October 2014 picture

The 1200 m Montserrat mountains in Catalonia, NE Spain (credit: Xavier Varela)

The 1200 m Montserrat mountains in Catalonia, NE Spain (credit: Xavier Varela)

The Montserrat mountains are part of the Pre-Coastal Range of Catalonia in Spain and rise close to the capital Barcelona to form a spectacular backdrop.

http://upload.wikimedia.org/wikipedia/commons/c/cd/Montserrat_des_de_Manresa.JPG

Their peculiar pinnacled form results from their comprising tough, well-cemented thick conglomerates, pink in colour and having formed in an early Cenozoic delta. The conglomerates are in very thick, homogeneous beds riven by vertical joints. These two features control the serrated and pinnacled topography, from which is derived the ranges’ Catalan name.

http://upload.wikimedia.org/wikipedia/commons/4/4c/Montserrat_cgl_with_sst_layer.jpg

Cenozoic conglomerates of the Monserrat mountains, Catalonia (credit: Wikipedia)

Human evolution news

Since discovery of its fossilised remains in Liang Bua cave on the Indonesian island of Flores was discovered in 2004 the diminutive Homo floresienesis, dubbed the ‘hobbit’ by the media, has remained a popular news item each time controversies surrounding it have flared. To mark the tenth anniversary  of its publication of a paper describing the remains Nature has summarised the recollections of many of those involved in trying to understand the significance of H. floresiensis (Callaway, E. 2014. Tales of the hobbit. Nature, v. 514, p. 422-426). Two main schools of thought continue in dispute, one holding that it is anatomically so different from anatomically modern humans and earlier members of the genus Homo that it constitutes a new species, despite its youngest member dating back only 18 ka, the other that it is H. sapiens, its tiny size having resulted from some kind of genetic disorder, such as microcephaly or Down’s syndrome. There have been so many attempts to expunge the idea of such an odd fossil cohabiting an island with fully modern humans yet being a different and perhaps extremely archaic species that such an outlook itself seems somewhat pathological.

English: Homo floresiensis, replica Deutsch: H...

Replica of the Homo floresiensis skull from Liang Bua cave, Flores, Indonesia (credit: Wikipedia)

The evidence presented to force H. floresiensis into a deformed human mould has never been convincing, and the best way of combating that view is to document from a ‘non-combatant ‘standpoint the many ways in which its anatomy differs from ours and how it might have arisen; a job to which Chris Stringer of the Museum of Natural History in London is amply qualified (Stringer, S. 2014. Small remains still pose big problems. Nature, v. 514, p. 427-429). He, like the original discoverers, feels this is a case of evolution of small stature due to a limited population being isolated for a long time on a relatively small island, which is just what happened to elephants that colonised Flores to become the pigmy Stegodon that H. floresiensis seemingly hunted. These tiny Flores dwellers (adults were about 1 m tall) used fire and made tools, similar ones dating as far back as ~1 Ma. Stringer mentions the possibility of first human colonisation about that time by Asian H. erectus but also the view that if it happened once there may have been several waves of immigration to Flores. The unusual ‘hobbit’ anatomy is not restricted to tiny size and a small skull and brain cavity (400 cm3), but includes odd hips, wrist bones, shoulder joint and collar bone. In fact the remains bear as much or more resemblance to australopithecines like ‘Lucy’ (3.2 Ma) than to other members of our genus, even H. erectus that has been proposed as its possible ancestor. Could they be far-travelled descendants of the 1.8 Ma old H. georgicus from Dmanisi in Georgia? More fossils clearly need to be found, and Stringer raises the possibility of the search being widened to other islands east of Java, such as Sulawesi, the Philippines and Timor. He hints that in such a tectonically active region tsunamis may have led to animals and humans saving themselves and then being current dispersed on rafts of broken vegetation, rather like some survivors of the 2004 Indian Ocean tsunami who ended up 150 miles from their homes by such a means.

