Introduction

The fossil record of man is scanty and discoveries are rare. It seems that the farther back in time we go, the fewer the fossil remains we can expect (Cooke 1983). There are probably several factors involved in diminishing the hard evidence of our own evolution. When a mammal dies in the open, scavengers will immediately work on the carcass, scatter pieces, and probably destroy bones. The surviving parts may be exposed to the hot sun during the daytime and cooled down at night. Extreme temperature differences between day and night lead to cracking due to loss of water and reduction of organic material in the bone fibers.

© Springer-Verlag Berlin Heidelberg 2007

Usually this process is accompanied by the activity of microorganisms and maybe also by physical transport in riverbeds or on floodplains during torrential rain falls. All of this modifies the original surface of bone or even breaks elements apart. Most of the time, the history of an animal body ends in its complete destruction, unless the bones are covered and protected, e.g., by rapid burial with sediments (Cooke 1983). Sometimes animals are trapped in cavities, like fissures or pits, and their bones are preserved articulated. But even if we have a favorable environment for rapid covering, most of the time postburial chemical and physical processes are destructive, with some rare exceptions favored by special geological environments.

For this reason, the search for hominid fossils depends at a larger scale on the geological setting because the sedimentary environment is a limiting factor upon chances of finding early hominids. Due to the multifarious influences for destruction, each fossil we collect seems to be a stroke of luck. In Africa, the hunt for the discovery of the origin of mankind concentrates on three major types of geological settings with a great potential for new discoveries. These are graben structures, cave deposits, and intracratonic basins that developed during the last 8 million years (Ma), after the hominids diverged from their last common ancestor with the African great apes. Consequently, the East African Great Rift Valley (EARV), with its widespread outcrops of fossil-rich Miocene to Pleistocene deposits, presents a unique situation for paleoanthropological field research. The Rift provided permanent water sources for our oldest ancestors and a rich fauna. A very similar situation is found in intracratonic basins like the Chad depression, occurring as relatively static systems with large lake transgression and regression cycles, varying in time, and sometimes producing fossiliferous deposits of enormous lateral extension. In contrast, the famous South African cave sites, Makapansgat, Sterkfontein, Swartkrans, and Kromdraai, were formed by extensive freshwater karstification and sedimentation processes and their complex cavities were used as shelters for hominids and probably functioned also as traps and scavenger caves.

Geological and paleoanthropological research at African hominid sites has produced a rich database during the last century. Today the geological background information influences the search for the origin of humankind, and the growing knowledge about geology helps us to understand the processes of fossilization and supports the interpretation of individual situations at fossil localities. And the application of digital geographical information systems (GIS), in combination with high-resolution satellite images, has provided a huge additional database for field investigations. Potential fossil localities, in Rift valleys, intracratonic basins, and karstified limestone, can be localized by satellite image analysis. Nevertheless, extensive surveys by experienced field teams will continue to be the fundamental basis for the discovery of new fossil hominid remains.

0 0

Post a comment