weren’t equipped to swim across it, but in due course they left the rainforest for the woodlands and savannahs of East Africa’s high plateau. 

The gorilla’s story is more sobering. It has to do with rising escarpments, rain shadows and changing river courses. Confined to more specific forests, as climate change caused a reduction in suitable habitats, gorillas got hit hard . 

Around half a million years ago two land bridges emerged across the Rift: the Rwenzori mountains and the Virunga volcanoes. Meantime the eastern escarpment began to rise, eventually cutting off the flow of the Katonga and Kagera rivers which diverted northward to create Lake Victoria. The rising escarpment also created a rain shadow that allowed the forests to thrive again. While the once vast Lake Obweruka then broke up into lakes Kivu, Edward and Albert and the Albertine Rift turned arid, the lush rainforests on the other side must have seemed inviting to the gorillas.

Genetic data confirms the mountain and lowland sub-species of the Eastern gorilla diverged 380,000 years ago. Presumably the mountain gorillas, the only populations found east of the western escarpment, used the Virunga land bridge to cross the Rift. Suffice to say by 10,000 BC, at the end of the last Ice Age, all Eastern gorilla populations had settled into their present habitats, though back then they were much larger and interconnected. Still, never again would mountain and lowland meet.

Then, between 1000 and 500 BC, the Second Bantu migration saw humans begin to move southward from the shores of Lake Victoria into the Congo Basin. The natural path for them to follow was the Albertine Rift Valley where they began to grow bananas. This brought them into direct conflict with the gorillas in the forests, who had hitherto seen  only the occasional Batwa pygmy. After that Eastern gorilla populations went into steady decline.

But when did the Virunga mountain gorillas settle on the cold, windy volcanic slopes that now connect the two sides of the Rift? Until 100,000 years ago the volcanoes were far too active for this to have been a viable habitat. Could it be they were on their way back to the Congo Basin from the forests of the eastern escarpment, and are the decedents of Bwindi gorillas? We will probably never know the answer.

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