Shifting adaptive landscapes: Progress and challenges in reconstructing early hominid environments

Abstract

Since Darwin situated humans in an evolutionary framework, much discussion has focused on environmental factors that may have shaped or influenced the course of human evolution. Developing adaptive or causal perspectives on the morphological and behavioral variability documented in the human fossil record requires establishing a comprehensive paleoenvironmental context. Reconstructing environments in the past, however, is a complex undertaking, requiring assimilation of diverse datasets of varying quality, scale, and relevance. In response to these difficulties, human evolution has traditionally been interpreted in a somewhat generalized framework, characterized primarily by increasing aridity and seasonality periodically punctuated by pulses or intervals of environmental change, inferred largely from global climatic records. Although these broad paradigms provide useful heuristic approaches for interpreting human evolution, the spatiotemporal resolution remains far too coarse to develop unambiguous causal links. This challenge has become more acute as the emerging paleoenvironmental evidence from equatorial Africa is revealing a complex pattern of habitat heterogeneity and persistent ecological flux throughout the interval of human evolution. In addition, recent discoveries have revealed significant taxonomic diversity and substantially increased the geographic and temporal range of early hominids. These findings raise further questions regarding the role of the environment in mediating or directing the course of human evolution. As a consequence, it is imperative to critically assess the environmental criteria on which many theories and hypotheses of human evolution hinge. The goals here are to 1) compile, review, and evaluate relevant paleoecological datasets from equatorial Africa spanning the last 10 Ma, 2) develop a hierarchical perspective for developing and evaluating hypotheses linking paleoecology to patterns and processes in early hominid evolution, and 3) suggest a conceptual framework for modeling and interpreting environmental data relevant to human evolution in Goals of paleoanthropology extend well beyond the discovery and description of fossil humans. The various species depicted in human phylogenies are not simply drifting through the geometry of morphological or taxonomic space but rather evolving in a dynamic, multidimensional environmental context. Early hominids represent components of a much larger framework of evolving ecosystems and shifting climatic patterns that ultimately directs the evolutionary trajectories of constituent biota. Interpreting and reconstructing early hominid behavior and morphology requires a detailed understanding of the adaptive landscapes navigated by our ancestors. Recognition of this need, however, is trivial relative to operationalizing the endeavor. How do we establish this multidimensional context that embraces all the environmental circumstances encountered by early hominids in the past, ranging from subtle intra-and interspecific biotic interactions...

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

“…Although many early hominin environments are interpreted as grassy or open woodlands (6)(7)(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).…”

“…The behavioral implications of local conditions require understanding of regional climate and biogeography (3)(4)(5)7), because hominin species likely had home ranges much larger than the extent of excavated sites at FLK Zinj. Lake sediments at Olduvai Gorge include numerous stacked tuffs with precise radiometric age constraints (23).…”

Late Miocene to Pliocene carbon isotope record of differential diet change among East African herbivores

“…These paleovegetation proxies integrate over different spatial and temporal scales, and each proxy has inherent strengths and weaknesses when used to reconstruct the relative amounts of C 3 and C 4 vegetation in ancient habitats (5,6).…”

Edaphic and Topographic Constraints on Exploitation of the Central Kenya Rift by Large Mammals and Early Hominins

“…This apparent association between Rift environments and finds of early hominins has given rise to the notion that there may be a causal relationship between the two phenomena, with the distinctive dynamics of rifting acting as a key agent in evolutionary change through one of three possible processes: (1) directly at a macro-geographical and temporal scale through the introduction of major barriers that have served to isolate populations along divergent evolutionary pathways (vicariance hypotheses, Coppens, 1994;Lewin & Foley, 2004); (2) indirectly by amplifying spatial and temporal variability in climate and vegetation (hypotheses of mosaic environments and variability selection, Potts, 1996Potts, , 1998Kingston, 2007;Maslin et al, 2014;Trauth & Maslin, 2009); or (3) directly but at a more localised spatio-temporal scale by the creation of a complex and dynamic regional topography that afforded opportunities for the evolution of the distinctive human niche with its emphasis on meat-eating, bipedalism and an extended period of juvenile dependency (the complex topography hypothesis or tectonic landscape model, King & Bailey, 2006;Winder et al, 2013). However, establishing robust correlations between environmental variables, especially features of the physical landscape, and distributions of fossil or archaeological sites has well-known difficulties: of accurately reconstructing ancient landscape features in regions subject to ongoing geological change; of biases introduced by differential deposition, preservation, exposure and discovery of bones and stones; of chronological correlation between disparate and stratigraphically unrelated features; and of integrating processes that operate over widely divergent spatio-temporal scales.…”

“…We describe our approach as landscape research, but emphasise that it differs in significant ways from the wellestablished tradition of landscape analysis commonly applied in the study of Plio-Pleistocene deposits in the African Rift (e.g., Isaac, 1977;Isaac & Harris, 1978;Bunn et al, 1980;Kroll, 1994;Stern, 1994;Blumenschine & Peters, 1998;Potts, Behrensmeyer & Ditchfield, 1999;Kingston, 2007;Blumenschine et al, 2012). For the most part, these studies have focused on stone artefacts and faunal materials in relation to their immediately surrounding sediments and surfaces at a spatial scale of hundreds of metres to kilometres, with the emphasis on localised distortions of the record by sediment accumulation and erosion, and on reconstruction of environmental features such as vegetational and palaeoclimatic indicators.…”