The earliest humans in Tibet

Modern Tibetans thrive in the rarefied air at altitudes above 4 km partly because they benefit from a genetic mutation of the gene EPAS1, which regulates haemoglobin production. Surprisingly, the segment of Tibetan’s DNA that contains the mutation matches that present in the genome of an undated Denisovan girl’s finger bone found in the eponymous Siberian cave. The geneticists who made this discovery were able to estimate that Tibetans inherited the entire segment sometime in the last 40 thousand years through interbreeding with Denisovans, who probably were able to live at high altitude too. Wherever and whenever this took place the inheritance was retained because it clearly helped those who carried it to thrive in Tibet. The same segment is present in a few percent of living Han Chinese people, which suggests their ancestors and those of the Tibetans were members of the same group some 40 ka ago, most of the Han having lost the mutation subsequently.

That inheritance would have remained somewhat mysterious while the existing evidence for the colonisation of the Tibetan Plateau suggested sometime in the Holocene, possibly by migrating early farmers. A single archaeological site at 4600 m on the Plateau has changed all that (Zhang, X.L. and 15 others 2018. The earliest human occupation of the high-altitude Tibetan Plateau 40 thousand to 30 thousand years ago. Science, v.  362, p. 1049-1051; DOI: 10.1126/science.aat8824). The dig at Nwya Devu, which lies 250 km NW of Lhasa, has yielded a sequence of sediments (dated by optically stimulated luminescence at between 45 to 18 thousand years) that contains abundant stone tools made from locally occurring slate. The oldest coincides roughly with the age of the earliest anatomically modern human migrants into northern China, so the earliest Tibetans may well have been a branch of that same group of people, as suggested by the DNA of modern Tibetan and Han people. However, skeletal remains of both humans and their prey animals are yet to emerge from Nwya Devu, which leaves open the question of who they were. Anatomically modern humans or archaic humans, such as Denisovans?

The tools do not help to identify their likely makers. Slate is easy to work and typically yields flat blades with sharp, albeit not especially durable, edges; they are disposable perhaps explaining why so many were found at Nwya Devu. None show signs of pressure flaking that typify tools made from harder, more isotropic rock, such as flint. Yet they include a variety of use-types: scrapers; awls; burins and choppers as well as blades. The lack of associated remains of prey or hearths is suggested by the authors to signify that the site was a workshop; perhaps that will change with further excavation in the area. The age range suggests regular, if not permanent, occupancy for more than 20 ka

Related articles: Gibbons, A. 2014. Tibetans inherited high-altitude gene from ancient human. Science News,2 July 2014, Zhang J-F. & Dennell, R. 2018. The last of Asia conquered by Homo sapiens. Science, v. 362, p. 992-993; DOI: 10.1126/science.aav6863.

Volcanism and the Justinian Plague

Between 541 and 543 CE, during the reign of the Roman Emperor Justinian, bubonic plague spread through countries bordering the Mediterranean Sea. This was a decade after Justinian’s forces had had begun to restore the Roman Empire’s lost territory in North Africa, Spain, Italy and the present-day Balkans by expeditions out of Byzantium (the Eastern Empire). At its height, the Plague of Justinian, was killing 5000 people each day in Constantinople, eventually to consume 20 to 40% of its population and between 25 to 50 million people across the empire. Like the European Black Death of the middle 14th century. The bacterium Yersinia pestis originated in Central Asia and is carried in the gut of fleas that live on rats. The ‘traditional’ explanation of both plagues was that plague spread westwards along the Silk Road and then with black rats that infested ship-borne grain cargoes. Plausible as that might seem, Yersinia pestis, fleas and rats have always existed and remain present to this day. Trade along the same routes continued unbroken for more than two millennia. Although plagues with the same agents recurred regularly, only the Plague of Justinian and the Black Death resulted in tens of million deaths over short periods. Some other factor seems likely to have boosted fatalities to such levels.

