Molecular and isotopic perspectives on early human habitats at Olduvai Gorge, Tanzania

Open Access
Author:
Magill, Clayton R
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 19, 2012
Committee Members:
  • Katherine Haines Freeman, Dissertation Advisor
  • Katherine Haines Freeman, Committee Chair
  • Lee Kump, Committee Chair
  • MARK E PATZKOWSKY, Committee Member
  • Nina G Jablonski, Committee Member
Keywords:
  • biomarker
  • isotope
  • evolution
  • Africa
  • biogeochemistry
Abstract:
Early human (hominin) evolution was linked to dramatic changes in regional hydrology and ecosystem composition. Emerging hypotheses linking hominin evolution and the environment present specific, testable predictions for hominin evolutionary responses to different modes of environmental change. Yet, discontinuous terrestrial sediment sequences coupled with indirect proxy indicators for plants and water obscure environmental perspectives on the hominin fossil record. Opportunity to overcome these challenges is presented by lake and soil sediments from a preeminent hominin archaeological locality – Olduvai Gorge, Tanzania. These extensive, well-constrained sediments have abundant organic matter, allowing for unparalleled resolution of biomarker data through time and space for an individual locality. To link the hominin archaeological record to local environmental conditions at Olduvai Gorge, I characterized distributions and stable isotope compositions (δ13C and δD) for a diverse suite of modern and sedimentary biomarkers. Towards this end, I established a quantitative framework for reconstructing plant functional type (PFT) relative abundances in tropical ecosystems based on biomarker δ13C values from living plants by compiling previously published data relating (a) PFT to tropical ecosystem structure, (b) PFT to soil organic matter δ13C values, and (c) soil organic matter δ13C values to biomarker δ13C values. Separation of previously published δD values for biomarkers from living plants according to PFT can elucidate differences in the biochemical and physiological influences on apparent fractionation between source-waters and biomarkers (εlipid/water). By extension, ‘landscape’ apparent fractionation factors are calculated from isotopic mass-balance of εlipid/water values via biomarker δ13C estimates for relative PFT abundances. Using this novel approach to establish fractionation factors in dynamic environments, I then reconstructed δD values for precipitation and lake waters at Olduvai Gorge during the past. The early Pleistocene is associated with a key adaptive juncture in hominin evolution – the emergence of our direct ancestor Homo erectus/ergaster. To constrain local environmental conditions during this juncture, I measured δ13C and δD values for sedimentary biomarkers from a continuous sequence of lake sediments deposited between about 2.0 and 1.8 million years ago (i.e., the early Pleistocene) at Olduvai Gorge. Biomarker δ13C values correlate strongly with changes in orbital geometry and tropical sea-surface temperatures during this interval, and reveal cyclic catchments-scale ecosystem shifts between closed woodlands and open grasslands. After correcting measured biomarker δD values for ‘landscape’ apparent fractionation factors, reconstructed source-water δD values indicate lower amounts of precipitation fell on open grasslands (about 250 mm yr-1) as compared to closed woodlands (about 700 mm yr-1). The scale and pace of environmental changes at Olduvai Gorge contrast with long-held views of directional or step-wise aridification in eastern Africa during the early Pleistocene. Faunal evolutionary responses to environmental change are strongly influenced by microhabitat (<100 m2) features. To reconstruct fine-scale spatial heterogeneity in plants and water associated with hominin habitation, we measured the distributions and δ13C values for plant biomarkers (leaf-waxes, lignin and 5-n-alkyresorinols) preserved in time-equivalent soil sediments across the iconic FLK Zinjanthropus archaeological Level 22 (FLK Zinj) locality at Olduvai Gorge. Central trenches of FLK Zinj are associated with low δ13C values for leaf-waxes and lignin monomers indicative of closed woodland habitat. About 200 m to the north of the central trenches, abundant aquatic-plant and sedge biomarkers occur in conjunction with mound-like tufa deposits, suggesting wetland habitat near freshwater springs. In contrast, southern trenches contain high δ13C values for leaf-waxes and lignin monomers indicative of open grassland habitat. Taken together, these data delimit a heterogeneous microhabitat mosaic that is obscured in catchment-scale records of environmental change.