Another story that is set to ‘run and run’ is that of ‘alien’ DNA in the human genome and productive relations between early out-of-Africa migrants with Neanderthals, Denisovans and perhaps yet a mysterious, earlier human species. The oldest (45 ka) anatomically modern human genome sequence so far charted is from a leg bone found by a mammoth-ivory prospector in Siberian permafrost (Fu, Q. and 27 others 2014. Genome sequence of a 45,000-year-old modern human from western Siberia. Nature, v. 514, p. 445-449). Like a great many living non-Africans this individual carried about 2 % Neanderthal DNA, but unlike living people the 45 ka genome has it in significantly longer segments. That allowed the authors to re-estimate the timing of the genetic flow from Neanderthals into the individual’s ancestors. Previous estimates from living DNA geve the possibility of that being between 37-86 ka, but this closer data suggests that it happened between 7 to 13 ka before the date of the fossil femur, i.e. narrowing it down to between 52 and 58 ka closer to the widely suggested time of African exodus around 60 ka (but see an Earth Pages item from September 2014)

New gravity and bathymetric maps of the oceans

By far the least costly means of surveying the ocean floor on a global scale is the use of data remotely sensed from Earth orbit. That may sound absurd: how can it be possible to peer through thousands of metres of seawater? The answer comes from a practical application of lateral thinking. As well as being influenced by lunar and solar tidal attraction, sea level also depends on the Earth’s gravity field; that is, on the distribution of mass beneath the sea surface – how deep the water is and on varying density of rocks that lie beneath the sea floor. Water having a low density, the deeper it is the lower the overall gravitational attraction, and vice versa. Consequently, seawater is attracted towards shallower areas, standing high over, say, a seamount and low over the abyssal plains and trenches. Measuring sea-surface elevation defines the true shape that Earth would take if the entire surface was covered by water – the geoid – and is both a key to variations in gravity over the oceans and to bathymetry.

Radar altimeters can measure the average height of the sea surface to within a couple of centimetres: the roughness and tidal fluctuations are ‘ironed out’ by measurements every couple of weeks as the satellite passes on a regular orbital schedule. There is absolutely no way this systematic and highly accurate approach could be achieved by ship-borne bathymetric or gravity measurements, although such surveys help check the results from radar altimetry over widely spaced transects. Even after 40 years of accurate mapping with hundreds of ship-borne echo sounders 50% of the ocean floor is more than 10 km from such a depth measurement (80% lacks depth soundings)

This approach has been used since the first radar altimeter was placed in orbit on Seasat, launched in 1978, which revolutionised bathymetry and the details of plate tectonic features on the ocean floor. Since then, improvements in measurements of sea-surface elevation and the computer processing needed to extract the information from complex radar data have show more detail. The latest refinement stems from two satellites, NASA’s Jason-1(2001) and the European Space Agency’s Cryosat-2 (2010) (Sandwell, D.T. et al. 2014. New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure. Science, v. 346. p. 65-67; see also Hwang, C & Chang, E.T.Y. 2014. Seafloor secrets revealed. Science, v. 346. p. 32-33). If you have Google Earth you can view the marine gravity data by clicking here.  The maps throw light on previously unknown tectonic features beneath the China Sea (large faults buried by sediments), the Gulf of Mexico (an extinct spreading centre) and the South Atlantic (a major propagating rift) as well as thousands of seamounts.

Global gravity over the oceans derived from Jason-1 and Cryosat-2 radar altimetry (credit: Scripps Institution of Oceanography)

Global gravity over the oceans derived from Jason-1 and Cryosat-2 radar altimetry (credit: Scripps Institution of Oceanography)

There are many ways of processing the data, and so years of fruitful interpretation lie ahead of oceanographers and tectonicians, with more data likely from other suitably equipped satellites: sea-surface height studies are also essential in mapping changing surface currents, variations in water density and salinity, sea-ice thickness, eddies, superswells and changes due to processes linked to El Niño.

‘Earliest’ figurative art now spans Eurasia

The first generally recognised piece of artwork is abstract in the extreme: a worked piece of hematite with a complex linear pattern etched into it. It comes from Blombos Cave  in South Africa, together with similarly engraved bone, shell ornaments and advances in stone tool kits.

Image copyright held by author, Chris Henshilw...