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Monk administering the last rites to victims of the Plague of Justinian

Five years before plague struck the Byzantine historian Procopius recorded a long period of fog and haze that continually reduced sunlight; typical features of volcanic aerosol veils. Following this was the coldest decade in the past 2300 years, as recorded by tree-ring studies. It coincides with documentary evidence of famine in China, Ireland, the Middle East and Scandinavia.. A 72 m long ice core extracted from the Colle Gnifetti glacier in the Swiss Alps in 2013 records the last two millennia of local climatic change and global atmospheric dust levels. Sampled by laser slicing, the core has yielded a time series of data at a resolution of months or better. In 536 an Icelandic volcano emitted ash and probably sulfur dioxide over 18 months during which summer temperature fell by about 2°C. A second eruption followed in 540 to 541. ‘Volcanic winter’ conditions lasted from 536 to 545, amplifying the evidence from tree-ring data from the 1990’s.

The Plague of Justinian coincided with the second ‘volcanic winter’ after several years of regional famine. This scenario is paralleled by the better documented Great Famine of 1315-17 that ended the two centuries of economic prosperity during the 11th to 13th centuries. The period was marked by extreme levels of crime, disease, mass death, and even cannibalism and infanticide. In a population weakened through malnutrition to an extent that we can barely imagine in modern Europe, any pandemic disease would have resulted in the most affected dying in millions. Another parallel with the Plague of Justinian is that it followed the ending of four centuries of the Medieval Warm Period, during which vast quantities of land were successfully brought under the plough and the European population had tripled. That ended with a succession of major, sulfur-rich volcanic eruption in Indonesia at the end of the 13th century that heralded the Little Ice Age. Although geologists generally concern themselves with the social and economic consequences of a volcano’s lava and ash in its immediate vicinity– the ‘Pompeii view’ – its potential for global catastrophe is far greater in the case of really large (and often remote) events.

Chemical data from the same ice core reveals the broad economic consequences of the mid-sixth century plague. Lead concentrations in the ice, deposited as airborne pollution from smelting of lead sulfide ore to obtain silver bullion, fell and remained at low levels for a century. The recovery of silver production for coinage is marked by a spike in glacial lead concentration in 640; another parallel with the Black Death, which was followed by a collapse in silver production, albeit only for 4 to 5 years.

Related article: Gibbons, A. 2018. Why 536 was ‘the worst year to be alive’. Science, v. 362,p. 733-734; DOI:10.1126/science.aaw0632

Subglacial impact structure: trigger for Younger Dryas?

Radar microwaves are able to penetrate easily through several kilometres of ice. Using the arrival times of radar pulses reflected by the bedrock at glacial floor allows ice depth to be computed. When deployed along a network of flight lines during aerial surveys the radar returns of large areas can be converted to a grid of cells thereby producing an image of depth: the inverse of a digital elevation model. This is the only means of precisely mapping the thickness variations of an icecap, such as those that blanket Antarctica and Greenland. The topography of the subglacial surface gives an idea of how ice moves, the paths taken by liquid water at its base, and whether or not global warming may result in ice surges in parts of the icecap. The data can also reveal topographic and geological features hidden by the ice (see The Grand Greenland Canyon September 2013).

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Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland (Credit: Kjaer et al. 2018; Fig. 1D)

Such a survey over the Hiawatha Glacier of NW Greenland has showed up something most peculiar (Kjaer, K.H. and 21 others 2018. A large impact crater beneath Hiawatha Glacier in northwest Greenland. Science Advances, v. 4, eaar8173; DOI: 10.1126/sciadv.aar8173). Part of the ice margin is an arc, which suggests the local bed topography takes the form of a 31km wide, circular depression. The exposed geology shows no sign of a structural control for such a basin, and is complex metamorphic basement of Palaeoproterozoic age. Measurements of ice-flow speeds are also anomalous, with an array of higher speeds suggesting accelerated flow across the depression. The radar image data confirm the presence of a subglacial basin, but one with an elevated rim and a central series of small peaks. These are characteristic of an impact structure that has only been eroded slightly; i.e. a fairly recent one and one of the twenty-five largest impact craters on Earth.. Detailed analysis of raw radar data in the form of profiles through the ice reveals  that the upper part is finely layered and undisturbed. The layering continues into the ice surrounding the basin and is probably of Holocene age (<11.7 ka), based on dating of ice in cores through the surrounding icecap. The lower third is structurally complex and shows evidence for rocky debris. Sediment deposited by subglacial streams where they emerge along the arcuate rim contain grains of shocked quartz and glass, as well as expected minerals from the crystalline basement rocks. Some of the shocked material contains unusually high concentrations of transition-group metals, platinum-group elements and gold; further evidence for impact of extraterrestrial material – probably an iron asteroid that was originally more than 1 km in diameter. The famous Cape York iron meteorite, which weighs 31 t – worked by local Innuit to forge harpoon blades – fell in NW Greenland about 200 km away.

