African genes

Published on by zooks777Leave a comment

Much of the interpretation of the growing database of human genetic variability has so far focused on migration out of Africa and across the habitable continents. To some extent the largest variability, of Africans themselves, has been undersampled, but a multinational team of Africans and non-Africans has now begun to redress the balance (Tishkoff and 24 others 2009. The genetic structure and history of Africans and African Americans. Science, v. 324, p. 1025-1043) partly to study genetically-linked epidemiology and partly anthropology. The study centres on African’s own ideas about their identity/ethnicity as well as documented cultural and linguistic division, and covers 3194 individuals from 121 populations in the continent, African-American populations in 4 US cities and 60 other populations from outside Africa. The team expands knowledge tremendously, as expressed by the many intricate diagrams. They use the statistical method of Bayesian clustering to tease out the ancestral bases for the genetic patterns preserved by Africans, which appear to be based on 14 major ancestral groups that mostly tally with cultural and linguistic divisions. Overall, the picture is one of repeated mixing of populations through migrations within the continent, many within historic times such as the shift of West Africans south-eastwards, but also much earlier movements such as the ancestors of the San people of southern Africa. These remaining gatherer-hunter people together with central African pygmies and the Hadza and Sandawe of Tanzania share ancestry and also, except for pygmies, language that involves click-sounds – the pygmies abandoned their original language in favour of that of the groups that now surround them in the Equatorial rain forests. Of the three groups, the Hadza most maintain the genetic structure of the earliest ancestors on the continent, but all three shared a common ancestor about 35 Ka ago. Interestingly, comparison with people outside Africa confirms earlier studies that indicated a source population for the out-of-Africa migration in East Africa close to the Red Sea.  The paper is necessarily condensed and so difficult to follow, but clearly opens up great vistas in understanding intricacies at which anthropologists have previously only guessed. Like the physical landscape of Africa, that of its population reflects the range of factors that have shaped human evolution and hence a great deal of its destiny.

See also: Gibbons, A. 2009. African’s deep genetic roots reveal their evolutionary story. Science, v. 324, p. 575.

Very old human footprints in Mexico?

In 2006 palaeoanthropologists in the Americas, already at loggerheads about evidence for pre-Clovis (pre 13 ka) colonisation, were rocked to their boots. A team from Liverpool John Moores University, Bournemouth University and the Mexican Geophysics Institute claimed to have found human footprints more than 40 ka old in a volcanic ash deposit (Gonzalez, S. et al. 2006. Human footprints in Central Mexico older than 40,000 years. Quaternary Science Reviews, v. 25, p. 201-222).  The extensive site exposed by quarrying carries many apparent footprints, both human and non-human. Moreover, some of the prints are in convincing-looking trackways. The very old date was obtained by optically stimulated luminescence dating of quartz-grains  that measures the time since the grains were last exposed to sunlight or thermal baking. Were it not for that result probably little fuss would have been made. Now this remarkable find is under serious challenge (Feinberg, J.M. et al. 2009. Age constrains on alleged ‘footprints’ in the Xa;nene Tuff near Puebla, Mexico. Geology, v. 37, p. 267-270). This US-Mexican team applied Ar-Ar dating to the ash and found an age of about 1.3 Ma, confirmed by its association with reversed magnetic polarity in the deposit – at 40 ka the geomagnetic field was as it is today. On that basis, Feinberg and colleagues claim to have refuted the identification of human footprints, and claim that they are merely quarrying marks degraded by later weathering. The Xalnene Tuff in which the footprints were found was deposited in a lake that has been periodically filled and dried out. If the disputed features can be shown irrefutably to be footprints, then there are only two possibilities: either they date from a 40 ka lowstand when the tuff was rewetted and soft, or they are of Homo erectus who somehow found their way to the Americas after leaving Africa around 1.7 Ma ago and crossed the drying lake bed shortly after the tuff was ejected from a nearby volcano.

