Review articleHominin-bearing caves and landscape dynamics in the Cradle of Humankind, South Africa
Highlights
► We provide constraints on the evolution of the landscape in the Cradle of Humankind, South Africa. ► Landscape dynamics can be linked to the distribution of hominin fossils in caves. ► In the past 4 My the landscape in the CoH has undergone major changes. ► Clustering of fossil-bearing caves reflects a Lévy flight patterns linked to availability of water. ► Complex interactions between multiple fractures controlled by uplift shaped the landscape.
Introduction
It was in Africa that anthropoid primates gave rise to a lineage of hominins, our ancestors, through a series of evolutionary steps recorded in the fossil record of mainly eastern and southern Africa. Here hominin fossils are preserved along rifts, in the sediments of ancient lacustrine and riverine systems and in caves, raising the fundamental question whether the distribution of these fossils is the result of preferential preservation in sedimentary trapping sites, or whether the fossil distribution reflects a deeper relationship between hominins and the African landscapes in which they lived (e.g. King and Bailey, 2006). Understanding the relationship between the evolving landscape and fossil sites is, therefore, fundamental to understanding hominin evolution. This paper investigates that relationship for the fossil sites in the Cradle of Humankind, South Africa, focusing on the landscapes near Malapa, which hosts the important Australopithecus sediba fossils which represent a possible ancestral species to the genus Homo (Berger et al., 2010, Pickering et al., 2011a).
The geological record suggests that key evolutionary events in Africa coincided with important changes in the African landscape and climate (e.g. Vrba, 1995, Reed, 1997, Bobe et al., 2002, Bobe and Behrensmeyer, 2004, Passey et al., 2010). Since 35 Ma, the Earth’s climate has cooled (Zachos et al., 2008) as the high plateaus in eastern and southern Africa rose to become one of the largest topographic anomalies on the planet’s surface characterized by at least 500 m of positive residual elevation, commonly referred to as the African “superswell” (Nyblade and Robinson, 1994). Paleoclimate modeling (Sepulchre et al., 2006) indicates a causal link between uplift of the African plateau, and climate and landscape changes, with strong aridification and an increase in open grasslands since the Miocene (Bobe and Behrensmeyer, 2004, Feakins et al., 2005). Climate change had profound effects on distribution patterns of flora and fauna (deMenocal, 1995, deMenocal and Bloemendal, 1995, Hill, 1995, Kingston et al., 2007) and could have played a key role in the appearance and dispersal of early hominins (White, 1995, Vrba, 1995, Potts, 1998, Trauth et al., 2007, Maslin and Christensen, 2007, deMenocal, 2004, deMenocal, 2011). Attempts have been made to link the onset of extreme climate variability since the mid-Pliocene with fundamental evolutionary changes in hominins, including cranial expansion (e.g. Trauth et al., 2007, deMenocal, 2011, Donges et al., 2011).
Large-scale, long-term climate and vegetation changes in Africa are readily attributed to tectonic drivers, but tectonic effects are rarely considered as influencing hominin evolution within timeframes of less than 1 My and at scales that coincide with the territorial distribution of individual animal groups. Yet, uplift of the African plateau had dramatic impacts on the landscape occupied by hominins at a wide variety of scales (e.g. Reynolds et al., 2011, Bailey et al., 2011). In East Africa uplift was accompanied by the formation of the East African Rift System (Tiercelin and Lezzar, 2002), intimately associated with significant hominin finds (e.g. Tobias, 1985, Asfaw et al., 2002, White et al., 2009). Uplift of the African plateau rerouted large river systems including the Nile, the Congo and the Zambezi (Stankiewicz and deWit, 2006, Roberts et al., 2012), and accommodated the development of ecological corridors as well as biogeographic barriers along rift valleys and their bounding escarpments, creating complex, dynamic landscapes (e.g. O’Brien and Peters, 1999, Bailey et al., 2011), and the sediment traps necessary for the preservation of hominin fossils.
Local dynamic landscapes are created and sustained by active tectonic settings, and involve heterogeneous topography with diverse habitats (Bailey et al., 2000, Bailey et al., 2011), with the potential to provide stable water sources, greater biodiversity and ample refuges (e.g. cliffs, gorges, caves etc.) that offer protection to hominins. Tectonically active zones rejuvenate the landscape at regular intervals (i.e. coincident with seismic events) and provide a buffer to long-term negative effects of climate change (e.g. Bailey et al., 1993, King et al., 1994, King and Bailey, 2006). Bailey et al., (2011) argue that dynamic landscape features characterize many of the South African hominid sites, including those in the Cradle of Humankind and suggest that the fossil distribution pattern represents a preference for hominins to occupy these sites, possibly even driving behavioral and morphological adaptations (Reynolds et al., 2011).