The central issue is not that Hiawatha Glacier conceals a large impact crater, but its age. It certainly predates the start of the Holocene and is no older than the start of Greenland glaciation about 2.6 Ma ago. That only Holocene ice layers are preserved above the disrupted ice that rests immediately on top of the crater raises once again the much-disputed possibility of an asteroid impact having triggered the Younger Dryas cooling event and associated extinctions of large mammals in North America at about 12.9 ka (see Impact cause for Younger Dryas draws flak May 2008). Only radiometric dating of the glassy material found in the glaciofluvial sediments will be able to resolve that particular controversy.

Oceanic hydrothermal vents and the origin of life

A range of indirect evidence has been used to suggest that life originated deep in the oceans around hydrothermal vents, such as signs of early organic matter in association with Archaean pillow lavas. One particularly persuasive observation is that a number of proteins and other cell chemicals are constructed around metal sulfide groups. Such sulfides are common around hydrothermal ‘smokers’ associated with oceanic rift systems. Moreover, Fischer-Tropsch reactions between carbon monoxide and hydrogen produce quite complex hydrocarbon molecules under laboratory conditions. Such hydrogenation of a carbon-bearing gas requires a catalyst, a commonly used one being chromium oxide (see Abiotic formation of hydrocarbons by oceanic hydrothermal circulation May 2004). It also turns out that fluids emitted by sea-floor hydrothermal systems are sometimes rich in free hydrogen, formed by the breakdown of olivine in ultramafic rocks to form hydroxylated minerals such as serpentine and talc. The fact that chromium is abundant in ultramafic rocks, in the form of its oxide chromite, elevates the possibility that Fischer-Tropsch reactions may have been a crucial part of the life-forming process on the early Earth. What is needed is evidence that such reactions do occur in natural settings.

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A white carbonate mound forming at the Lost City hydrothermal vent field on the Mid-Atlantic Ridge (Credit: Baross 2018)

One site on the mid-Atlantic ridge spreading centre, the Lost City vent field, operates because of serpentinisation of peridotites exposed on the ocean floor, to form carbonate-rich plumes and rocky towers; ‘white smokers’. So that is an obvious place to test the abiotic theory for the origin of life. Past analyses of the vents have yielded a whole range of organic molecules, including alkanes, formates, acetates and pyruvates, that are possible precursors for such a natural process. Revisiting Lost City with advanced analytical techniques has taken the quest a major step forward (Ménez, B. et al. 2018. Abiotic synthesis of amino acids in the recesses of the oceanic lithosphere. Nature, advance online publication; DOI: 10.1038/s41586-018-0684-z). The researchers from France and Kazakhstan focused on rock drilled from 170 m below the vent system, probably beyond the influence of surface contamination from living organisms. Using several methods they detected the nitrogen-containing amino acid tryptophan, and that alone. Had they detected other amino acids their exciting result would have been severely tempered by the possibility of surface organic contamination. The formation of tryptophan implies that its abiotic formation had to involve the reduction of elemental nitrogen (N2) to ammonia (NH3). Bénédicte Ménez and colleagues suggest that the iron-rich clay saponite, which is a common product of serpentine alteration at low temperatures, may have catalysed such reduction and amino-acid synthesis through Friedel–Crafts reactions. Fascinating as this discovery may be, it is just a step towards confirming life’s abiogenesis. It also permits speculation that similar evidence may be found elsewhere in the Solar System on rocky bodies, such as the moons Enceladus and Europa that orbit Saturn and Jupiter respectively. That is, if the rock base of hydrothermal systems thought to occur there can be reached.

Related article: Baross, J.A. 2018. The rocky road to biomolecules. Nature, v. 564, p. 42-43; DOI: 10.1038/d41586-018-07262-8.