‘Hobbit’ news

Bones of at least 6 or 7 small people have turned up in the now famous Liang Bua cave on the island of Flores, Indonesia. Their stratigraphic positions span the period from 95 to 17 ka. There have been numerous claims that they do not represent a dwarfed human species – i.e. Homo floresiensis – but individuals who suffered from some form of pathological condition. The strongest evidence supporting that sceptical view is that the one near-complete skull does not fall on the well-established brain –body-size distribution that covers many species: it seems too small for either a normal pigmy modern human or a similarly diminutive H. erectus. Now crucial new anatomical evidence seems set to swing the balance. (Jungers, W.L. et al. 2009. The foot of Homo floresiensis. Nature, v. 459, p. 81-84; Weston, E.N. & Lister A.M. 2009. Insular dwarfism in hippos and a model for brain size reduction in Homo floresiensis. Nature, v.  459, p. 85-88). The foot bones of the most recent and most complete specimen are not like those of humans but more ape-like, although they show clear evidence of bipedalism. Interestingly, they seem to be more primitive than those of H. erectus, raising the possibility of an undocumented dispersal of perhaps from Africa into Eurasia as an ultimate ancestor. Curiously, the foot is disproportionately long compared with the rest of the skeleton; another bonus for ‘hobbit’ fans. Not having a snout, H. floresiensis certainly was no ape, indeed the skull is best expressed as a scaled-down version of either H. erectus or H. habilis. As to extremely small brain size in relation to the body size of H. floresiensis, insular dwarfism of fossil hippos in Madagascar provides a useful analogue, as Weston and Lister suggest. In adulthood they also have disproportionately small brains. As with many puzzles in human evolution, the stir caused by these new discoveries maintains H. floresiensis as a ‘hot topic’ and further excavations are inevitable – Flores has plenty of caves, as do many islands in the Indonesian chain.

See also: Lieberman, D.E. 2009. H. floresiensis from head to toe. Nature, v.  459, p. 41-42.

Quaternary snatched from jaws of extinction

Published on by zooks7771 Comment

At a stormy meeting in August 2004at the 32nd International Geological Congress in Florence, a rearguard action was mounted by a group of stalwart geologists to thwart an attempt to expunge the last remnant of the stratigraphic divisions inspired by Giovanni Arduino’s work in the 18th century from the minds of all future geologists (see December 2004 issue of EPN). The Quaternary was under siege. Despite the fact that the International Commission on Stratigraphy (ICS) of the IUGS had already prepared the ground for a coup de gras by stating that, “This composite epoch [the “Quaternary”] is not a formal unit in the chronostratigraphic hierarchy”, its defenders seem to have won (Mascarelli, A.L. 2009. Quaternary geologists win timescale vote. Nature, v. 459, p. 624). The ICS voted on 21 May 2009 to formally define the base of the Quaternary at 2.6 Ma when the Earth began to cool, glaciation began in the Northern Hemisphere and stone tools first appeared in Africa (it was formerly set at 1.8 Ma, for no obvious reason) and to pass that to IUGS for ratification. Another minority group is enraged, with rumours of chewed carpets, as the Quaternary has annexed 800 ka of what previously was designated as Pliocene: ‘It’s kind of a land grab’, commented Philip Gibbard, a Quaternary expert from Cambridge University, possibly with a hint of glee. To me, it is a milestone decision that gives a proper place to tool making, bipedal apes – ourselves – which makes a great deal more sense that the absurd notion of the Anthropocene (see Epoch, Age, Zone or Nonsense? in March 2008 issue of EPN), whose base some deluded colleagues are trying to set at the beginning of the Industrial Revolution!

