Dietary options and behavior suggested by plant biomarker evidence in an early human habitat

Abstract

The availability of plants and freshwater shapes the diets and social behavior of chimpanzees, our closest living relative. However, limited evidence about the spatial relationships shared between ancestral human (hominin) remains, edible resources, refuge, and freshwater leaves the influence of local resources on our species' evolution open to debate. Exceptionally well-preserved organic geochemical fossils-biomarkers-preserved in a soil horizon resolve different plant communities at meter scales across a contiguous 25,000 m 2 archaeological land surface at Olduvai Gorge from about 2 Ma. Biomarkers reveal hominins had access to aquatic plants and protective woods in a patchwork landscape, which included a spring-fed wetland near a woodland that both were surrounded by open grassland. Numerous cut-marked animal bones are located within the wooded area, and within meters of wetland vegetation delineated by biomarkers for ferns and sedges. Taken together, plant biomarkers, clustered bone debris, and hominin remains define a clear spatial pattern that places animal butchery amid the refuge of an isolated forest patch and near freshwater with diverse edible resources.biomarker | leaf wax | carbon isotope | paleoecology | human evolution S patial patterns in archaeological remains provide a glimpse into the lives of our ancestors (1-5). Although many early hominin environments are interpreted as grassy or open woodlands (6-8), fossil bones and plant remains are rarely preserved together in the same settings. As a result, associated landscape reconstructions commonly lack coexisting fossil evidence for hominins and local-scale habitat (microhabitat) that defined the distribution of plant foods, refuge, and water (7). This problem is exacerbated by the discontinuous nature and low time resolution often available across ancient soil (paleosol) horizons, including hominin archaeological localities. One notable exception is welltime-correlated 1.8-million-y-old paleosol horizons exposed at Olduvai Gorge. Associated horizons contain exceptionally preserved plant biomarkers along with many artifacts and fossilized bones. Plant biomarkers, which previously revealed temporal patterns in vegetation and water (8), are well preserved in the paleosol horizon and document plant-type spatial distributions that provide an ecosystem context (9, 10) for resources that likely affected the diets and behavior of hominin inhabitants.Plant biomarkers are delivered by litter to soils and can distinguish plant functional type differences in standing biomass over scales of 1-1,000 m 2 (11). Trees, grasses, and other terrestrial plants produce leaf waxes that include long-chain n-alkanes such as hentriacontane (nC 31 ), whereas aquatic plants and phytoplankton produce midchain homologs (e.g., nC 23 ) (12, 13). The ratio of shorter-versus long-chain n-alkane abundances distinguish relative organic matter inputs from aquatic versus terrestrial plants to sediments (13): Lignin monomers provide evidence for woody and nonwoody plants. This refracto...

The FLK Zinj paleolandscape: Reconstruction of a 1.84 Ma wooded habitat in the FLK Zinj-AMK-PTK-DS archaeological complex, Middle Bed I (Olduvai Gorge, Tanzania)

“…Despite this, the conservation of some GSSC phytoliths and, above all, the preservation of hexagonal platelets (hat-shaped) phytoliths of Cyperaceae (sedges), rarely preserved even in surface soil samples (Novello et al, 2012), suggest that paleovegetation patterns, rather than partial dissolution of phytoliths due to taphonomical processes, are responsible for the phytolith assemblages obtained here. We note, however, that using organic biomarkers Magill et al (2016) obtained a strong signal for C 4 grasses in the central area (between FLK Zinj and PTK), which is not corroborated by the abundance of e.g. saddle, rondel, or bilobate grass silica short cell phytoliths.…”

“…Despite the low presence of fern phytoliths in the assemblages, their remains were found in all sites and their presence suggests shady and wet habitat (Kamau, 2012). The presence of ferns in samples from FLK Zinj is also attested by the presence of specific biomarkers (Magill et al, 2016).…”

“…It is mineralogically distinct from Tuff IC, but both tuffs yield indistinguishable argon-argon ages of 1.848 ± 0.008 Ma and 1.848 ± 0.003 Ma (Deino, 2012). Hence, sediments and paleosols between tuffs IB and IC were likely deposited within a time interval of 0.011 Ma (Magill et al 2016). Between Tuffs IB and IC there is a clayey tuff/tuffaceous clay formed primarily of clay and a variable proportion of ashes, silt, and fine sand.…”

“…δ 13 C values for sedimentary leaf lipids and total organic carbon also indicated recurrent ecosystem variations, where open C 4 grasslands abruptly transitioned to closed C 3 forests within several hundreds to thousands of years (Magill et al 2013a) in response to varying precipitation (Magill et al, 2013b). More recently, plant biomarkers from the Zinj level confirmed the presence of a dense forest patch with sedges and shadeloving ferns near the freshwater source at FLK NN and more open vegetation with C 4 grasses at FLK S (Magill et al, 2016).…”

Leaf Wax δD and δ13C in Soils Record Hydrological and Environmental Information Across a Climatic Gradient in Israel

“…Isotopic analysis of leaf wax molecules from soil horizons (paleosols) and sediments from paleontological, paleoanthropological, and archaeological sites can potentially offer climatic and landscape context directly associated with the archaeological record (e.g., Magill et al, ; Uno et al, ). Leaf wax analysis can add to the existing environmental proxies (e.g., δ 13 C and clumped isotopes in soil carbonates; Levin, ; e.g., Quade, ), pollen and phytoliths (for recent compilation see Bonnefille, ) and lignin (Magill et al, ), and assist in understanding the possible links between the environment and biological and cultural evolution.…”