Artifacts from Blombos Cave, South Africa (credit: Wikipedia; copyright held by Chris Henshilwood)

Dated at 100 ka, the Blombos culture is regarded by many palaeoanthropologists as the start of the ‘First Human Revolution’. Yet most believe that such a massive cultural shift only properly manifested itself around 40 ka in Europe shortly after its colonisation by anatomically modern humans. It was then that lifelike pictures of animals began to appear on the walls of caves, such as those discovered in Chauvet Cave in France and radiocarbon dated to between 35.5 to 38.8 ka.

Drawing of horses in the Chauvet cave.

Drawing of horses in the Chauvet cave. (credit: Wikipedia)

Such a Eurocentric view is based on the lack of evidence for precedent art of this kind from elsewhere. The adage that 'absence of evidence is not evidence of absence' - attributed to Carl Sagan - recently popped up with sophisticated dating of cave art in the Indonesian island of Sulawesi. The cave-riddled limestones of southern Sulawesi have long been known for artwork on the roofs of caves and in some of their darker recesses, including sketches of local animals, humans and a great many stencils made by blowing a spray of pigment over a hand placed on a rock face. The pictures were thought to be relatively recent.

Painting of a dwarf water buffalo and stencils of human hands from a cave in SW Sulawesi (credit: Maxim Aubert, Griffith University, Australia)

Painting of a dwarf water buffalo and stencils of human hands from a cave in SW Sulawesi (credit: Maxim Aubert, Griffith University, Australia)

A joint Australian-Indonesian  group of Archaeologists used a specialist technique to date them (Aubert, M. and 9 others 2014. Pleistocene cave art from Sulawesi, Indonesia. Nature, v. 514, p. 223-227. See also Roebroeks, W. 2014. Art on the move. Nature (News & Views), v. 514, p. 170-171). Like many paintings in limestone caves, with time they become coated with calcite film deposited from water flowing over the rock surface, known as flowstone or speleothem. It is possible to date the film layers  using the uranium-series method to derive a maximum age for the encased pigment from speleothem beneath it and a minimum age from the layer immediately overlaying it. One of the hand stencils proved to be the oldest found anywhere, with a minimum age of 39.9 ka, while sketches of animals ranged from 35.4 to 35.7 ka. To see more images and view an interactive video about the Sulawesi finds click here.
The discovery by Maxime Auberts and his colleagues has set the cat among the pigeons as regards the origin of visual art. The paintings’ roughly coincident age with the earliest in Europe raises three possibilities: the artistic muse struck simultaneously with people widely separated since their ancestors’ emergence from Africa; somehow the skills were quickly carried a third of the way around the world from one place to the other; the original migrants from Africa took artistic ability of this kind with them to Eurasia, perhaps as early as 125 ka ago.
Three points need to be considered: whether in Europe or eastern Indonesia, cave art is preserved either on the roofs or in the deep recesses of caves, where it is more likely to survive then in more exposed sites; preservation by speleothem enhances longevity and the oldest works are in limestone caves; many more archaeologists have researched caves in Europe than in the far larger areas of Asia and Africa. A view worth considering is that art may have begun outdoors, in a well-lit site on whatever ‘canvas’ presented itself. The artists’ choice of cave walls in Europe and Indonesia may have resulted from the need for shelter from rain and/or cold, whereas much of Africa and Australia poses little need for ‘interior design’. Besides, what if art began on the most easily available canvas of all – human skin! My guess is that the record will widen in space and deepen in time.
See also here

Signs of lunar tectonics

Large features on the near side of the Moon give us the illusion of the Man-in-the-Moon gazing down benevolently once a month. The lightest parts are the ancient lunar highlands made from feldspar-rich anorthosite, hence their high albedo. The dark components, originally thought to be seas or maria, are now known to be large areas of flood basalt formed about half a billion years after the Moon’s origin. Some show signs of a circular structure and have been assigned to the magmatic aftermath of truly gigantic impacts during the 4.1-3.8 Ga Late Heavy Bombardment. The largest mare feature, with a diameter of 3200 km, is Oceanus Procellarum, which has a more irregular shape, though it envelopes some smaller maria with partially circular outlines.

Full Moon view from earth In Belgium (Hamois)....