Early signs of oxygen…but in the wrong place
The so-called ‘Great Oxidation Event’ is marked by the first occurrence of iron-oxide bearing subaerial sediments or palaeosols, widely regarded as occurring at around 2400 Ma. That is probably around the time that photosynthesis overtook the rate of oxidation reactions that previously consumed the oxygen that it produced, so that oxygen could build-up continually in the air. But that date is far earlier than the origin of subaerial photosynthesis and oxygenic photosynthesis must have arisen among oceanic bacteria before then, but only those inhabiting shallow water where the sunlight is. Banded iron formations that go back into the Archaean are often cited as evidence for when such photosynthesis got underway. Their dominant mineral hematite probably formed by oxidation of soluble iron-II and combination of iron-III with free biogenic oxygen, presumed by most workers to be in shallow water. Among the oldest hematite-rich formations is the Marble Bar Chert of Western Australia, dated to 3460 Ma (Hoashi, M. et al. 2009. Primary haematite formation in an oxygenated sea 3.46 billion years ago. Nature Geoscience, v. 2, p. 301-306). The hematite crystals in the chert seem to have formed at above 60ºC in ocean-floor hydrothermal springs that were discharging abundant dissolved iron-II. The authors estimate the basin in which the cherts formed to be between 200 to 1000 m deep. Since at such depths photosynthesis would not be possible, they claim that sufficient oxygen was produced by shallow-water photosynthesis to form oxygenated intermediate and deep ocean waters, reminiscent of far later times in Earth’s history. This is a minority view, and hinges on whether or not the hematite did form directly on the sea floor. One possibility is that it could have been precipitated colloidally from iron-II-rich ocean water in the photic zone where early photosynthesisers would be, to sink to the deeper sea floor. Eventually very fine iron oxide might recrystallise.
See also: Konhauser, K. 2009. Deepening the early oxygen debate. Nature Geoscience, v. 2, p. 241-242.

At last, a geoscientific April Fool joke?

Published on by zooks777Leave a comment

Maybe it was a coincidence, but the April issue of Geology contain a paper whose title looked suspiciously unreal (White, K. et al. 2009. Hydrologic evolution of the Edwards Aquifer recharge zone (Balcones fault zone) as recorded in the DNA of eyeless Cicurina cave spiders, south-central Texas. Geology, v. 37, p. 339-342). Seemingly, the Cretaceous Edwards Aquifer now flows through cavern systems at the base of a fault-controlled escarpment. At higher levels in the unit are air-filled caves, that are relics of previous karstic events. It is in these dark, dry caves that the arachnid troglobites dwell. Troglobitic animals (those that inhabit totally dark caves and have no eyes) originate as normal surface dwellers, which through successive generations lose functioning eyes and coloration. Conversely, they evolve improved senses of smell, taste and vibration detection. The species that emerge are among the rarest of creatures, for they often occur in only a single cave: a special case of allopatric speciation that may happen when small populations are cut off from one another. Technically, then, this study is no joke, for analysis of mtDNA from the spiders in different caves ought to show evidence of microcosmic evolution, and possible provide a molecular ‘clock’ to chart the times of cave colonisation. And this is what the authors from the University of Mississippi and the endangered invertebrate group of a Texan consulting company have tried to do. The spiders in the higher caves are more evolved than those at progressively lower levels. Moreover, since the karst evolution has developed in a structurally active setting, the spider data correlates with tectonic history…

The ancestral animal

Published on by zooks777Leave a comment

The Cambrian Explosion of shell-bearing animals and the preceding, diverse and very odd Ediacaran fauna that left imprints and moulds in the Late Neoproterozoic both posed two puzzles for early palaeontologists. What organisms evolved so that unmistakable traces of animal life were able to leave fossils after about 600 Ma, and what pace did evolution take to present us with virtually all the animal phyla, including some not around nowadays, ‘fully separated’? Molecular genetic studies of living animals are beginning to throw up some answers (Holmes, R. 2009. The mother of us all. New Scientist, v. 202 (2 May Issue), p. 38-41). It is a complex and growing field, so Bob Holmes’ review of current ideas on the last common ancestor of the animals is welcome for non-specialists. It does look as though the radiation was long before the Ediacaran, but may well have been very rapid. The genetically closest single-celled organism to metazoan animals are the rare choanoflagellates; filter feeders with a collar-like structure and a tail. They bear some resemblance to the feeding cells of sponges, but sponges in their current form seem highly unlikely as the Ur-creature, totally lacking any organs and really just a coexistence of clone-like cells. Gene sequencing from 42 animal groups puts sponges at the bottom of a relatedness tree, yet at the bottom of two of the main branches. So the sponges do indeed seem to have it as our ultimate ancestors. Yet the flurry of ever-more detailed sequencing, for more and more groups using increasingly sophisticated statistical analysis has fired up controversy. Jellyfish-like ctenophores now have a look-in too, as do mysterious placozoans, according to one or other researcher. This field is throwing up an object lesson for hubristic scientists used to counting their chickens… No, the votes are never all in, and surprises always lie ahead for both the unwary and the patient. 