The risk of landslides in Africa

The most widespread risk from natural hazards is, with little doubt, that posed by ground instability; landslides and landslips; mudflows; rock avalanches and a range of other categories in which large volumes of surface material are set in motion. They can be triggered by earthquakes, volcanism or heavy rainfall that changes the physical properties of rock and soil. Not only steep slopes pose a risk, for some affect ground with quite gentle topography, as witness the terrible scenes from Sulawesi triggered by the 28 September 2018 magnitude 7.5 earthquake beneath the Minhasa Peninsula. This set in motion mudflows on gently sloping ground when the seismic waves caused liquefaction of unconsolidated sediments that not only shattered dwellings by the lateral motion, but whole communities sank into the slurry with little trace. The rapid events left a death toll confirmed at 2010 people with about 5000 missing, feared to have been swallowed by the earth. In the last 9 months mass movement has resulted in fatalities in many places, the most publicised being in Uganda, Japan, Philippines, Sulawesi, Ethiopia, Sumatra, South India, Bangladesh, California, Nepal, and the list grows as it does every year.

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Types of mass movement (Credit: US Geological Survey

As well as purely natural causes, human activities, such as deforestation, excavations and dumping of materials, greatly exacerbate risks. The South Wales former coal-mining communities commemorate every year the collapse of a mine spoil heap on a steep hillside on 21 October 1966 that engulfed a primary school at Aberfan, killing 116 small children and 28 adults. Wherever they occur, there seems to be little chance of escape for those in their path. Slowly it has become possible for geoscientists to outline areas that are potentially at risk from catastrophic mass wastage, sometimes from the distribution of scars of previous events on remotely sensed images, but increasingly by multivariate analysis of landscapes in terms of the factors that may contribute to future ground failures. The principal ones are: topographic slope and relief; annual rainfall, especially the likely precipitation in a single day; vegetation cover, particularly by trees; strength of surface rock and soils, including degrees of consolidation, interbedding and water content; geological structure, such as the trajectory of faults, degree of  jointing and the dip of strata. Modelling risk has to grapple with the global scale of the problem, which cannot be addressed in the least developed regions by piecemeal local studies, although those are urgent, of course, in areas with recorded instances of catastrophic ground failure. Regional studies can screen vast areas of probably low risk so that meagre resources can focus on those that appear to be most dangerous to populated places.

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Degree of risk from landslides of all types in the northern part of the East African Rift System (Credit: Broeckx et al. 2018; Fig. 6)

Belgian engineering geologists and GIS specialists have assembled a monumental risk assessment of Africa, together with a bibliography of all published work on mass movement across the continent (Broeckx, J. et al. 2018. A data-based landslide susceptibility map of Africa. Earth-Science Reviews, v. 185, p. 102-121; DOI: 10.1016/j.earscirev.2018.05.002). They point out that Google Earth’s 3-D viewing potential at fine spatial resolution provides a free and rapid means of mapping scars of previous earth movements in considerable detail over areas that data analysis suggests to be susceptible. Their paper provides continent-scale maps of the parameters that they used as well as maps showing several versions of their risk analysis. The supplementary data to the paper include downloadable, full-resolution maps of landslide susceptibility.

German global DEM now freely available

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TerraSAR-X and Tandem-X satellites fly close to each other some 500km above the Earth

In  2007 and 2010 two radar-imaging satellites were launched by the German space agency DLR, TerraSAR-X and Tandem-X respectively. After 2010 both orbited in close, side-by-side formation, sometimes as little as 200 m apart. With one acting as a both a transmitter and receiver of microwave pulses, the other as a receiver, this set up allowed the two signals returning from the Earth’s surface to be matched. The slightly different positions of the platforms results in a time difference at which a pulse reflected from a point on the Earth’s surface reaches the two receiving antennas. This difference varies according to the topographic elevation of the point – in effect analogous to the parallax shift captured in conventional stereoscopic images but measured by the interference between the two signals. Although involving far more complex computation, such radar interferometry produces estimates of each point’s elevation and ultimately a 3-dimensional image of the Earth’s surface. After a period of commercial operation, DLR has decided to make part of the data available free of charge. Both systems use microwaves with a wavelength of around 3 cm (9.65 GHz frequency), which allows topographic elevation to be measured to a precision of ±1 m. Using orbits that cross the poles, each at an angle to the Equator, allows swaths from the dual system eventually to cover the whole planet, in the manner of winding a ball of string. Eventually, the data will permit the detection of vertical movements of one kind or another when multiple coverage of the Earth becomes available. However, the expected lifetime of the platforms is limited, so DLR plans to launch two 23.6 cm interferometric radar satellites to assess dynamic processes occurring on the Earth’s surface.

