A spring forward for hominin evolution in East Africa.

Mark O Cuthbert,Gail M Ashley,Richard G Taylor

doi:10.1371/journal.pone.0107358

Mark O Cuthbert, Gail M Ashley + Show 1 more

Open Access

https://doi.org/10.1371/journal.pone.0107358

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Abstract

Groundwater is essential to modern human survival during drought periods. There is also growing geological evidence of springs associated with stone tools and hominin fossils in the East African Rift System (EARS) during a critical period for hominin evolution (from 1.8 Ma). However it is not known how vulnerable these springs may have been to climate variability and whether groundwater availability may have played a part in human evolution. Recent interdisciplinary research at Olduvai Gorge, Tanzania, has documented climate fluctuations attributable to astronomic forcing and the presence of paleosprings directly associated with archaeological sites. Using palaeogeological reconstruction and groundwater modelling of the Olduvai Gorge paleo-catchment, we show how spring discharge was likely linked to East African climate variability of annual to Milankovitch cycle timescales. Under decadal to centennial timescales, spring flow would have been relatively invariant providing good water resource resilience through long droughts. For multi-millennial periods, modelled spring flows lag groundwater recharge by 100 s to 1000 years. The lag creates long buffer periods allowing hominins to adapt to new habitats as potable surface water from rivers or lakes became increasingly scarce. Localised groundwater systems are likely to have been widespread within the EARS providing refugia and intense competition during dry periods, thus being an important factor in natural selection and evolution, as well as a vital resource during hominin dispersal within and out of Africa.

Highlights

Hominin fossil discoveries in the last few decades have shown that humans evolved in Africa and migrated in waves to other parts of the world, starting as early as 1.85 Ma [1]
The climate is likely to have varied on the millennial timescale due to tropical expressions of Dansgaard-Oeschger cycles [12], as well as on a decadal timescale governed by variations in El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) variations [13,14]
Whereas it is not possible to be precise about the absolute rates of recharge and discharge, the groundwater models we have developed here, based on our paleoenvironmental reconstruction and hydrological conceptual model, are able to realistically estimate the plausible range of variation in flows as well as their persistence during dry periods

Summary

Introduction

Hominin fossil discoveries in the last few decades have shown that humans evolved in Africa and migrated in waves to other parts of the world, starting as early as 1.85 Ma [1]. The strengthening of the Walker circulation around this time [13] is likely to have led to more extreme climate variability on the annual to decadal timescale This may have been significant for providing an intensification of rainfall and increased frequency of runoff-recharge events [14,45]. ‘Diffuse’ groundwater recharge across the wider region was unlikely given the predominant semi-arid climate conditions unless there were occasional very prolonged and intense rainfall events Such events may have enabled soil moisture deficits to be overcome locally and/or triggered preferential flow pathways through the soils to enable water to escape the zone of evapotranspiration [14,47]. Using Brutsaert’s [48] solution for a unit response to a step change in recharge as follows: uðtÞ~

Hi2 z z

A C ð8Þ with

Results and Discussion