A KINEMATIC MODEL FOR AFAR DEPRESSION LITHOSPHERIC THINNING AND ITS IMPLICATIONS FOR HOMINID EVOLUTION: AN EXERCISE IN PLATE TECTONIC PALEOANTHROPOLOGY

We present a detailed Nubia-Arabia-Somalia (NU-AR-SOM) kinematic reconstruction based on sea floor isochrons in the Gulf of Aden and Red Sea and piercing points along the Red Sea margins. The reconstruction is combined with digital topographic and depth-to-Moho data to constrain in 4D the Late Oligocene to present day evolution of the Afar supra-Moho crust. Opposite end member models for crustal evolution are described. We conclude less than 14% of the Afar supra-Moho crust was constructed by magmatic processes such as underplating. The reconstructions indicate the greater percentage of crustal thinning occurred before 6.2 Ma. We model the thinning of the effective elastic lithosphere that accompanied extension and show the regional scale topographic development of the Afar depression was virtually complete by mid Pliocene time.

The plate tectonic model has paleoanthropological implications. Prior to 6.2 Ma the proximal positions of NU-SOM, AR, and the Danakil block suggest sub-aerial conditions prevailed between Yemen and Ethiopia. Uninhibited Africa-Eurasia faunal exchange through Afar and Arabia (corroborated by published geologic and paleontologic data) was tectonically permissible until the time of the earliest hominids. Continued stretching caused the Afar land bridge(s) to disappear during early to mid Pliocene time. Primitive hominid populations living within the Afar Depression became isolated from AR sometime before ~3.2 Ma. With the plateau becoming less habitable due to long term late Neogene cooling, hominids that remained in the Afar Depression were forced to adapt to a much smaller range that was effectively bounded by the already well-developed NU-SOM escarpments and the newly opened Straits of Bab el Mandeb. Increased population pressure exacerbated by potentially severe fluctuations in local climate (well documented by land and marine paleoclimate proxies) may have caused hominids living in Afar to undergo physical and cultural evolution more rapidly and successfully than hominids inhabiting less confined ranges elsewhere in Africa. We suggest that plate tectonic induced isolation caused the Afar Depression to become the cauldron within which our genus homo arose to prominence.

If our interpretation is correct, continental drift played a major role in hominid-to-human evolution.