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Side illuminated, colour-coded TanDEM-x elevation model of part of the Sahara desert, in the Tamanrasset province of central Algeria

The resolution of radar interferometry in the two dimensions of a map depends on many factors, some of which stem from the complex processing of the raw data. DLR global data is presented at three resolutions (pixel size): 12 m, the finest; 30 m and 90 m. For local acquisition even finer resolution is possible. Only the 90 m version is being released for free use. The first interferometric radar elevation data to be made freely available was from the NASA Shuttle Radar Topography Mission (SRTM) that was accomplished from the US Space Shuttle Endeavour in 2000, using a single instrument that incorporated two antennas separated by a 60 m long mast deployed from the Shuttle. SRTM acquired data only between latitudes 60° N and 60° S, using 23.6 cm L-band radar. As well as omitting high latitudes, the SRTM design limited actual elevation precision to about 4 m compared with the ±1 m from TerraSAR-X/TanDEM-X. SRTM data with a two-dimensional resolution of 30 m are freely available from the US Geological Survey.

Full global elevation data with a 30 m 2-D resolution and elevation precision of ±9 m have also been produced by the optical stereoscopic potential of the US-Japan ASTER imaging system and are freely available to all via the US Geological Survey. Unlike data produced by radar missions, the optical stereoscopic data from ASTER depend on cloud-free, daytime conditions, and accurate derivation of parallax can be prevented by areas of rugged terrain in deep shadow at the 10 am local-time when images are acquired.

Despite the limitation of TerraSAR-X/TanDEM-X elevation data to a 90 m 2-D resolution, and the consequent loss of textural detail in landscapes, they appear to have the edge in terms of completeness and vertical precision. To get elevation data from DLR requires personal registration after reading a lengthy screed of documentation about data acquisition.

More early art from South Africa?

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Silcrete flake from Blombos with crosshatching drawn in red ochre. (Credit: C. Foster)

The Blombos Cave 300 km east of Cape Town is where the earliest signs of art produced by anatomically modern humans were found (see Snippets on human evolution October 2011). The most publicized was a shaped piece of ochre etched with a hashed pattern of lines (Henshilwood, C.S. et al. 2018. An abstract drawing from the 73,000-year-old levels at Blombos Cave, South Africa. Nature v. 561, online; DOI: 10.1038/s41586-018-0514-3). This and the ochre-processing workshop where it was found gave a date of about 100 ka, Now another item has hit the newsrooms; a  ground piece of flinty silcrete that shows signs of being the product of knapping, on which has been drawn a similar pattern, which resembles the now ubiquitous ‘hashtag’ associated with Twitter. The level in the excavation from which it was removed gives an age of about 75 ka. Like the earlier artifact, it involved the use of ochre but in a way that has been said to be an example of drawing or painting, rather than etching. It is likely to have been produced by a sharpened piece of solid ochre, perhaps a kind of crayon

For some reason the object has been hyped as the earliest example of art and of advanced cognitive abilities. But the pattern is not as complex as that on the original etched ochre block from Blombos, or even those on a freshwater mussel from Trinil in Java that could have =been made by associated Homo erectus between 430 and 500 ka ago. This does not take the context at Blombos into account. There is ample evidence that ochre, along with charcoal and burnt seal bone, was being ground there and made into paint found in an abalone shell. It can be surmised that such paint was used for some kind of decoration that has not yet been discovered. That is quite possibly because it was used for body paint as similar materials are still widely used.  Now anyone – male or female – who uses cosmetics today, be it foundation, lipstick, eye-liner and -shadow or the truly fabulous make-up used by the Kathakali performers of Kerala, takes an age to try and to decide on which of an almost imperceptible range of shades to apply. Ochres are like that, as any native Australian artist will tell you.

