Evolution humaine et paysages | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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Tectonique et mécanique de la lithosphère

  Evolution humaine et paysages



The view accepted by many palaeoanthropologists and the general public is that as the African climate dried starting in the late Pleistocene, forest apes were forced to abandon trees to live on the flat savannah plains of Africa. On these plains a branch known as hominins evolved from the clumsy, but upright Australopithecus to the fully upright striding Homo erectus whose body form modern humans have inherited. This project challenges this narrative for human evolution.

 

Figure 1. Schematic illustration of the effects of tectonic activity creating landscape features illustrated by examples in this paper.
Figure 1. Schematic illustration of the effects of tectonic activity creating landscape features illustrated by examples in this paper. The scale bar shown in the figure is approximate. a) Features typical of contractional (reverse-faulting) environments where the surface expression of faulting appears in the form of folds as well as faults that cut the surface. b) Features typical of extensional (normal-faulting) environments. These produce similar features as in (a) but the faults usually fully cut the surface.

 

Over more than 20 years the IPGP tectonics lab has developed techniques to understand the landscapes created in tectonically active regions and applied this to understanding the mechanics of the deformation of the continental lithosphere in general and specifically to establish earthquake hazard. This project adapts these techniques to understand the character of regions where our ancestors lived. Many hominin sites in the East African Rift are associated with tectonic or volcanically active landscapes. The relation has been explained to be the result of activity first promoting burial of fossil material and its later exhumation. Our closer examination of sites regions around the Mediterranean and Africa has revealed that the dynamic landscapes resulting from tectonic activity have been particularly favourable to hominins. Some important features related to active faults are shown in Figure 1 which can be compared with Figure 2 with shows lake or river edge traditionally favoured as hominin habitats.

 

Figure 2. Schematic illustration of effects of changing water table in flat landscapes
Figure 2. Schematic illustration of effects of changing water table in flat landscapes: a) In regions of low relief, pans, waterholes, and lakes are sensitive to changes of climate and can become dry if the water table drops; b) Rivers in regions of low relief are also sensitive to changes of water table resulting from climate change and can be perennially or completely dry. If there is a reliable source of water from a high rainfall headwater region such rivers can be more reliable.

 

Our progressive understanding of site regions has led us to propose a new "complex topography hypothesis" wherein hominin bipedalism arose in environments that had been
disrupted by tectonic and volcanic processes. These areas, with frequent sudden and severe changes in gradient, and a wide variety of landforms (e.g. rocky outcrops, cliffs, gorges, ridges, etc.) offered security (plenty of look-out and observation points, shelter, protection on ledges from most predators) and could therefore have been attractive for hominins. They would also have acted as a powerful stimulus to improving locomotor skills for moving around the landscape by climbing, balancing, scrambling and moving rapidly over broken ground. These demands, we argue, would have facilitated the emergence of bipedalism. Additionally, this type of broken, disrupted terrain offered greater chances of trapping prey species in cul-de-sacs or on very broken ground, and would thus have been additionally attractive to early hominins. This type of complex topography would have placed additional cognitive demands on early hominins for more complex management of co-operation and team-work (e.g. navigational skills, not getting lost, keeping in touch, effective communication, etc.)

 

These skills first appeared in Homo erectus a species with sufficient flexibility to disperse out of Africa into Europe and Asia. Figure 3 shows early Homo sites plotted on a
map of topographic roughness with high altitude and high latitude regions excluded. The sites are associated with the interface regions between plains and mountain edges
where our ancestors could take advantage of large resources of prey species while simultaneously exploiting their adaptation to complex landscapes.

 

Figure 3. Early sites outside Africa. Terrain roughness (complexity) provides a proxy for tectonic activity.
Figure 3. Early sites outside Africa. Terrain roughness (complexity) provides a proxy for tectonic activity. Roughness at high latitude and altitude is filtered out. Sites appear at boundaries between rough and smooth terrain. The smooth terrain could provide a supply of large herbivores (mega herbivores) until their extinction starting about 300 ka. These animals carry substantial body fat essential to the diet of Homo erectus and successor species.

 

Recently our work has received support from the European Research Council in a joint project with the Department of Archaeology, York University, UK. A major part of the project known as DISPERSE examines how Homo erectus and more modern humans could have left Africa. Particular attention is directed to western Arabia with the possibility that our ancestors could have crossed to Arabia from Africa at times of low sea level. Sea levels are higher now than at almost any time and the Arabian Red Sea margin would have provided an attractive environment. Figure 4 (see the video in the end) shows the extent of this margin at different times in the last 120 ka. This part of the project includes marine survey methods and diving to search for sites that are now submerged. A second initiative concerns establishing how animals that hominins prey on must move around the landscape. The traditional view is that herbivorous species move between places where they find water and vegetation. However if the soils lack certain essential trace elements then grazing or browsing animals cannot exploit the associated vegetation. Dense vegetation can result form the inability of animals to eat it while more scarce vegetation can result from heavy grazing or browsing. We have developed these ideas in the southern Levant and are now applying them in Kenya. The capacity of soils in Kenya to provide the necessary elements is being studied in a joint project with the Kenya Agricultural Research Institute (KARI) and the Kenya National Museum. Early results suggest that soils in the Rift can be very poor except in local areas and that these are where important hominin sites are found. We are taking two approaches to this study. First we are collecting numerous soil samples that are analysed by KARI. In the second we use questionnaires to interview local people (Masai goatherds) about where they take their animals (sheep, goats, cattle and donkeys - grazers and browsers. Even to the untrained eye animals on good soils are healthier than where soils are poor (Figure 5).

 

Figure 5. Healthy Maasai goats browsing vegetaion on soils rich in Calcium and Magnesium
Figure 5. Healthy Maasai goats browsing vegetaion on soils rich in Calcium and Magnesium. In surrounding regions the soils are deficient in these elements and animals are obviously less health. A major homo site (Olorgasailie) is associated with the mineral rich soils.

 

More details on the projects of DISPERSE on Maud Devès’s website.

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