frontiers of geobiology
The recent paper on the genetic structuring of African lions – see this Post invoking geological and palaeoclimatic evidence – has fascinating implications for hominin evolution. These implications centre around links between biodiversity in the wetland archipelago of the African hinterland and geological evolution (see Cotterill 2005, 2006). The finding that the mega-wetlands of the Afrotropical Realm have a keystone role in the evolution of Panthera leo underscores these geobiology interests besides the many biological issues it flags. For example, besides the core significance to the ecology and behaviour of these superpredators, and their conservation, it spills over into carnivore biology, and obviously mammalogy and tropical ecology.
In this post I pick up another line of evidence that’s long captivated my interest under the aegis of geoecodynamics. This is the link between hominin evolution and tropical wetlands in the evolving landscape. The discovery of the importance of wetlands in the origins of lions is portentous to human origins too; and the science of the entire tapestry is no less fascinating, when we consider the important place lions hold in the modern human psyche in so many cultures.
First, to summarize the phenomenon of Panthera leo in south-central African wetlands, and paraphrase part of my post on the subject. Geographical isolation of the lion populations in the Etosha and Okavango basins invokes regional climatic forcing mediated by regional vegetation shifts across the African subontinent, which were magnified throughout the immense Mega-Kalahari basin. Indeed, we see this in the major portion of the northern and eastern Kalahari supporting the broad forest and moist savanna belts, which cover much of western Zambia, northern Angola and the southern Congo. Rainfall is not only relatively high, but more reliable in persisting from November into May. The contrast to the deserts centered on the Kalakgadi desert in the southwest Kalahari could not be starker. It can be argued that Kalakgadi-type conditions dominated the subcontinent during the climatic shifts driven by the coldest, and thus most arid, Pleistocene glacials. A Mega-Kalahari desert in these arid periods would have covered much of the subcontinent: extending from the southern Congo basin to the Orange River, and from Zambia’s Plateau to the Angolan Escarpment.
This means the biodiversity of subcontinental Africa has experienced pervasive, recurring vegetation shifts over least 3 million years. Mega-Kalahari Expansions and Contractions since the Pliocene presents as the most plausible hypothesis for the geographic isolation of the wetland lions as the flagship species in the wetland assemblages of the Etosha Basin and the Okavango Delta.
Enter our hominin ancestors on this wetland stage in the African Plio-Pleistocene: set within the immense sandsea of the Mega-Kalahari. This is where two additional lines of evidence stand out. They have a profound bearing on the origins of hominins in the Late Cenozoic. Barham (2000) highlights the role of Mega-Kalahari expansions and contractions to the evolution of Stone Age cultures over the Plio-Pleistocene – especially the Sangoan and Lupemban Industries centred on the south-central African plateau. Moreover, recent attention to the Delta Hypothesis of human evolution (Wrangham 2005) and the surrounding dambos (O’Brien and Peters 1999) underscores the pivotal role of the wetland archipelago (Cotterill 2005) in hominin evolution.
So it follows that the nature of the stage on which our ancestors interacted with lions needs to be rethought, especially the scenarios in which hominins dodged lions and stole and scavenged their kills. Anthropologists focus on Africa’s savannas and deserts, and the steppes and cooler climes of the Palaearctic as the setting of ape-carnivore interactions. This could be misplaced in choosing the wrong landscape model. The often fraught association between apes and the Panthera leo species complex was likely inaugurated in wetland archipelagos – and thus across the Kalahari Plateau.
The map I have picked out above, so well known to many African geologists, highlights two major features of the MegaKalahari basin. Both stand out as ecological controls on the Late Cenozoic of the continent’s biota. One is the geographical extent of the sandy sediments. The second are the depths of the underlying isopachs approximating concentrations of deeper patches of sediments. Each of these at over 200-500 + m depth below the landsurface represents an ancient regional depocentre that was maintained by its respective drainage net. It is assumed that, overall, the regional topology was dynamic and so links between the smaller catchments changes over time. Nevertheless, our knowledge of the Palaeo-Kalahari drainage net remains very poor. It is presumed that each of these basins was infilled by a palaeodrainage system whose origins and topology remain guesswork at best. Nevertheless, the geochronology of the deepest the surfaces of these depocentres became established long before Neogene times – a minimum age – as wetlands, and in the case of the Bulozi (=Upper Zambezi – see Moore et al 2012), Etosha, and Okavango have persisted as wetlands of dynamic extent ever since. See this list of publications [also ResearchGate] for additional summaries, advances and challenges that this remarkable wetland archipelago of south-central Africa presents to interdisciplinary science.