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Lord Rama face paint in Kathakali

To me, the most likely origins of both kinds of Palaeolithic hashtag are: in the case of the ‘drawing’, checking the colour and ‘grindability’ of a sharpened piece of red ochre before use; and for the etched block, using a sharp tool to grind off small amounts from what may have been a well-used block of an especially valued hue.

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Neanderthal Mum meets Denisovan Dad

Two bone fragments from the Denisova Cave – the former abode of an 18th century Russian hermit called Denis – in the Altai region of Siberia yielded ancient  DNA. One matches that from previously analysed Neanderthal remains and the other a genome that could only be ascribed to a hitherto unknown ancient-human population, now known as the Denisovans. Since their discovery further analysis of both modern and ancient DNA has shown that modern humans living outside of Africa contain a few percent of DNA from both ancient-human groups. Soon after leaving Africa some of their ancestors interbred with both; indeed a 40 ka-old modern-human jaw from Romania revealed genetic evidence that the individual had a Neanderthal great-great grandparent. Their descendants spread far and wide to populate Eurasia, Australasia and the Americas. Using the ancient DNA to peer back in time suggests that Neanderthals and Denisovans diverged from a common ancestor between 470 and 380 ka, itself having split from modern-human ancestry between 770 to 550 ka. Denisovan DNA also contains evidence that its ancestry included segments that could only have come from a totally unknown hominin species. Interestingly, DNA from the Neanderthal bone fragment found at Denisova contains fragments from an anatomically modern-human.

Tourists at the entrance to Denisova Cave, Rus...

Tourists at the entrance to Denisova Cave, Russia (credit: Wikipedia)

With such riches from tiny fragments of human bones unearthed from the Denisova Cave, it is no surprise that the team led by Svante Pääbo at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, has subsequently analysed others that showed signs of human proteins. The latest ‘takes the biscuit’. A fragment of limb bone from someone who was at least 13 years old yielded DNA commensurate with their having been the child of a Neanderthal mother and a Denisovan father (Slon, V. and 18 others 2018. The genome of the offspring of a Neanderthal mother and a Denisovan father. Nature, v. 560, published on-line; doi: 10.1038/s41586-018-0455-x). Their child was a girl, who has been nicknamed ‘Denny’ by the team, though ‘Denise’ might seem more appropriate. The only clues to what her father, or any Denisovan, might have looked like stem from a few teeth and a skull fragment from the cave that have yielded Denisovan DNA. The teeth are much larger and the skull fragment is thicker than those of Neanderthals, suggesting that Denisovans were distinctly bigger and more robust than even the sturdy Neanderthals.

The father came from a population related to a later Denisovan found in the cave – the first to be sequenced. This suggests long-term occupancy of the area by Denisovans. But his genome also carries traces of Neanderthal ancestry. Surprisingly, the mother is more closely related to Croatian Neanderthals, rather than to an earlier Neanderthal found in the cave. Neanderthals were clearly capable of migrating between Europe and eastern Eurasia; more than 5000 km in this case. Even though very few archaic humans have been genetically sequenced it is beginning to look as if genetic mixing between diverse hominin groups in the last half million years was common, when they actually met. A custom of marrying outside a closely related group (exogamy) has been popular throughout recorded history; indeed it makes sound genetic sense. With the tiny human population density during the Late Pleistocene, it may then have been cause for mutual celebration.  As documented in Chapters 2 and 3 of David Reich’s Who We Are and How We Got Here (Oxford University Press, 2018) human origins since about 470 ka until the present chart a history of episodic migrations and genetic mixing that certainly makes nonsense of earlier ideas of ‘racial purity’ and casts doubt even on the term ‘species’ as regards members of the genus Homo.

If we are ever to discover who the Denisovans were and what they looked like, the evidence is likely to come from East Asia at latitudes where climate favours preservation of DNA. Advanced sequencing equipment and techniques are now operational in China, where suspected Denisovan remains have been found

See also: Warren, M. 2018. First ancient-human hybrid. Nature, v. 560, p. 417-418; doi: 10.1038/d41586-018-06004-0); Sample, I. 2018. Offspring of Neanderthal and Denisovan identified for first time. The Guardian (22 August 2918).

