African paleoclimate and early hominin evolution.

Olduvai Gorge. From Wikimedia Commons

Over the last ten million years the landscape of East Africa has been altered dramatically. It has changed from a relatively flat, homogenous region covered with tropical mixed forest, to a heterogeneous region, with mountains over 4 km high and vegetation ranging from desert to cloud forest. Long-term climate change seems to be modulated primarily  by tectonic changes. The progressive formation of the East African Rift Valley led to increased aridity and the development of numerous lake basins.

Five major transitions have influenced African climate during the early stage of human evolution: 1)  the emergence of  and expansion of C4 biomes (~8 Ma); 2) The Messinian Salinity Crisis (~ 5.3 Ma); 3)  the Intensification of Northern Hemisphere Glaciation during the Pliocene epoch between 3.6 and 2.7 million years ago;  4) the development of the Walker Circulation; 5) the Early-Middle Pleistocene Transition.

Map of East Africa with modern lake and paleolake basins (from Maslin et al., 2014)

It has been hypothesized that both the uplift of the Tibetan Plateau about 8 Ma ago and the reduction of the Paratethys Sea intensified the seasonal Indian monsoon climate,  and that the more seasonal climate favored grasses over trees.

The isolation of the Mediterranean Sea from the Atlantic Ocean was caused by the tectonic closure of the Strait of Gibraltar. During the Messinian Salinity Crisis, the Mediterranean Sea went into a cycle of partly or nearly complete desiccation and removed nearly 6% of all dissolved salts in the oceans.

The Intensification of  Northern Hemisphere Glaciation (iNHG), the third regional climate event,  was characterised by periodic advances and retreats of ice sheets on a hemispherical scale and was the culmination of long-term high latitude cooling, which began with the Late Miocene.

Diatomites of the genera Stephanodiscus and Aulacoseira. (From Kingston et al., 2007)

The Early-Middle Pleistocene Transition, represents a major global climatic reorganization that profoundly affected ocean and atmospheric circulation, ice sheets and the distribution and evolution of biota.

The diatomite deposits from Pliocene lakes in the Baringo Basin suggest that the lakes appear rapidly, remain part of the landscape for thousands of years, then disappear in a highly variable and erratic way. Two dominant genera of diatoms present in East African lakes and Pliocene-Recent deposits helps to understand the dynamic of these humidity/aridity cycles: Aulacoseira predominates under cool windy conditions, while Stephanodiscus predominates under warmer, less windy conditions. The segregation of Aulacoseira and Stephanodiscus into subtle layers on a scale of < 100 mm and the presence of micro-laminae on a scale of one hundred to a few hundred microns suggest cyclic variation in a time frame of one to a few years (Kingston et al., 2007).

Early human evolutionary theories and climate change. From Maslin et al. 2014

The major events in hominin evolution have occurred in East Africa. Several theories have been developed to explain the interaction between African paleoclimate and early hominid evolution. The savannah hypothesis suggested that hominins were forced to descend from the trees and adapted to life on the savannah facilitated by walking erect on two feet. This idea was already outlined by Lamarck in his Philosophie zoologique (1809], where he describes in details how an early ancestor of primeval human abandons an arboreal life to adapt itself to open plains.

More recent, the pulsed climate variability hypothesis  highlights the role of short periods of extreme climate variability specific to East Africa in driving hominin evolution and subsequent dispersal events (Maslin and Trauth, 2009). These periods of ‘pulsed climate variability’ are characterized by the appearance and disappearance of large, deep lakes in the East African Rift Valley. Paleoclimatic information derived from benthic foraminifera, regional aeolian dust flux data and the East African lake record indicates that hominin speciation events and changes in brain size seem to be statistically linked to the occurrence of ephemeral deep-water lakes (Shultz and Maslin, 2013).

References:

Maslin M.A., C. Brierley, A. Milner, S. Shultz, M. Trauth, K. Wilson “East African climate pulses and early human evolution” Quaternary Science Reviews (2014).

Maslin M.A., ‘Cascading uncertainty in Climate Change models and its implications for policy’ Geographical Journal 179, 264-271 (2013)

Ashley, G., Bunn, H., Delaney, J., Barboni, D., Domínguez-Rodrigo, M., Mabulla, A., Gurtov, A., Baluyot, R., Beverly, E., Baquedano, E., 2014. Paleoclimatic and paleoenvironmental framework of FLK North archaeological site, Olduvai Gorge, Tanzania. Quat. Int. 322e323, 54-65.

Shultz S, Maslin M (2013) Early Human Speciation, Brain Expansion and Dispersal Influenced by African Climate Pulses. PLoS ONE 8(10): e76750. DOI: 10.1371/journal.pone.0076750

John D. Kingston et al., Astronomically forced climate change in the Kenyan Rift Valley 2.7- 2.55 Ma: implications for the evolution of early hominin ecosystems, J Hum Evol (2007), doi:10.1016/j.jhevol.2006.12.007