Luckily, Holmes closes by looking at a careful proposal for the ‘How’. Claus Nielson of the University of Copenhagen, a major ‘player’ in this field, has suggested how starting with a slab-like choanoflagellate, with all its function cells on the outside, might have evolved be curling to enclose a tube of inward facing cells; a precursor of a gut. One next step from there could be specialisation of some cells as nerves, then the development of a ‘mouth’ and ‘anus’ – the basis for the bilateral symmetry of all higher animals including ourselves. As for the ‘When’, there are sufficient leads from a molecular clock approach to settle on the oddest climatic events of the last 1.5 Ga of the Proterozoic, the near global glaciations or ‘Snowball Earth’ events that began around 750 Ma ago.

Photosynthesis from way back when: the hunt for RuBisCO

Charles Darwin had an abiding fascination with plants, though one that was essentially practical through observation and breeding. That is sufficient excuse in his bicentenary for reviews, but a good way to honour his legacy is again to push essays to the leading edge of present understanding (Leslie, M. 2009. On the origin of photosynthesis. Science, v. 323, p. 1286-1287). Being able to convert sunlight, water and carbon dioxide to the basis of their own life and that of the rest of the planet, plants and other photosynthesising organisms are the fundamental essence of the living world. Land plants are recent developments, emerging in the Silurian around 425 Ma ago with presumed terrestrial spores some 50 Ma earlier. Their forbears were almost certainly marine algae. Yet they are highly evolved, and it is not to separate precursors that palaeobotanists can look  for origins, but to the internal chloroplasts that look remarkable like cells in their own right with separate DNA and RNA. They perform the astonishing trick of breaking the extremely strong OH-H bonds that form the water molecule otherwise achieved either by extremely high temperatures or by electrolysis. The trick is for an organism to grab an electron thereby releasing the bond and both hydrogen and oxygen. The hydrogen links to carbon and oxygen from CO2, and the other oxygen is freed. Similar to a magician’s trick with smoke and mirrors, photosynthesis uses pigments. Colour in any object or material results from photons of one wavelength range in sunlight being absorbed so that those reflected make up the colour. The most familiar is chlorophyll which absorbs two wavelength ranges: the red and the blue regions to leave green to be reflected for us to see. It is actually a bit of quantum mechanics, as the absorbed photons carry the energy needed to stoke up that of electrons so that they can break free of the OH-H bond in water and split the molecule. The chain of organic chemistry which follows this trick is hugely complex, and it seems to have taken several forms reflected in specific genes in a growing array of photosynthesising bacteria of various genetic antiquities. There are green ones, blue ones, the reds, yellows and oranges.

Luckily the chemical remnants of photosynthesising bacteria are pretty robust, and also distinctive. The central one for most photosynthesising organisms is an enzyme that is complicated, called Ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO for short. Euan Nisbet of Royal Holloway, University of London has been hunting RuBisCO for most of the latter part of his career as a Precambrian geologist. he and colleagues found relics of it in 2.7 Ga Archaean sediments from Zimbabwe and Canada (Nisbet, E.G. et al. 2007. The age of Rubisco: the evolution of oxygenic photosynthesis. Geobiology, v. 5, p. 311-335) and claim there are signs far older. Needless to say.

