Part I
The preceding chapters have described the physical features distinguishing Africa from other continents. Africa is mostly high-lying in its eastern and southern interior with almost equal sections north and south of the equator. Its high interior elevation is due initially to its central location within Gondwana, amplified by tectonic uplift continuing from the Miocene into the Pleistocene. Eastern escarpments generated rain shadows, lowering precipitation leeward in tropical and subtropical latitudes and especially via season restrictions. Both rivers and lakes have fluctuated widely in flow regimes and water levels. All east-flowing rivers may be crossed without swimming in their upper reaches during dry seasons. Ramifying rift valleys disrupted surface topography through the east and generated the local depositional basins that have retained fossils. The major depositional basin in the west accumulated loose sands while in the east sediments extended coastlines. Africa’s eroding plateau surface exposed soils to bedrock influences, fostered additionally by reduced leaching due to the lowered rainfall. Volcanic activity associated with rifting contributed mafic geological substrates generating relatively fertile soils for the tropics. Southern Africa retains volcanic contributions from feeder pipes to the basalt eruptions that accompanied the breakup of Gondwana. Within regions underlain by basement granitic substrates, local hotspots of fertility are associated with wetlands, former lake beds and smaller-scale drainage basins.
Water redistribution within landscapes and geological influences on soils contribute to the spatial heterogeneity that is a notable feature of African savannas. Fluctuations in rainfall contribute to temporal variation driven by the Earth’s shifting orbital geometry. As the Earth has progressively cooled since the late Miocene, conditions within Africa have become increasingly dry, intensifying the seasonal aridity.
Summarising, the distinguishing features of Africa are:
1.contributions of the high interior to rain shadows and bedrock influences on soils;
2.continuing tectonic influences on local precipitation and lake levels;
3.pervasiveness of volcanic enrichment of soil fertility;
4.reductions in locally effective rainfall due to rain shadows and surface run-off;
5.erratic temporal variation in rainfall seasonally and between years;
6.lack of river barriers to block dispersal between north and south.
In combination, these features have contributed to the prevalence of savanna grasslands and the consequent radiation of large grazing ungulates. They also shaped the evolutionary transitions that led from forest-dwelling apes to the sophistication of modern humans, the story to be developed in the chapters that follow.
Figure I.2
Formerly perennial Letaba River reduced to scattered pools in the dry season, Kruger NP, South Africa.