The Afar Depression of Ethiopia, especially the middle reaches of the Awash River, has become world renowned as the cradle for early humanity. After the revolutionising discovery in 1974 in the Hadar area of the 3.3 Ma old Australopithecus afarensis remains that became known as ‘Lucy’, other finds – Ardepithecus, Orrorin and Sahelanthropus hit the headlines, pushing back the age of possible human ancestors to almost 7 Ma. None of these had Lucy’s degree of preservation, and the vital issue for the origin of humanity – bipedalism – could only be addressed by scanty evidence about the position of attachment of the cranium to the spine. Much else had to be inferred from teeth and facial shape, and odd bits of long limb bones. Lucy and remains of other A. afarensis individuals that rain progressively washes from the badlands of Hadar provide an embarrassment of riches by comparison. There is little doubt that could walk upright, but a question that has lingered is whether or not it also clambered habitually in trees. The other missing information is the vital one of development, for one big difference between apes and us is the grossly extended infancy of modern humans during which the attributes of consciousness, language and much else that is unique arise. To get a grip on developmental issues demands near-complete juvenile remains. The oldest infant fossils that come close are those of a Neanderthal child from 100 ka ago. A dramatic paper (Alemseged, Z. et al. 2006. A juvenile early hominin skeleton from Dikika, Ethiopia. Nature, v. 443, p. 296-301) brings the spotlight back to Middle Awash and to A. afarensis.
The drama has been long in the making. Zeresenay Alemseged, an Ethiopian working in Germany, made the initial find in 2000, collecting more material and painstakingly exposing bones from their sandstone matrix, grain by grain, over the last 5 years. The skull and dentition are complete, and bar the pelvis, lower spine and some limb bones, so is the rest of the skeleton. Morphology points unerringly to A. afarensis, and the stratigraphic position is the same as that entombing ‘Lucy’. Even without the inferences that can be drawn from it, preservation of a complete body is a near-miracle that ranks with that of the ‘Turkana Boy’ (H. ergaster) and ‘Lucy’. The entombing sediments are those of a small stream, which discharged to a large lake that occupied parts of the Middle Awash area during the Pliocene, so that the body was quickly enclosed in fine sands, possibly after the child was washed away in a flash flood. The jaws contain adult teeth waiting to erupt and, by comparison with chimpanzees, they suggest an age at death of about three years, although comparison with human children would probably give an older estimate. The shape of the adult teeth is similar to those of female, so the infant is a ‘she’. Much more work needs to be done on ‘Selam’ (Peace in Amharic), but that reported so far bears strongly on the issue of bipedalism. The shoulder blades and semi-circular canals of the ear, on which balance depends, are ape-like, and a finger bone is curved like that of a chimpanzee. ‘Selam’ was equipped for climbing, but she has leg and foot bones with more human affinities, which would enable upright walking as well. Being a near-complete individual, ‘Selam’ can be compared with whole adult A. afarensis specimens, notably ‘Lucy’, and with modern apes and humans, to assess the crucial issue of development that should throw light on just how close the species was to a transition to the human species that arose about a million years later.
Interestingly, the same issue of Nature includes a mini-biography of the Tunisian-born geologist Maurice Taib. He was the first to work on the terrestrial Pliocene sediments of the middle reaches of the Awash River, thereby opening to road to palaeoanthropolical fame for the likes of Don Johanson, Tim White and two generations of Ethiopian scientists, whom Taib played a major role in training and encouraging (Dalton, R. 2006. The history man. Nature, v. 443, p. 268-269).
See also: Wood, B. 2006. A precious little bundle. Nature, v. 443, p. 278-281. Wynn, J.G. et al. 2006. Geological and palaeontological context of a Pliocene juvenile hominin at Dikika, Ethiopia. Nature, v. 443, p. 332-336.
Drying East Africa
The 7 Ma recorded history of humans and their hominin ancestors was almost exclusively East African, until early members of the genus Homo began to migrate in pulses after about 1.8 Ma. Exodus from Africa on several occasions has been linked with climate change or the opening of routes by falls in sea level during periods of massive ice accumulation at high northern latitudes. Likewise, the evolutionary adoption of a bipedal gait by formerly forest-dwelling apes was probably driven by climate change that saw the spread of more open savannah ecosystems. Records from fossil assemblages in river and lake-bed sediments of East Africa, and from pollen in nearby sea-floor sediments do show a reduction in woodland cover and a spread of grasslands since the Upper Miocene (6 to 8 Ma) – the period of hominin adaptive radiation. Most workers on African climate change in the Neogene attribute the shift to cooling, either through a fall in atmospheric CO2 or the onset of Northern Hemisphere glaciation. Yet East Africa has its own engine for climate and ecosystem change: the formation of the great Rift system and the uplift associated with it. While recognised as a climatic influence tectonics in the region has been downplayed by comparison with global shifts. That is surprising, since in the last 20 Ma, and perhaps more recently, what was an area of low relief has been transformed while rift shoulders rose to more than 3 km, from Eritrea in the north to Malawi 6000 km to the south.
