Evolution and dispersal of the genus Homo: A landscape approach
Introduction
The timing, geographical location and causes of the origins of the genus Homo (which is usually presumed to have split from one of the australopithecine lines) and the subsequent expansion of our lineage in Europe and Asia are the foci of ongoing interest and debate. The broad consensus is that the genus originated in Africa between two and three million years ago (Ma), followed by a wide-spread dispersal of Homo to lower and middle latitudes in Europe and Asia at or after about 1.8 Ma (Grine et al., 2009, Joordens et al., 2013). The presumption is that australopithecine adaptations were more narrowly confined by ecological, physical or climatic conditions to habitats in Africa, and that these constraints ceased to impede dispersal of the Homo lineage, either because it had acquired a new adaptive capacity that was able to overcome them, or because environmental change had removed them. Given uncertainties about the confounding effects of differential visibility, we cannot be sure that australopithecines were not more widely distributed outside Africa (or within it) (e.g., Dennell, 2003, Dennell, 2008, Dennell and Roebroeks, 2005). Nevertheless, the pattern of Homo dispersal seems to have been genuinely different, expanding into new territory, albeit with total population sizes that appear to have remained small (Sherry et al., 1997, Huff et al., 2010). We therefore take the existing pattern as a legitimate starting point for exploring the relationship between external environmental variables and the distribution of known hominin sites, while recognizing the ever-present uncertainties posed by factors of differential survey and preservation.
A variety of explanations have been proposed to explain these broad differences in distribution, drawing variously on external environmental factors, particularly climate change, or intrinsic changes in biological or cultural potential. In this paper we propose an approach to Homo dispersal which introduces spatio-temporal variation in the physical landscape as an important factor. We draw, in particular, on the role of complex topography and its relationship to active tectonics as a potential additional factor driving the hominin evolutionary trajectory, an approach variously referred to as ‘the tectonic landscape model’ or the ‘complex topography hypothesis’ (King and Bailey, 2006, Reynolds et al., 2011, Winder et al., 2013; see also the debate in Thorpe et al., 2014 and Winder et al., 2014). We summarize the features of the model and further develop it to provide an explanation of dispersal in the Homo lineage, which, we argue, avoids some of the difficulties raised by alternative explanations. We explore the implications of this model by comparing the distribution of landscape features, hominin site locations, and other environmental variables, and outline the difficulties of implementing such an approach and remaining issues in need of further investigation.
Section snippets
Background
Since Darwin (Darwin, 1859, Darwin, 1872) and Wallace (Wallace, 1876, Wallace, 1880), dispersal has been conceived of as a process integral to biological evolution, with the clear implication that both dispersal and evolution should be explicable in relation to the same principles. According to modern biological theories of dispersal (e.g., Bowler and Benton, 2005, Dytham, 2009), species that expand beyond the margins of their pre-existing habitat tend to move into areas closely similar to
The tectonic landscape model
The tectonic landscape model (Reynolds et al., 2011: 281) focusses on the potential role in human evolution of complex topography, meaning a land surface characterized by irregularities in surface morphology or ‘roughness’ (Bailey et al., 2011: 260). Rough land surfaces are commonly (though not uniquely) associated with tectonically active landscapes, characterized by ongoing earthquake activity, faulting and volcanism. Winder et al. (2013) draw attention to the following six characteristics of
Mapping landscape features
For mapping complex landscape features, we use measures of topographic roughness, defined as irregularities in surface morphology, using digital elevation models (DEMs) derived from satellite data. The full details of the concepts and the mathematical methods for transforming digital elevation data into measures of roughness are set out elsewhere (King and Bailey, 2006, Bailey and King, 2011, Bailey et al., 2011, Reynolds et al., 2011), and in summary form here, and in the figure captions (
Mapping landscapes and dispersal routes
In producing low-resolution maps of roughness at the world scale, we use slope angles as a proxy measure of roughness (Fig. 2b). In addition, we show maps that exclude complex topography in the northern hemisphere at high altitude (Fig. 2c) and high latitude (Figure 2, Figure 3) on the assumption that the earliest Homo dispersals out of Africa would have avoided very cold climate regimes even if topographic conditions were otherwise favourable. We have further refined this modification to
Discussion
The key to the complex topography hypothesis as applied to early Homo dispersal lies in two factors. The first is the attractions afforded by complex topography in terms of tactical advantage, access to biomass-rich areas where large mammals could be targeted with relatively high chances of success, predictable water supplies and other resources. The second is the constraint imposed on dispersal, in confining the main pathways of dispersal to regions of complex topography with these advantages.
Conclusions
We have argued here that spatio-temporal variation in the physical landscapes in which hominins lived would have been a significant part of their environment and we have shown how simple techniques taking advantage of satellite data can be used to map these variables in relation to locations of fossil and archaeological evidence. An important outcome is the demonstration of how topographic and climatic variables can interact to accentuate constraints or opportunities for hominin settlement and
Acknowledgements
We acknowledge funding support from the European Research Council (ERC project 269586 DISPERSE of the Seventh Framework Programme). I.C.W. also acknowledges additional support from the Holbeck Charitable Trust, the Leathersellers' Company Charitable Fund, the Department of Archaeology (University of York) research fund and a Charles A. Lockwood Memorial Grant administered by the Primate Society of Great Britain. This paper is DISPERSE contribution no. 24 and IPGP contribution no. 3653.
References (142)
- et al.
Early to Middle Pleistocene Homo dispersals from Africa to Eurasia: geological, climatic and environmental constraints
Quatern. Int.
(2012) - et al.
Climate forcing of first hominid dispersal in Western Europe
J. Hum. Evol.
(2009) - et al.
Archaeology of the continental shelf: marine resources, submerged landscapes and underwater archaeology
Quat. Sci. Rev.
(2008) - et al.
Dynamic landscapes and human dispersal patterns: tectonics, coastlines, and the reconstruction of human habitats
Quat. Sci. Rev.
(2011) - et al.
Landscapes of human evolution: models and methods of tectonic geomorphology and the reconstruction of hominin landscapes
J. Hum. Evol.
(2011) - et al.
Symbolic behaviour and the peopling of the southern arc route to Australia
Quatern. Int.
(2009) - et al.
Following Pleistocene road signs of human dispersal across Eurasia
Quatern. Int.
(2013) - et al.
Way out of Africa: Early Pleistocene paleoenvironments inhabited by Homo erectus in Sangiran, Java
J. Hum. Evol.
(2009) - et al.
ESR at Treugol'naya Cave, Northern Caucasus Mt., Russia: dating Russia's oldest archaeological site and paleoclimatic change in oxygen isotope stage 11
App. Radiat. Isotopes
(2005) - et al.
Paleoenvironmental and paleoclimatic proxies to the Early Pleistocene hominids of Barranco León D and Fuente Nueva 3 (Granada, Spain) by means of their amphibian and reptile assemblages
Quatern. Int.
(2011)