Full Moon viewed from Earth. Oceanus Procellarum is the large, irregular dark feature at left. (credit: Wikipedia)

A key line of investigation to improve knowledge of the lunar maria is the structure of the Moon’s gravitational field above them. Obviously, this can only be achieved by an orbiting experiment, and in early 2012 NASA launched one to provide detailed gravitational information: the Gravity Recovery and Interior Laboratory (GRAIL) whose early results were summarised by EPN in December 2012. GRAIL used two satellites orbiting in a tandem configuration similar to the US-German Gravity Recovery and Climate Experiment (GRACE) launched in 2002 to measure variations over time in the Earth’s gravity field. The Grail orbiters flew in a low orbit and eventually crashed into the Moon in December 2012, after producing lots of data whose processing continues.

The latest finding from GRAIL concerns the gravity structure of the Procellarum region (Andrews-Hanna, J.C. and 13 others 2014. Structure and evolution of the lunar Procellarum region as revealed by GRAIL gravity data. Nature, v. 514, p. 68-71) have yielded a major surprise. Instead of a system of anomalies combining circular arcs, as might be expected from a product of major impacts, the basaltic basin has a border made up of many linear segments that define an unusually angular structure.

The topography and gravity structure of the Moon. Oceanus Procellarum is roughly at the centre. Note: the images cover both near- and far side of the Moon. (credit: Andrews-Hanna et al 2014)

The topography and gravity structure of the Moon. Oceanus Procellarum is roughly at the centre. Note: the images cover both near- and far side of the Moon. (credit: Andrews-Hanna et al 2014)

The features only become apparent from the gravity data after they have been converted to the first derivative of the Bouguer anomaly (its gradient). Interpreting the features has to explain the angularity, which looks far more like an outcome of tectonics than bombardments. The features have been explained as rift structures through which basaltic magma oozed to the surface, perhaps feeding the vast outpourings of mare basalts, unusually rich in potassium (K), rare-earth elements (REE) and phosphorus (P) know as KREEP basalts. The Procellarum polygonal structure encompasses those parts of the lunar surface that are richest in the radioactive isotopes of potassium, thorium and uranium (measured from orbit by a gamma-ray spectrometer) – thorium concentration is shown in the figure.

Tectonics there may be on the Moon, but the authors are not suggesting plate tectonics but rather structures formed as a huge mass of radioactively heated lunar lithosphere cooled down at a faster rate than the rest of the outer Moon. Nor are they casting doubt on the Late Heavy Bombardment, for there is no escaping the presence of both topographic and gravity-defined circular features, just that the biggest expanse of basaltic surface on the Moon may have erupted for other reasons than a huge impact.

Earthquakes and radar interferometry

A friend recently moved to the Napa Valley in California, almost certainly motivated by the vast area given over to the grape and the quality of Napa wines. Shortly after the flit, in the middle of some minor refurbishment, he had quite a shock; a Magnitude 6.0 earthquake at 3:20 a.m. local time on 24 August, the worst in northern California for 25 years. My friend lives only 15 km from the epicentre in South Napa, but his house was undamaged. His confidence in the move remains unshaken, however, as there was no effect on this year’s grape harvest.

The event was monitored by the European Space Agency’s Sentinel-1A high-resolution radar satellite that entered orbit in April 2014. Sentinel revisits any area on the ground every 12 days, has all-weather/day-night imaging capacity, a 250 km-wide image swath with 10 m spatial resolution and is designed to analyse ground movements using interferometry between radar data before and after events. Interferometric radar imaging or InSAR relies on changes in the phase of radar waves between two dates of ‘illumination’ of the ground – radar images normally use only the amplitude of a radar wave, ignoring its phase – and potentially can measure shifts in ground elevation of the order of centimetres.

Interferometric radar image of the area around San Francisco showing the ground movement for the period before and after the

Interferometric Sentinel-1A radar image of the area around San Francisco showing the ground movement for the period before and after the Napa Valley earthquake (NE corner) of 24 August 2014 (credit: ESA)

The image records ground movement in small steps of elevation that are assigned colours, the sequence blue-green-yellow-red-magenta spans a ground shift of about 3 cm. If several of these ‘fringes’ are closely spaced over relatively small areas this is due to significant motions locally. Broad areas with little change in colour have barely moved in the period between the dates of the two images.