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Multiregional human evolution in Africa

Africa is not only a large continent, but is subdivided into many different climatic zones and ecosystems and these have changed drastically over the last 2 Ma. It is further subdivided by terrain features, such as the courses of major rivers, large plateaus, tectonic rift systems and the mountains that frequently define their flanks. Getting around Africa is not easy today, was more difficult before modern transport, and many geomorphic provinces may have been mutually inaccessible in the distant past. For instance, the Sahara Desert forms a major barrier to travellers on foot because access to surface water is non-existent except at widely spaced oases. Without boats or rafts the Nile and Congo cannot be crossed for a thousand miles or more. Migration was perhaps a very rare event outside of periods of widespread humid climates or when great environmental stress forced people either to move or perish. Despite these physical and ecological divisions and barriers palaeoanthropologists have, until recently, tended to regard the evolution of Homo sapiens and earlier human and hominin species as having occurred within single populations: a linear view forced on them by scanty fossil remains and limited methodologies. Logically, when human numbers were small Africa probably had several isolated population Physical isolation would have engendered genetic isolation in which our ancestors evolved for tens of thousand years.

Anatomically modern human (AMH) remains found at Jebel Irhoud in Morocco turned out to be 315 ka old, displacing those from Ethiopia (190 ka) as the earliest known examples of AMH. Several more archaic H. sapiens fossils have turned up in southern Africa and as far afield as the Middle East, suggesting that the early evolution of AMH was in an Africa-wide context rather than in one area – the rift system of Ethiopia and Kenya – from which a new species radiated outwards. This breadth of finds has encouraged Eleanor Scerri of Oxford University and her many international colleagues to resurrect what was once a widely discarded hypothesis; a multiregional model of modern human origins, originally proposed to have arisen from pre-sapiens groups in Eurasia by Milford Wolpoff but which was sunk once genetic connections among living humans turned out to be rooted in Africa. (Scerri, E.M.L. and 22 others 2018. Did our species evolve in subdivided populations across Africa, and why does it matter? Trends in Ecology & Evolution, v. 33, p. 582-594; (PDF) doi: 10.1016/j.tree.2018.05.005). Scerri et al’s model is sited in Africa and the paper’s authors include several leading palaeoanthropologists who once opposed multiregionalism and established the Recent African Origin hypothesis on the back of the early genetic data.

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Different early AMH cranium shapes: left Jebel Irhoud, Morocco (315 ka), right Qafzeh, the Levant (85 ka) (credit: Scerri et al, 2018; Figure 1)

From region to region in Africa, the oldest AMH crania show significant differences from each other, but within a distinct combination of features that clearly distinguish us from our fossil relatives and ancestors, such as Homo heidelbergensis from Zimbabwe and the primitive-looking H. naledi found in a South African cave in 2015. Improved dating now shows that the Zimbabwean H. heidelbergensis and H.naledi remains are roughly the same age as the Jebel Irhoud AMH specimens. The first has long been held as the progenitor of AMH and descended from H. antecessor, perhaps the common ancestor for AMH, Neanderthals and Denisovans about 700 ka ago. The three human species cohabited Africa early in the evolutionary history of AMH. It is now abundantly clear from ancient and modern genomes that AMH, Neanderthals and Denisovans interbred in Eurasia. The proximity in time and space of earlier African AMH to two more ancient human species opens up a similar possibility earlier in the emergence of all living humans. There is evidence for that too: Yoruba people living in West Africa, whose genomes have been analysed, carry up to 8% of genetic ancestry that originated in an unidentified ancient population that was non-sapiens. At present, DNA analysis with the same high precision and information content from other living Africans has not been performed, and deterioration of ancient DNA in African climates has so far thwarted genomic studies of ancient African fossils.