Before rifting began, flood volcanism poured out a basaltic veneer in the late Eocene to mid-Oligocene, to achieve a thickness of more than 2 km in Ethiopia. Rather than creating high ground the flood basalts, being denser than continental crust, probably caused subsidence that roughly maintained low surface elevations. The achieved their present high elevations in the Ethiopian Plateau no earlier than the late Miocene. Large plateaux deflect low altitude winds and seem certain to have influenced climate on a regional scale, as did the Tibetan Plateau. The timing and pace of East African uplift remains poorly constrained, partly because geological evidence shows highly episodic tectonics, with periods of seeming quiescence between episodes of extensive and profound faulting and uplift, and partly because many of the rocks involved are sparsely dated. Yet the present topography and geological infrastructure are sufficiently well known that modelling any morphological influence on climate is possible. By considering several plausible tectonic scenarios, a team of French palaeoclimatologists have modelled the possibilities (Sepulchre, P. et al. 2006. Tectonic uplift and eastern Africa aridification. Science, v. 313, p. 1419-1423). Their models show that uplift may have shifted atmospheric circulation drastically to establish the strong seasonality that dominates the region nowadays. Applying their results to likely ecosystems results in a pattern of decreased tree-cover.
While convincing, Sepulchre and colleagues’ work demands more precise timing for the establishment of sufficient tectonic topography. Nevertheless, it shows that events, arguably beginning at the core-mantle boundary, that triggered East Africa’s dominant tectonic influence, the Afar plume, probably conditioned our own eventual emergence.
A lot closer in time is an analysis of climate change in the Eastern Sahara desert since the end of the Younger Dryas (<12 ka) that devotees of the ‘English Patient’ will find revealing (Kuper, K. & Kröpelin, S. 2006. Climate-controlled Holocene occupation in the Sahara: motor of Africa’s evolution. Science, v. 313, p. 803-807. Being based on 150 archaeological excavations, the account of sudden humidity after 8.5 ka and then slow aridification since 5.3 ka is persuasive background to the rise of the pharaonic kingdoms of the Nile once nomadic Saharan pastoralism slowly became impossible.
Asian migrations reviewed
Sometime between 100 and 60 ka, fully modern humans found their way from Africa to the Far East and beyond. The timing and the issue of how many migrations were involved are topics in turmoil, now that genetic analyses help trace linkages among modern people. That was semi-popularised by Steven Oppenheimer’s The Peopling of the World (2003, Constable, London), which remains the genetically based ‘straw man’ of human migrations. Like Oppenheimer, Paul Mellars also of the Dept of Archaeology at Cambridge University, argues for single exodus and rapid eastward dispersal, but leaves open the route either via the Straits of Bab el Mandab or through Mesopotamia (Mellars, P. 2006. Going East: new genetic and archaeological perspectives on the modern human colonization of Eurasia. Science, v. 313, p. 796-800). While genetic lines of descent are a most powerful tool, any conclusions need confirmation through ‘hard’ evidence from excavations, and both Arabia and the India subcontinent are irritatingly blank in that regard. However, there are a few coastal sites that whet the appetite. As Jonathan Kingdon first suggested, in Self-made Man and His Undoing (1993, Simon and Schuster, London), the most likely routes for migrants would have been along the shoreline. ‘Strandlopers’ would have had easy pickings from littoral food sources, even during periods of aridity related to global cold spells. But there is the problem: with sea levels well below the present ones, most truly ancient sites will now be hidden below the sea. As regards the route taken, much depends on what the Nile valley has to offer archaeologically, for that is the natural way to the eastern Mediterranean and access to the Arab Gulf either across Syria or skirting the mountains of Kurdistan. The route across the Red Sea already has excellent support by the discovery by the Gulf of Zula in Eritrea of abundant evidence for habitation by ‘strandlopers’ around 100 ka.