The epicentre of the South Napa earthquake clearly shows up at the NE corner of the image, like half a ‘bull’s eye’. A closer look at the enlarged image (click on the image) shows two such features sharply bounded to the west by a line: that coincides with the West Napa Fault.

My friend lives to the west of the faults where the broad areas of colour signify much smaller motions than in the main affected area. He woke and left the building thinking this was a foreshock of a much more destructive event, and had an anxious few days. The United States Geological Survey  estimated that during the main ‘quake 15,000 people experienced severe shaking, 106,000 people felt very strong shaking, 176,000 felt strong shaking, and 738,000 felt moderate shaking. But there was only one fatality and 13 hospitalised casualties.

Image

September’s picture: Iceland eruption

MoreHolurThis month’s stunning image from Earth Science Picture of the Day, taken on 8 September this year is of Iceland’s biggest fissure eruption (video clip) since 1875, in the Holuhraun lava field, which began on 31 August this year. The flow is about to meet the Jokulsa a Fjollum, a large river flowing from Iceland’s largest ice cap Vatnajokull. At the time of writing (29 September) lava is flowing along the river bed at around 1 km each day. So far, the flow has spread over 44 square kilometres, and risks blocking the Jokulsa a Fjollum where it flows through a narrow channel bounded by older lava flows. If that happens the river will form a substantial lake until it is able to flow over and erode the bedrock, and will also leave one of the country’s spectacular waterfalls (Sellfoss) dry.

Aerial View of Jökulsá á Fjöllum

Aerial View of Jökulsá á Fjöllum, Iceland, downstream of Holuhraun (credit: Wikipedia)

The fissure is connected to the large Bárðarbunga stratovolcano that lies beneath Vatnajokull, which is currently showing signs of subsidence, at about 40 cm each day, and seismicity. There are concerns that this activity may presage an eruption there which may melt large volumes of ice and perhaps release a flood or jökulhlaup from beneath the icecap. Such a flood would likely follow the course of the Jokulsa a Fjollum river.

Newly discovered signs of Archaean giant impacts

It is barely credible that only two decades ago geoscientists who argued that extraterrestrial impacts had once had an important role in Earth history met with scorn from many of their peers; slightly mad, even bad and perhaps dangerous to know. Yet clear evidence for impacts has grown steadily, especially in the time before 2.5 billion years ago known as the Archaean (see EPN for March 2003 , April 2005, July 2012 , May 2014). Even in the 1990s, when it should have been clear from the golden years of lunar exploration that our neighbour had been battered at the outset of the Archaean, claims for terrestrial evidence of the tail-end of that cataclysmic event were eyed askance. Now, one of the pioneer researchers into the oldest terrestrial impacts, Don Lowe of Stanford University, California has, with two colleagues, reported finds of yet more impact-related spherule beds from the famous Archaean repository of the Barberton Mountains in South Africa (Lowe, D.R. et al. 2014. Recently discovered 3.42-3.23 Ga impact layers, Barberton Belt, South Africa: 3.8 Ga detrital zircons, Archaean impact history and tectonic implications. Geology, v. 42, p. 747-750).

Barberton greenstone belt, South Africa (credit: Barberton World Heritage Site)

Barberton greenstone belt, South Africa (credit: Barberton World Heritage Site)

Like four other such layers at Barberton, those newly described contain several types of spherules, degraded to microcrystalline alteration products of the original glasses. Some of them contain clear evidence of originally molten droplets having welded together on deposition. Their contrasted geochemistry reveals target rocks ranging in composition from well-sorted quartz sands to intermediate, mafic and ultramafic igneous rocks. Some beds are overlain by chaotic deposits familiar from more recent times as products of tsunamis, with signs that the spherules themselves had been picked up and transported.

