Why are early hominin fossils rare outside the Great Rift Valley and South African caves?

Early hominin fossils are rare outside of specific regions like the Great Rift Valley and South African limestone caves because geological conditions favorable for the preservation and excavation of such remains are uncommon.1

The preservation of fossils requires specific environments that protect organic remains from rapid decomposition. In many regions, particularly within tropical rainforests, acidic soils and humid climates accelerate the breakdown of organic matter, making fossilization unlikely. Conversely, areas like the Great Rift Valley benefit from unique geological activity, such as volcanic eruptions that deposit layers of ash to cover and protect remains, followed by seismic activity and erosion that eventually expose these fossil records for discovery. Similarly, South African cave sites acted as natural traps that accumulated bone remains over geologic time.123

Because these specific conditions—which facilitate both the long-term preservation of bone and its eventual exposure—are geographically restricted, our current understanding of early hominin diversity is largely limited to the few regions where such favorable taphonomic processes have occurred.1