A fluffy grazing dinosaur

The Cretaceous of NE China is becoming a favoured destination for palaeobiologists interested in well-preserved vertebrates, little dinosaurs, especially. An increasing number turned up by fossil hunters have skin relics covered in feathers, although they are rarely if at all equipped for flight, are. Recently, something even more bizarre was unearthed (Zheng, X.-T. et al. 2009. An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature, v. 458, p. 333-336). In plain-speak, Tianyulong confuciusi was fluffy. And as readers really ought to know, the heterodontosaurs were largely Jurassic herbivorous creatures, 70 Ma older than T. confuciusi; a good example of a ‘living fossil’ in its own time. They evolved to large Cretaceous herbivores, such as the famous duck-billed hadrosaurs, Triceratops and Stegosaurus, members of the Ornithischia as opposed to the more commonly carnivorous Saurischia. It was the latter that were widely believed to have been evolutionary branch from which birds sprang. There is a complex argument surrounding T. Confucius, based on which is a proposal that the ancestral dinosaurs were themselves fluffy. First, thoughts of brightly coloured ‘monsters’ and now the possibility that some may even have looked cuddly.

See also: Witmer, L.M. 2009. Fuzzy origins for feathers. Nature, v. 458, p. 293-295.

The swaddled mantle

Published on by zooks777Leave a comment

A great deal of both theoretical petrology and tectonics hinges on how temperature changes with depth within the Earth. The geotherm, as this variation is termed, depends on how heat is conducted – by conduction, convection or radiation – and where it is produced – either as a relic of original heat of Earth’s accretion or through decay of radioactive isotopes. There are plenty of imponderables, and it would be safe to say that, below the depths at which we can measure temperature (a few km), geotherms are guesswork. Metamorphism, partial melting in crust and mantle, and the rigidity of rock depend on temperature and pressure. Rocks that are too cool to act in a plastic manner tend only to conduct heat, and they are poor conductors. This applies to most of the crust, especially the lower continental crust, which is also low in heat producing radioactive K, U and Th isotopes and rigid. The upshot of this is that the crust acts to insulate the mantle, and that implies build-up of heat and temperature just below the crust. A new means of measuring a rock’s thermal conductivity has revealed that thermal conductivity actually decreases as temperature rises (Whittington, A.G et al. 2009. Temperature dependent thermal diffusivity of the Earth’s crust and implications for magmatism. Nature, v. 458, p. 319-321). The range of crustal temperatures in both continental and oceanic crust roughly halves conduction in the lower crust from previously measured values. This further increases insulation of the mantle, boosting the chances of partial melting.

This tallies with a coincidentally published account of how seismic shear waves change speed with depth beneath the oceanic crust (Kawakatsu, H. et al. 2009. Seismic evidence for sharp lithosphere-asthenosphere boundaries of oceanic plates. Science, v. 324, p. 499-502). As well as sharply showing up the lithosphere-asthenosphere boundary, thought to be a transition from brittle to ductile behaviour, it detects thin layers of partially melted peridotite, which facilitates plate tectonics. A further coincidence is publication of an analysis of 15 years of global earthquake records that focuses on the base of the lithosphere (Rychert, C.A. & Shearer, P.M 2009. A global view of the lithosphere-asthenosphere boundary. Science, v. 324, p. 495-498). As well as its thickness this effectively maps the top of the asthenosphere and therefore the thickness of tectonic plates across the planet, albeit crudely (previously both had been estimated from surface heat flow and theoretical models). Beneath cratons that have remained sluggish for more than a billion years, the asthenosphere is deep (~95 km) and thin, shallowing and thickening appreciably beneath more recently active continental belts. Despite being the uppermost Earth and the stuff of plates and the medium upon which they move, respectively, the lithosphere and asthenosphere are less-well known than the mantle and even the core in terms of the mechanical properties. That may sound odd, but there is a good reason why it is so: more deeply travelled seismic waves are a great deal easier to record by the global network of seismic stations than are shallow regions.

On the edge of chaos in the Younger Dryas

Published on by zooks777Leave a comment

Around 13 thousand years ago, the world was warming rapidly and the great northern ice sheets in retreat. Plants, animals and humans in Europe were able to and did migrate northwards. Sea level still being low, there was nothing to stop decolonisation of Britain by crossing the huge fluviatile plain of Doggerland where the southern North Sea now stands.(see Return to ‘Doggerland’ in September 2008 issue of EPN). At 12.9 ka there came the shock of a rapid temperature fall at the start of the Younger Dryas episode, when ice sheets began to re-establish themselves in the upland areas of Britain and Scandinavia. What happened to those intrepid migrants we may never know, but what they would have faced had they chosen to remain in the game-teeming NW Europe of that episode has become clearer with detailed investigations in sediments at the bottom of a Norwegian lake supplied by melt water from glaciers (Bakke, J. et al. 2009. Rapid oceanic and atmospheric changes during the Younger Dryas cold period. Nature Geoscience, v. 2, p. 202-205).