Dated by their stratigraphic relations to local felsic igneous rocks, the spherule beds arrived in pulses over a period of about 240 Ma between 3.42 to 3.23 Ga. Even more interesting, the overlying tsunami beds have yielded transported zircons that extend back to 3.8 Ga spanning the Archaean history of the Kaapvaal craton of which the Barberton greenstone belt rests and indeed that of many Eoarchaean cratons; the Earth’s oldest tangible continental crust. The zircons may reflect the depth to which the impacts penetrated, possibly the base of the continental crust. It isn’t easy to judge the size of the responsible impactors from the available evidence, but Lowe and colleagues suggest that they were much larger than that which closed the Mesozoic at the Cretaceous-Palaeogene boundary; perhaps of the order of 20-70 km across. So, although the late, heavy bombardment of the Moon seems to have closed at around 3.8 Ga, from evidence yielded by the Apollo programme, until at least half a billion years later large objects continued to hit the Earth more often than expected from the lunar record. Lowe has suggested that this tail-end of major bombardment on Earth may eventually have triggered the onset of plate tectonics as we know it now.

Ants and carbon sequestration

Aside from a swift but highly unlikely abandonment of fossil fuels, reduction of greenhouse warming depends to a large extent, possibly entirely, on somehow removing CO2 from the atmosphere. Currently the most researched approach is simply pumping emissions into underground storage in gas permeable rock, but an important target is incorporating anthropogenic carbon in carbonate minerals through chemical interaction with potentially reactive rocks. In a sense this is a quest to exploit equilibria involving carbon compounds that dominate natural chemical weathering and to sequester CO2 in solid, stable minerals.

The two most likely minerals to participate readily in weathering that involves CO2 dissolved in water are plagioclase feldspar, a calcium-rich aluminosilicate and olivine, a magnesium silicate. Both are abundant in mafic and ultramafic rocks, such as basalt and peridotite, which themselves are among the most common rocks exposed at the Earth’s surface. The two minerals, being anhydrous, are especially prone to weathering reactions involving acid waters that contain hydrogen ions, and in the presence of CO2 they yield stable carbonates of calcium and magnesium respectively. Despite lots of exposed basalts and ultramafic rocks, clearly such natural sequestration is incapable of absorbing emissions as fast as they are produced.

One means of speeding up weathering is to grind up plagioclase- and olivine-bearing rocks and spread the resulting gravel over large areas; as particles become smaller their surface area exposed to weathering increases. Yet it doesn’t take much pondering to realise that a great deal of energy would be needed to produce sufficient Ca- and Mg-rich gravel to take up the approximately 10 billion tonnes of CO2 being released each year by burning fossil fuels: though quick by geological standards the reaction rates involved are painfully slow in the sense of what the climatic future threatens to do. So is there any way in which these reactions might be speeded up?

Two biological agencies are known to accelerate chemical weathering, or are suspected to do so: plant roots and animals that live in soil. Ronald Dorn of Arizona State University set out to investigate the extent to which such agencies do sequester carbon dioxide, under the semi-arid conditions that prevail in Arizona and Texas (Dorn, R.I. 2014. Ants as a powerful biotic agent of olivine and plagioclase dissolution. Geology, v. 42, p. 771-774). His was such a simple experiment that it is a wonder it had not been conducted long ago; but it actually took more than half his working life. Spaced over a range of topographic elevations, Dorn used an augur at each site to drill five half-metre holes into the root mats of native trees, established ant and termite colonies and bare soil surfaces free of vegetation or animal colonies, filling each with sand-sized crushed basalt.

Empire of the Ants (film)

Film poster for Empire of the Ants (starring Joan Collins) (credit: Wikipedia)

Every five years thereafter he extracted the basalt sand from one of the holes at each site and each soil environment. To assess how much dissolution had occurred he checked for changes in porosity, and heated the samples to temperatures where carbonates break down to discover how much carbonate had been deposited. That way he was able to assess the cumulative changes over a 25 year period relative to the bare-ground control sites. The results are startling: root mats achieved 11 to 49 times more dissolution than the control; termites somewhat less, at 10 to 19 times; while ants achieved 53 to 177 times more dissolution. While it was certain that the samples had been continuously exposed to root mats throughout, the degree of exposure to termites and ants is unknown, so the animal enhancements of dissolution are probably minima.