The new view of our origins points to repeated hybridisation involving other coexisting human species, as well as evolution in isolation, from the outset. It continued through later times while Neanderthals and Denisovans survived. Even recent human genetic history is peppered with intermingling of a great variety of migrants passing through all the habitable continents. Another issue: In the earliest times, were cultures exchanged as well as genes? The first appearance of AMH coincides with that of a new stone technology (Levallois technique), moving away from the earlier dominance by handaxes towards more delicate, leaf-shaped points, that characterise the African Middle Stone Age. Similar techniques reached Europe with the Neanderthals. Was this an invention of the earliest AMH or a joint venture?

You can find an excellent review of these issues in the September 2018 issue of Scientific American (Wong, K. 2018. Last hominin standing.  Scientific American, v. 319(3), p. 56-61) along with several other articles on human evolution.

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

How does subduction start?

Robert Stern of the University of Texas at Dallas, USA, and Taras Gerya of ETH, Zurich, have produced a masterly review of how subduction gets started from place to place, and from time to time in geological history (Stern, R.J. & Gerya, T. 2018. Subduction initiation in nature and models: A review. Tectonophysics, v. 744 (in press); (PDF). It is the foundering of oceanic lithosphere into the mantle and gravity that give modern plate tectonics the bulk of energy that drives it along by slab pull. Yet the mantle’s consumption of a lithospheric slab somehow has to be set in motion from the symmetrical spreading of ocean floor as occurs either side of a constructive margin. It could not happen were the lithosphere to retain its low bulk density relative to mantle peridotite for all time. Moreover, it wouldn’t last for long were the lithosphere not to retain its strength through hundreds of kilometres depth as it sinks into the mantle. Active subduction zones have consumed vast amounts of oceanic lithosphere, for more than 65 million years, especially in fast-spreading ocean basins such as the western and eastern Pacific. The record is held by the destructive margin on the west flank of South America where more than 150 million years-worth of eastern Pacific lithosphere has been swallowed. Yet in order for oceanic lithosphere, which is stronger than that beneath the continents, somehow to fail and begin to sink a linear weak zone must develop at the interface between two incipient new plates. On top of that, all subduction on Earth is one-sided. A simple mechanism involving just thermal convection predicts that both plates either side of a break would have similar density so both should sink, more or less symmetrically.

subduction types

Various ways in which subduction may start. (Credit: Stern and Gerya 2018 – in press – Figure 4)

Geophysical observations reveal that terrestrial subduction can be divided into that which is induced by plate motions and changes in force balance within spreading plates, or spontaneously due to unique conditions developing along the line of initiation. In the first class are cases where a microcontinent is driven into another continental margin and extinguishes the subduction responsible, while spreading continues behind the accreted microcontinent drive older lithosphere beneath the suture (this may have happened in the past but is not seen today). Another, similar, induced case occurs where an oceanic island arc accretes by subduction beneath it so that subduction flips in polarity to consume the driving sea-floor spreading. The loading of oceanic lithosphere by sediments piled onto it by erosion of a continental margin may spontaneously collapse to result in subduction beneath the sedimentary wedge and the continent (again, not happening today, but inferred from examples inferred by earlier geological history). Spontaneous failure may also occur where old, cold lithosphere is juxtaposed with younger by transform faulting, or where a mantle plume heats up lithosphere to create a thermally weakened zone.

Stern and Gerya do not leave the issue at simple mechanics but discuss how plates may develop weak zones or inherit them from earlier tectonic events. The role of water released by metamorphism of descending materials may encourage the observed one-sidedness of subduction by reducing frictional resistance and plate strength and make the process self-sustaining. The paper also discusses the various permutations and combinations that affect the style of induced destructive margins in compressional and extensional environments and a whole variety of nuanced cases of spontaneous initiation. Numerical modelling of the subduction process plays an important, though somewhat bewildering role in discussion, as do considerations of the forces likely to be at play. Applying theoretical considerations to actual examples from the geological record are sublimely enlivening, as are speculations about the future evolution of the passive margins of the Atlantic. Clearly, there is a healthy future for field and mathematical study on the processes at destructive plate margins, such as building in the aspects of magmagenesis. Since Stern has built his career on study of long dead collusions zones, products of arc accretion etcetera, development of their understanding is undoubtedly the main thrust of his and Gerya’s tour de force. Stern provides a full PDF at his University of Texas website for the benefit of anyone who wants to delve deeper than space at Earth-pages and my limited intellect permit!

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook