Earth-logs

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.

Humans left Africa with a meagre tool kits at a remarkably early date, possibly around 1.9 Ma from finds of primitive stone tools in Pakistan and Central China, and certainly before 1.7 Ma in the case of the now celebrated human remains at Dmanisi in Georgia and in Java. Around 1.7 Ma sites with evidence for human occupation extend from southern to north-western Africa and over 2/3 of the width of southern Eurasia. Despite the increased chances of preservation in later times, such a wide-ranging expansion seems not to have recurred until the fully modern human diaspora from Africa that began around 70 to 100 ka. Fossil evidence suggests that descendants of these earliest known migrants thrived until as recently as 20 ka in south-east Asia, and perhaps longer, if tiny Homo floresiensis prove to be other than symptomatic of congenital dwarfism. They represent a puzzle, and absence of evidence has deterred palaeoanthropologists from sticking out their necks, until a recent review of possibilities (Dennell, R. & Roebroeks, W. 2005. An Asian perspective on early human dispersal from Africa. Nature, v. 438, p. 1099-1104).

For a long time all human remains dated between 1 and 1.9 Ma were ascribed to H. erectus, whose type specimen hails from Java, not Africa. Anatomical re-evaluation of specimens from Africa, notably the famous, 1.6 Ma old Turkana Boy from Kenya, shows that they are sufficiently different from Eugéne Dubois’s Javan H. erectus type specimen to warrant a different species name – ‘Action Man’ or H. ergaster. The Dmanisi humans have close affinities, but are older. Therein lies one puzzle: apart from the very much more primitive (and very rare) H. habilis of east Africa, there is no obvious African candidate as an ancestor for H. ergaster there. Dennell and Roebroeks speculate that they migrated back to Africa after evolving there from some unknown earlier species. Another puzzle centres on the tools carried by the early migrants from Africa.

Simple chopper and rough flake tools first appear in north-east Ethiopia at 2.6 Ma, but with no clear sign of who made them. The first discovery of the earliest known tool kit was at Olduvai Gorge in Tanzania – hence their name, Oldowan. They are associated with remains of the earliest known human species H. habilis, but date only to 1.8 Ma. Since Oldowan tool use is now known to have extended over a huge range of Africa and Eurasia at that time, the original emigrants must have carried the culture with them sometime after its first appearance in Ethiopia at 2.6 Ma. The emblematic artefact of ‘H. erectus’ is the beautiful pear-shaped biface axe, yet it first appeared at 1.5 Ma in Africa, and did not make an appearance outside the continent until about 700 ka and never made it to east Asia until carried their by fully modern humans: it was an African invention. Oddly, these highly crafted tools are often found with little sign of wear, and indeed opinion about what they were for is divided.

The great problem in palaeoanthropology is absence of fossils, which is hardly surprising. Dennell and Roebroeks comment that most Late Pliocene to Early Pleistocene terrestrial faunas are nearly always of large, robust animals, and even they are uncommon. The ravages of erosion and transportation also make it difficult to date finds of stone tools, as they may have been mixed with younger dateable materials. With confidence, they rely on the old adage (not well liked by the Popperian school of scientific methodology) that, ‘Absence of evidence is not evidence of absence’, and also that the earliest evidence for a new migrant is bound to be younger than its first presence. They look to the palaeoecological record of the period, which suggests a vast extent of open savannah covering much of Africa and southern Asia in the period when the climatic effects of glacial-interglacial cycles had not gripped low latitudes to create the desert barriers of later Pleistocene times. For species adapted to savannah life there was little to prevent their very wide migration, indeed simple diffusion would have moved them across the entire savannah range. Once thought to be confined to the East African Rift, australopithecines have turned up as far afield as modern Chad, 2500 km away, and as long ago as 3.5 Ma. If such diminutive creatures with no tools could diffuse so far, then what might have been the geographic limitation to the earliest tool users? Moreover, diffusion has no direction in the area that presents its possibility: movement could have been back and forth. An intriguing point made by Dennell and Roebroeks is that climatic instability first appeared around 2.6 Ma in Central China, so any emigrants moving north would have been subject to greater evolutionary-selective pressures for longer. Homo ergaster might have evolved in Asia and returned to Africa in the face of worsening conditions. This approach raises as many plausible hypotheses as a stick can be poked at, and should re-vitalise palaeoanthropological research outside Africa as a means of testing them.

Dee also: Kohn, M. 2006. Made in Savannahstan. New Scientist, v. 191 (1 July 2006 issue), p. 34-39.

Finds in Kenya, Ethiopia and Chad during the first few years of the 21^st century suggest that bipedal hominins, perhaps on the human clade, emerged as long ago as 7 Ma. Even using the previously accepted molecular-clock age for separation of chimpanzees and hominins, this is dangerously close to the time of the last common ancestor of both (5-10 Ma). Results from comparison of more detailed chimp and human genomics (Paterson, N. et al. 2006. Genetic evidence for complex speciation of humans and chimpanzees. Nature, doi:10.1038/nature04789, online) throw up a bewildering series of possibilities. On Patterson et al’s reckoning, our descent split from that of our nearest relatives no more than 6.3 Ma ago and perhaps as recently as 5.4 Ma, implying an overlap between tangible evidence and that based on DNA. Of even greater concern is the fact that human and chimp X-chromosomes are more similar than the rest, and seem to have diverged even later. One way in which this greater similarity could have arisen is if natural selection had been operating more strongly on X-chromosome genes, which studies of other related species show to have stemmed from hybridisation. Genes found on X-chromosomes that make hybrids less fertile can create strong selection pressures on this chromosome. An explanation that takes into account the young date of apparent splitting and strong selection operating on X-chromosomes is that the actual speciation(s) did take place before the time when the oldest hominin fossils were preserved, but that there was common interbreeding between the two closely related lines. 

Understandably, palaeoanthropologists and geneticists are arguing heatedly, but failing to recognise the great differences between fossils and extant genetic evidence: each is bound to tell a different part of the story. Yet another is the ecology connected to either lineage, the end point being a regional separation into creatures of forest and open savannah, separated by considerable distances in Africa – basically west and east of the East African Rift system. Before that climatic and vegetation-cover schism what would there have been to stop a great many branchings from either lineage of very closely related animals? The rarity of fossils from either may leave the true relationships early in the history of both clades completely impenetrable. One thing is for sure, although chimps and humans today do make close friendships, that is as far as it goes…

See also: Holmes, B. 2006. Did humans and chimps once merge?. New Scientist, v. 190 20 May 2006, p. 14. Pennisi, E. 2006. Genomes throw kinks in timing of chimp-human split. Science, v. 312, p. 985-986.

Hobbit matters

Debate about the significance of the tiny hominid fossils from the Indonesian island of Flores (H. floresiensis) continues to escalate. The remains are sufficiently complete for analysis of other things than size and morphology of skull and brain. It seems that the shoulder structure is different from that of modern humans, but more similar to that of full-sized H. erectus (see Culotta, E. 2006. How the hobbit shrugged: tiny hominid’s story take a new turn. Science, v. 312, p. 983-984). In ourselves, when standing straight, our inner elbows face slightly forwards so that we can work with both hands in front of the body. The necessary twist in the humerus is somewhat less in H. floresiensis, and by itself that would inhibit being able to make tools. However, the shoulder bones of the fossil articulate differently with the hobbit humerus so that a hunched posture would allow intricate work, but not an overarm throwing action. Much the same features characterise the well-preserved upper bodies of H. erectus fossils from Africa and Georgia. Incidentally, like J.R.R Tolkien’s fictional Hobbit, H. floresiensis also had disproportionately large feet.

It seems inescapable that H. floresiensis did make tools. As well as the 90-12 ka artefacts found in the Liang Bua cave with the hominid remains, which some have reckoned to be too complex for the small people to have made the, large numbers of similarly sophisticated stone tools have been found at other sites in Flores. These occur with similar prey species, but not hominid remains, from as long ago as 800 ka; a time at which only H. erectus was present in the Indonesian archipelago (Brumm, A. et al. 2006. Early stone technology on Flores and its implications for Homo floresiensis. Nature. V. 441, p. 624-628).

The minute size of H. floresiensis, with a brain capacity of a mere 400 cm³, continues to cause some researchers to doubts that the fossils – in fact 9 sets of remains from Luing Bua – were other than congenitally deformed modern humans: microcephalics. Anatomist Robert Martin of the Chicago Field Museum of Natural History (see www.sciencemag.org/cgi/content/full/312/5776/999b) used scaling factors of other dwarfed mammals from island faunas to model the body versus brain size to be expected for similarly dwarfed hominids that might arise from isolated H. erectus. He calculated that the 400 cm³ brain of H. floresiensis should be associated with a creature with around 11 kg body mass: about the size of small monkey. But that conflicts with the fact that the famous skull shows no signs of other deformities associated with microcephaly (See Culotta, E. 2006. How the hobbit shrugged: tiny hominid’s story take a new turn. Science, v. 312, p. 983-984).

From the late 19^th century it has been clear that two species of our genus Homo inhabited Europe and the Middle East: modern humans and Neanderthals. Recent partial sequences of Neanderthal genetic material, compared with the human genome, confirm that the two did not interbreed; at least, no trace of Neanderthal genetics remains in that of modern humans. The discovery in Indonesia that fully modern immigrants occupied the same territory as Homo erectus from 70 to 20 thousand years ago adds more weight to the hypothesis of multiple occupancy of the world by different kinds of humans until recent times. The astonishing discovery in 2003 of the remains of tiny hominids (Homo floresiensis) on Flores whose occupancy lasted from at least 840 ka to as recent as 12 ka (see The little people of Flores, Indonesia, November 2004 issue of EPN) confirms mixed occupancy late in hominid evolution. That includes several different representatives of Homo – habilis, eragster and erectus – and also paranthropoids in Africa around 2 Ma years ago. As regards Homo, this cohabitation, especially that in Africa, supports two hypotheses: that our lineage was bush-like and involved separate extinctions and sudden appearances of new species (cladogenesis), or that the great variability in physiognomy (polymorphy) of modern humans extended back for a considerable time. The second is the view of Jonathan Kingdon, who believes insufficient hominid fossils have been collected to rule out polymorphism among tool-using and tool-creating beings. The idea of a single lineage since the first appearance of bipedal apes that led unerringly through gradual changes to modern humans (phyletic evolution) has been largely discarded. For at least part of the 6-7 Ma hominid record, that abandonment of phyletic evolution may have to be reconsidered, following a report of remarkably productive excavations in the Awash Valley of NE Ethiopia (White, T.D. and 21 others 2006. Asa Issie, Aramis and the origin of Australopithecus. Nature, v. 440, p. 883-889).

The Middle Awash is the single most productive area for hominid remains and other fossils that help establish changes in their environment. That is so because of consistent collecting for more than two decades by a multinational team, co-led by Ethiopian and US palaeoanthropologists, from a sequence of flood plain sediments over 1 km thick, liberally interlayered with dateable volcanic horizons. Its middle parts record three species, Ardepithecus ramidus, Australopithecus anamensis and Australopithecus afarensis (of which ‘Lucy’ was a member), in an age range from 4.42 to 3.88 Ma. White and the other members of the team have unearthed 30 new fossils of all three species, but, so far, no examples of more than one in a particular thickness of sediments. Of course, ‘absence of evidence is not evidence of absence’, but this massive addition to the Pliocene hominid record is a challenge to the prevailing hypothesis of cladogenesis – Steven J. Gould’s idea of punctuated equilibrium, in which species arise by sudden appearance of new characteristics from earlier ancestors. Its test is whether or not ancestral species co-exist with new species for a time. In the Middle Awash, it seems that they do not, even though the critical 300 m of sediments represents only 200 thousand years.

The three species, and their predecessor Ardepithecus ramidus kadabba (5.5-5.8 Ma), show variations in their teeth, with Ar. r. kadabba and Ar. ramidus sharing some similarities, and Au. anamensis and Au. afarensis others. The shift between the two sets of common dentition can be explained by either gradual changes in a single lineage over about 2.5 to 3.0 Ma, or a sudden speciation event, perhaps around 4.5 Ma. The lack of overlap favours the first hypothesis. Complicating factors are rife, however, for there may have been migrations (Ar. Ramidus is known from far to the south in Kenya), and yet more evidence will undubtedly be found from the vast amount of sediment of this age in the Afar Depression.

See also: Dalton, R. 2006. Feel it in your bones. Nature, v. 440, p. 1100-1101.

Palaeodentistry

Those of a nervous disposition should not read this item.

A 7500 to 9000 year-old Neolithic graveyard in Pakistan has yielded remains of about 300 people who cultivated wheat, barley and cotton, and herded cattle. There is nothing remarkable in that, except that nine individuals have teeth that have clearly been drilled neatly (Coppa, A. et al. 2006. Nature, v. 440, p. 755). The holes are between 1-3 mm in diameter and up to 3.5 mm deep, and would have exposed sensitive parts of the tooth. In excavations of the nearby village of Merhgarh are found tiny flint drill heads associated with beads of various ornamental materials. The drills are of the same size as the tooth holes. Quite probably, miniature bow-drills tipped with flint would have been used by Neolithic dentists for at least 1500 years – there is no evidence for tooth drilling from younger cemeteries in the area, despite abundant evidence of dental decay. Experiments show that such drills would take less than a minute to produce the neat holes, probably wielded by jewellers rather than dentists.

Asian Homo erectus skilled in tool making

The 1.8 Ma emigrants from Africa who first populated the Far East have not been regarded as having been especially inventive. While their ‘cousins’ in Africa developed the aesthetically stunning bi-face axe about 1.6 to 1.4 Ma ago (the first instance of visualising a finished object within a rough piece of raw material), H. erectus in East Asia is associated with the most primitive stone tools made by simply breaking flinty stones. That seemed to have been the extent of their stone-using skills up to their final demise about 20 thousand years ago –not a lot of progress in 1.8 million years. A report in March at the Indo-Pacific Prehistory Association Congress (Manila) of yet to be published work by Harry Widianto of Indonesia’s National centre of Archaeology may force a revision of this less than charitable view of early Asians (Stone, R. 2006. Java Man’s first tools. Science, v. 312, p. 361). In the Solo district of Java, made famous by Renée Dubois who found the first fossils of H. erectus there, a wealth of finely worked flake tools has been discovered in sediments that are about 1.6 Ma old. Most are small and made from blood-red to beige, translucent chalcedony. It seems that necessity was the mother of invention in this case, because suitable materials for sharp tools are very scarce in Java.