The late-Pliocene to Quaternary cave deposits in the Cradle of Humankind (CoH), UNESCO, World Heritage area (Fig. 1), South Africa are one of the world’s most important geological settings hosting hominin fossils and associated faunal and archaeological remains. From caves including Sterkfontein, Swartkrans, Malapa, Drimolen and Kromdraai (Fig. 1) abundant hominin fossils have been recovered, ascribed to a range of species including Au. africanus; Au. robustus; Au. sediba and early Homo (e.g. Brain, 1993, Tobias, 2000, Berger et al., 2010). Hominin remains in the caves are encased in clastic, cave-fill deposits situated in stromatolite-rich, dolomite sequences deposited on a late-Archaean continental shelf (Martini, 2006, Eriksson et al., 2006). The dolomite units outcrop along an arch-like, antiformal structure cored by Archaean basement gneiss referred to as the Johannesburg Dome (Fig. 2). Detailed geological descriptions of the most important fossil sites are available (e.g. De Ruiter et al., 2009, Pickering and Kramers, 2010, Dirks et al., 2010, Pickering et al., 2011b), but these studies focus on providing geological context to the immediate environment within which individual fossils are found. Perhaps surprisingly, only few studies (e.g. Partridge, 1973, Martini et al., 2003, Dirks et al., 2010) have looked at the broader geological context of the evolving cave systems, how these systems are expressed within the landscape and how the landscape may have influenced the distribution pattern of the fossils. Several recent, large-scale studies by Bailey et al. (2011) and Reynolds et al. (2011) used satellite imagery to interpret the tectonic geomorphology of the CoH, arguing for fundamental tectonic fault controls, but these studies do not link the features observed from space to the actual geology on the ground; leaving questions regarding the accuracy of their interpretations.
When considering the caves, the distribution of fossils within them and the geology and landscape in which the caves occur, many pertinent questions remain unanswered. Formation, and exposure of the cave systems in the CoH is linked to uplift and denudation of the high-elevation plateau of southern Africa (e.g. Partridge, 1973, Kavalieris and Martini, 1976, Martini, 2006, Dirks et al., 2010). Caves formed in vadose conditions before they were drained by lateral incision of eroding streams of the Crocodile drainage net, allowing phreatic processes to sculpt caves further through collapse and cave propagation along fractures and joints, through accumulation of cave sediments and through erosion allowing access to previously closed cave systems. The caves are an expression of extensional tectonics, which, though less dramatic than the rift valleys of east Africa, also resulted in dynamic, high-relief landscapes (e.g. Partridge and Maud, 2000, Moore et al., 2009, Bailey et al., 2011), with significant uplift since the Pliocene.
But when looking at local scales it is harder to understand the interplay between tectonic drivers and caves, and fossils buried within them. For example, is the presence of fossils near caves a reflection of preservation potential, or does the karst landscape present a habitat enjoyed by our ancestors, like bats in caves?; are the caves simply fortuitous traps in the landscape where fossils including those of hominins became deposited, and by extension, is the distribution of fossiliferous caves random?; or does the distribution of hominid fossils in caves, in some way reflect a fundamental underlying geological or biological process that makes certain caves more suitable then others; e.g. caves frequented by predators and scavengers may collect more bones (Brain, 1981, De Ruiter and Berger, 2000, Pickering et al., 2004); or perhaps groups of hominins sheltered in caves for security similar to troops of baboons today.
Such questions do not just relate to distribution patterns of fossils but also to timing, i.e. to the dynamic nature of the landscape: how fast did the landscape change?; is the landscape we see today an accurate reflection of the landscape 1, 2 or 3 My ago?; when did caves open to allow animal remains to accumulate?; are erosion and exposure of caves occurring at predictable rates that will allow us to estimate the age of fossils by looking at the position of caves in the landscape?