Microscopic examination of mineral grains exposed to ant activity shows clear signs of surface pitting and other kinds of decay. Chemically, the samples showed that exposure to ants consistently increased levels of carbonate in the crushed basalt sand compared with controls, with levels rising by 2 to 4% by mass, with some variation according to ant species. Clearly, there is some scope for a role for ants in carbon sequestration and storage; after all, there are estimated to be around 1013 to 1016 individual ants living in the world’s soils. In the humid tropics the total mass of ants may be up to 4 times greater than all mammals, reptiles and amphibians combined. There is more to learn, but probably a mix of acid secretions and bioturbation by ants and termites is involved in their dramatic effect on weathering. One interesting speculation is that ants may even have played a role in global cooling through the Cenozoic, having evolved around 100 Ma ago.

Arabia : staging post for human migrations?

English: SeaWiFS collected this view of the Ar...

The Arabian Peninsula from the SeaWIFS satellite (credit: Wikipedia)

From time to time between 130 and 75 ka fully modern humans entered the Levant from Africa, which is backed up by actual fossils. But up to about 2010 most palaeoanthropologists believed that they moved no further, because of the growth of surrounding deserts, and probably did not return to the Middle East until around 45 ka. The consensus for the decisive move out of Africa to Eurasia centred on crossings of the Straits of Bab el Mandab at the entrance to the Red Sea, when sea level fell to a level that would have allowed a crossing by rafting over narrow seaways. The most likely time for such n excursion was during a brief cool/dry episode around 67 ka that coincided with an 80 m fall in global sea level: the largest since the previous glacial maximum (see Evidence for early journeys from Africa to Asia).

In 2011 finds reported from the United Arab Emirates of ‘East African-looking’ Middle Palaeolithic tools in sediment layers dated at 125, 95 and 40 ka led some to speculate that there must have been an eastward move from the Levant by anatomically modern humans (see Human migration – latest news). That view stemmed from the fact that the earliest date was during the last interglacial when sea level would have been as high as it is today, and around 95 ka it would have been little different. That report coincided with others about freshwater springs having emanated from uplifted reefs around the edges of the Arabian Peninsula during the last interglacial, and the existence of substantial lakes deep within the subcontinent around that time (see Water sources and early migration from Africa). Substantial funding followed such exciting news and results of new research are just beginning to emerge (Lawler, A. 2014. In search of Green Arabia. Science, v. 345, p. 994-999).

Oasis of Green Mubazzarah near Al Ain

Al Ain, a rare spring-fed oasis in the eastern Rub al Khali near the UAE-Oman border (credit: Wikipedia)

A team led by Michael Petraglia of the University of Oxford has used field surveys and remote sensing to reveal a great many, now-vanished lakes across the Arabian Peninsula, including many in the fearsome Rub al Khali or Empty Quarter. They are linked by an extensive, partly sand-hidden network of palaeochannels, which include several of the major wadis; a system that once drained towards the Persian Gulf. As well as abundant freshwater molluscs and other invertebrates, former lakeshore sediments are littered with huge numbers of stone tools, also with East African affinities (Scerri, E.M.L. et al. 2014. Unexpected technological heterogeneity in northern Arabia indicates complex Late Pleistocene demography at the gateway to Asia. Journal of Human Evolution, In Press http://dx.doi.org/10.1016/j.jhevol.2014.07.002). Using optically stimulated luminescence dating, which shows how long stone objects have been buried, the British team has found tools dating back as long as 211 ka, with a cluster of dates between 90 to 74 ka. Modern humans, Neanderthals and even Denisovans may have made these tools; only associated fossil remains will tell. Yet it is already clear that for lengthy periods – perhaps of a few hundred or thousand years – the hyper-arid interior of Arabia was decidedly habitable. It may have been a thriving outpost of emigrants from Africa, whose abandonment as climate shifted to extreme dryness as the last interglacial gave way to Ice Age conditions, could well have been the source of the great migration that colonised the rest of the habitable world. Petraglia’s team has already courted controversy with their claim for anatomically modern humans’ tools in South Indian volcanic ash beds that date to the Toba eruption around 74 ka: considerably earlier than the more widely accepted post-65 ka dates of human eastward migration.