East Africa is a global biodiversity hotspot and exhibits distinct longitudinal diversity gradients from west to east in freshwater fishes and forest mammals. The assembly of this exceptional biodiversity and the drivers behind diversity gradients remain poorly understood, with diversification often studied at local scales and less attention paid to biotic exchange between Afrotropical regions. Here, we reconstruct a river system that existed for several millennia along the now semiarid Kenya Rift Valley during the humid early Holocene and show how this river system influenced postglacial dispersal of fishes and mammals due to its dual role as a dispersal corridor and barrier. Using geomorphological, geochronological, isotopic, and fossil analyses and a synthesis of radiocarbon dates, we find that the overflow of Kenyan rift lakes between 12 and 8 ka before present formed a bidirectional river system consisting of a “Northern River” connected to the Nile Basin and a “Southern River,” a closed basin. The drainage divide between these rivers represented the only viable terrestrial dispersal corridor across the rift. The degree and duration of past hydrological connectivity between adjacent river basins determined spatial diversity gradients for East African fishes. Our reconstruction explains the isolated distribution of Nilotic fish species in modern Kenyan rift lakes, Guineo-Congolian mammal species in forests east of the Kenya Rift, and recent incipient vertebrate speciation and local endemism in this region. Climate-driven rearrangements of drainage networks unrelated to tectonic activity contributed significantly to the assembly of species diversity and modern faunas in the East African biodiversity hotspot.

中文翻译：

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沿肯尼亚裂谷的全新世双向河流系统及其对东非动物群交换和多样性梯度的影响

东非是全球生物多样性热点地区，淡水鱼类和森林哺乳动物从西到东呈现出明显的纵向多样性梯度。这种特殊生物多样性的组合和多样性梯度背后的驱动因素仍然知之甚少，多样化经常在局部范围内进行研究，而对非洲热带地区之间生物交换的关注较少。在这里，我们重建了一个河流系统，该河流系统在潮湿的全新世早期沿现在半干旱的肯尼亚裂谷存在了几千年，并展示了该河流系统由于其作为扩散走廊和屏障的双重作用而如何影响鱼类和哺乳动物的冰期后扩散。使用地貌学、地质年代学、同位素和化石分析以及放射性碳年代的综合，我们发现，在距今12-8ka之间，肯尼亚裂谷湖的溢流形成了一个双向水系，由一条与尼罗河盆地相连的“北河”和一条封闭的盆地“南河”组成。这些河流之间的分水岭代表了唯一可行的穿越裂谷的陆地传播走廊。过去相邻河流流域之间水文连通性的程度和持续时间决定了东非鱼类的空间多样性梯度。我们的重建解释了现代肯尼亚裂谷湖中尼罗河鱼类的孤立分布、肯尼亚裂谷以东森林中的几内亚-刚果哺乳动物物种，以及该地区最近的初期脊椎动物物种形成和地方特有现象。