At the moment we do not know the answers to these fundamental questions. Many attempts have been made to reconstruct what the African Plio–Pleistocene landscape and its habitats looked like in the CoH. Based on faunal, flora and isotope studies a mozaic environment has been reconstructed, which in the early-mid-Pleistocene, was more forested than today, with gallery forests along watercourses and nearby patchy open grasslands or woodland habitats in which Australopiths enjoyed a mixed C3–C4 diet on dolomite or mixed dolomite-shale-granite substrate (e.g. Vrba, 1982, Bamford, 1999, Lee-Thorp et al., 2003, Sponheimer and Lee-Thorp, 2003, Sponheimer et al., 2005, Sponheimer et al., 2006, Reynolds, 2007, Bamford et al., 2010, Copeland et al., 2011). Au. sediba lived in the center of this broadly C4 environment, and enjoyed a dedicated C3 diet (Henry et al., 2012), indicating strong variability in hominin behavior. Whilst such reconstructions provide a snap-shot view of the landscape, they do not provide answers to the dynamic relations that may exist between the evolving physical environment and the non-random, evolutionary pressures it exerted on hominins.
The object of this paper is to provide geological and geomorphological context to cave sites in the CoH that contain fossil remains including hominins. By focussing on the distribution of fossil-bearing cave sites, in relation to key geological and geomorphological features, an attempt is made to highlight the dynamic nature of the landscape, and illustrate how physical and/or biological controls exerted a fundamental influence on the distribution of fossil remains. Evidence will be presented for the denudation history of the CoH in the upper Crocodile River catchment along the western margin of the Johannesburg Dome. The Grootvleispruit catchment will be used as a type area (Fig. 1). This well-constrained river catchment in dolomite hosts the important and precisely dated fossil site of Malapa (Dirks et al., 2010, Pickering et al., 2011a), and was visited, if not inhabited, by a family group of Au. sediba individuals 1.977 ± 0.002 Ma ago (Berger et al., 2010, Dirks et al., 2010, Pickering et al., 2011a). Malapa was found in August 2008 during a systematic geological survey of the area and differs from other nearby hominin-bearing sites in that a high concentration of hominin remains of at least six individuals occurs in a small outcrop (∼60 m2) of clastic sediment deposited along the lower parts of a now almost entirely eroded cave system that was originally >40 m deep (Dirks et al., 2010).
Section snippets
Geology
The CoH World Heritage area is situated in a region of the Kaapvaal Craton referred to as the Johannesburg Dome (Fig. 2). The Johannesburg Dome consists of a near circular, antiformal structure cored by Mesoarchaean (>3.1 Ga) basement gneiss (Anhaeusser, 2006, Robb et al., 2006) surrounded by outward dipping platform sequences of sedimentary and volcanic rocks of Neoarchaean to Paleoproterozoic (3.0–2.1 Ga) age. These include stromatolite-rich dolomite of the late-Archaean (2.64–2.50 Ga) Malmani
The evolving landscape of the CoH
The ephemeral Grootvleispruit in the center of the CoH forms a catchment, 15.51 km2 in size, situated within the Malapa Nature Preserve on the farm Diepkloof (Fig. 1). Malapa cave with its spectacular fossils of Au. sediba occurs towards the geographical center of the Grootvleispruit catchment approximately 15 km NNE of Sterkfontein. Numerous caves, several springs and complex landscape features occur within this catchment (Fig. 3), which provides a well-constrained setting to reconstruct the
General setting of caves in Malmani dolomite
About 750 cave systems (∼80% of caves in South Africa) are known in the dolomite sequence of the Transvaal Supergroup (Martini, 2006), not counting inaccessible sinkholes, erosion remnants of caves and caves entirely filled with sediment. The highest concentration of caves occurs in the Malmani dolomite between Pretoria and Potchefstroom; i.e. across the area in which the CoH occurs (Fig. 2). For this study, we compiled a database of 597 caves, sinkholes and cave erosion remnants in and around
Discussion
We will start the discussion by briefly summarizing the main points made earlier. The landscape in the CoH over the past 4 My has been dynamic in the sense that it has undergone major changes in its physical appearance, reflected in a general denudation of the geology, and incision of the upper reaches of the Crocodile River into the African erosion surface. Geomorphological features such as chert breccia dykes, which weather differentially from surrounding dolomite, can be calibrated with 10Be
Conclusion
Our understanding of hominin evolution is critically dependent on understanding the sites where fossils are found in relation to the landscape in which the fossil sites occur. In this context no question is more important than whether fossil sites merely represent convenient trapping sites with superior taphonomic characteristics, or whether the fossil sites are a reflection of habitation and land-use patterns by animals that occasionally got trapped within them.
With a detailed description of
Acknowledgements
This research has been the result of 4 years of mapping in the Cradle of Humankind. The project started in early 2008 when we decided to map the distribution of caves in the CoH in an attempt to better understand the geological controls on cave formation and their distribution in the landscape. Early discussions benefited immensely from interactions with Geoffrey King. In August 2008 our mapping resulted in the discovery of the Malapa site and the fossils of Australopithicus sediba, and the
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