Academic journal article Antiquity

Complex Topography and Human Evolution: The Missing Link

Academic journal article Antiquity

Complex Topography and Human Evolution: The Missing Link

Article excerpt

Introduction

There are many hypotheses about hominin environments (Ports 1998a, 2007), each drawing on different evolutionary theories and palaeoenvironmental data sets to characterise the selective regimes driving hominisation. They fall into two groups. One emphasises climate, positing either simple climatic shifts (Dart 1925; Morgan 1972) or changes in climatic variability (Foley 1987; Potts 1998b) as drivers of corresponding changes in the niches available to hominins. The other focuses on specific vegetation types that might have selected for hominin traits by generating unique ecological opportunities for these species to exploit (Blumenschine et al. 1987; Thorpe et al. 2007). In recent years the role of woodlands in producing some human characteristics has gained acceptance (O'Higgins & Elton 2007), but the original savannah hypothesis still underpins many discussions of human origins (Cerling et al. 2011; Feibel 2011a).

In this paper, we focus on the anatomical features associated with locomotion, proposing the physical landscape and particularly the complex land forms typical of rifting and active tectonics as a key driving factor. We highlight the limitations of hypotheses based solely on climate or vegetational change in accounting for the evolutionary transition from tree-dwelling to ground-dwelling bipedalism, show how complex topography provides a better explanation for the specific anatomical features associated with the human evolutionary trajectory and divergence from other primates, and emphasise the need for new research that takes account of the long-term history of rift dynamics and provides reconstructions of the physical landscape at an appropriate scale.

Here we propose that conceptualising hominin environments as 'landscapes' of complex topography brings into focus a variable that has been missing from evaluations of anatomical evolution, and one that helps to explain the inconsistencies in existing theories. This 'complex topography hypothesis' supplements and complements vegetational and climatic alternatives rather than completely replacing them. It entails predictions about the hominin evolutionary trajectory which can be tested against those produced by alternative hypotheses, and opens up a new research agenda of field investigation.

Limitations of existing hypotheses

The original savannah hypothesis proposed that aridification thinned out the forests and forced hominins out of the trees onto savannah plains via an intermediate stage involving the use of the remaining trees for security (Dart 1925). Terrestrialisation was identified as the driver of the hominin-panin split, responsible for the appearance of most characteristic hominin features including upright bipedalism. However, the theory ultimately lost support precisely because no savannah niche exploitable by semi-terrestrial and relatively defenceless apes was identified.

Newer 'woodland' hypotheses (Blumenschine et al. 1987; Ports 2007; Thorpe et al. 2007) have proposed that upright posture or even bipedal gait evolved in the ancestral hominids within a closed, forested environment, and that this served as a pre-adaptation that later facilitated the hominins' transition from arboreality to terrestriality as the forests disappeared (O'Higgins & Elton 2007).

In these newer theories, the relative timing of evolutionary changes is different, but climatically driven vegetational change remains the key driver promoting evolutionary diversification within the hominin lineage (Figure 1). However, an upright climbing adaptation, evolved within the context of tree-dwelling, would not produce all the features required for effective rapid, long-distance terrestrial bipedalism. Explaining how our ancestors survived a locomotor transition in a relatively dangerous semi-open habitat remains a critical challenge to these hypotheses.

Palaeoenvironmental evidence is insufficient to distinguish between these alternative vegetational hypotheses because preservational biases, time-averaging and the post-depositional transport of remains make it impossible to obtain precise dates for the evolutionary and climatic events that constitute their key predictions. …

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