And the logical thought springs to mind – the MegaKalahari presents an immensely rich hunting ground for Geoecodynamics. Its remarkable biota holds a plethora of biotic indicators. To date, only the wetland variants of the evolving archipelago have begun to be explored (cf Cotterill 2006; Goodier et al. 2011; Moore et al. 2016).
Last but not least, remarkably few biological studies recognize the wider causal links between the MegaKalahari landscape, its palaeoenvironments, and biodiversity dynamics. One exception is the recent paper by Sithaldeen et al. (2015) . These authors invoke the temporal dynamics of MegaKalahari aridity to account for fascinating phylogeographic structuring in the Papio ursinus species complex of southern Africa. It is most interesting how today baboons do not occur in the Kalakgadi and central deserts (equally outside of narrow riparian refuge of the Namib’s Kuiseb river): the riparian gallery woodlands provide vital resources. The latter provide a lifeline for a baboon population to cling on in deserts characterized by acute surface water scarcity that can endure for even a decade (the historical rainfall record tells us this! So consider the climes experienced by organisms over longer episodes in Africa….). The technology and psychological tenacity of the Bushmen of the region enable these hunter-gatherers to persist in the Kalahari desert, but nonetheless they depend on access to key seasonal resources.
Well, to conclude, this Post summarizes a more formal presentation of some of the ideas I am synthesizing into a forthcoming scientific paper.
Barham, L. (2000) The palaeobiogeography of central Africa. In: L. Barham. The Middle Stone Age of Zambia, South Central Africa. Western Academic and Specialist Press, Bristol. pp. 223-236.
Cotterill F P D. (2005) The Upemba lechwe Kobus anselli: an antelope new to
science emphasizes the conservation importance of Katanga, Democratic
Republic of Congo. J Zool (Lond). 265:113–132.
Cotterill F P D. (2006) The evolutionary history and taxonomy of the Kobus
leche species complex of south-central Africa in the context of Palaeodrainage
dynamics [PhD thesis]. [Stellenbosch (South Africa)]: University
Goodier, S.A.M., F.P.D. Cotterill, C. O’Ryan, P. H. Skelton, M. J. de Wit. 2011. Cryptic diversity of African tigerfish (Genus Hydrocynus) reveals palaeogeographic signatures of linked Neogene geotectonic events. PLoS ONE 6(12): e28775. doi:10.1371/journal.pone.0028775
Haddon, I. G. & T. S. McCarthy (2005) The Mesozoic-Cenozoic interior sag basins of central Africa: The Late Cretaceous-Cenozoic Kalahari and Okavango basins. J. Afr. Earth Sci. 43: 316-333.
Moore, A. E., F.P.D. Cotterill & F.D. Eckardt 2012. The evolution and ages of Makgadikgadi palaeo-lakes: consilient evidence from Kalahari drainage evolution. S. Afr. J. Geol. 115: 385-413. doi:10.2113/gssajg.115.3.38
Moore A.E., F.P.D. Cotterill, C.W. Winterbach, H.E.K. Winterbach, A. Antunes & S.J. O’Brien. (2015) Genetic evidence for contrasting wetland and savannah habitat specializations in different populations of lions (Panthera leo). Journal of Heredity 22 DOI: 10.1093/jhered/esv097
O’Brien, E. M. & C. R. Peters (1999) Landforms, climate ecogeographic mosaics, and the potential for hominid diversity in Pliocene Africa. In: T. G. Bromage & F. Schrenk (eds). African Biogeography, Climate Change, and Human Evolution. University Press, Oxford. pp. 115-137.
Sithaldeen R, Ackermann R R, Bishop J M (2015) Pleistocene Aridification Cycles Shaped the Contemporary Genetic Architecture of Southern African Baboons. PLoS ONE 10(5): e0123207. doi:10.1371/journal.pone.0123207
Wrangham, R. W. (2005) The delta hypothesis: hominoid ecology and hominin origins. In: D. E. Lieberman, R. J. Smith & J. Kelley. Interpreting the past: Essays on Human, Primate and Mammal Evolution: in Honor of David Pilbeam. Brill Academic Press, Boston. pp. 231-242.