The research by Norwegian and German scientists used two interesting proxies for glacial advance and retreat: the amount of sedimentary titanium and the density of the sediment, both of which would have varied with the rate of glacial erosion. The data were calibrated to time by 96 14C dates, and the sampling frequency (every 0.06 mm for Ti and 5 mm for density through a 1.4m core that represents 1700 years) was sufficient potentially to resolve events to a few days and 6 months respectively. Allowing for background ‘noise’ effects, certainly monthly and annual changes should show up, and indeed they do. The pattern is one of rapidly changing conditions between warm and frigid, which the authors interpret as a result of repeated ‘boom and bust’. At 12.18 ka, further cooling occurred and the late Younger Dryas is the more chaotic part of the record. The hypothesis is that the fluctuations reflect growth and shrinkage of sea ice in the North Atlantic, matched by growth and melting of glaciers. Brief warming during periods of prevailing westerly winds melted glaciers, but fed vast amounts of fresh water to the North Atlantic that in turn encouraged surface waters to freeze. Sea-ice formation and the build-up of a polar high pressure area drove weather systems conducive to westerlies southwards, when glaciers grew. Something suddenly stopped this chaotic behaviour and high latitudes rapidly emerged from frigidity at 11.7 ka: the Holocene had begun and, soon, so would humanity in an equally chaotic manner.

 

Climate at the Eocene-Oligocene (E-O) boundary

Oxygen isotopes from benthic foraminifera in deep-sea sediment cores show an abrupt increase in δ18O at around 34 Ma, which spanned a mere 300 ka. This is taken to indicate withdrawal of ocean water to polar ice caps on land that diamictites from high southern latitudes link to the beginning of glaciation of Antarctic. Then as now, the south polar region was thermally isolated, probably as a result of its having become surrounded by seaways and development of the Antarctic Circumpolar Current from the Palaeocene onwards as a result of the final break-up of Gondwana when it became separated from Australia and South America. Other factors at the E-O boundary seem to have been decreasing atmospheric CO2 and low solar heating as a result of the Milankovich effect. Cooling due to such factors was disrupted and delayed by the spectacular global warming at the Palaeocene-Eocene boundary (55.8 Ma) as a result of massive methane release to the atmosphere. Detailed proxy records from both high- and low-latitude sea-floor sediment cores now resolve fine detail of the shifts in sea-surface temperature (SST) at the E-O boundary (Liu, Z.  et al. 2009. Global cooling during the Eocene-Oligocene climate transition. Science, v. 323, p. 1187-1190). The most profound shift in SST took place at high latitudes (in both Northern and Southern Hemispheres) with a drop of around 5 to 9ºC between 34 to 33.5 Ma. This was followed by slight rise to about 3ºC below pre-E-O conditions. Surprisingly, data from low latitudes ‘flat-lined’ at around 28ºC across the transition, suggesting steady evaporation of seawater, more of which would have precipitated as snow at high latitudes. The ‘hothouse’ conditions of the Cretaceous and early Cenozoic saw estimated high-latitude sea-surface temperatures rise from about 7ºC to 12ºC by the Early Eocene. The protracted global cooling that followed reached about 7ºC by about 42 Ma, which stabilised until 40 Ma when SST fell to about 4ºC just before the E-O boundary (see http://www.learner.org/courses/envsci/visual/visual.php?shortname=cenozoic).

The sudden start of Antarctic glaciation at 34 Ma looks increasing like an example of a chaos-like ‘flip’ in global climatic conditions brought on by a blend of factors that collectively reached a threshold, which once crossed permitted no escape, at least not over the last 30 Ma or so (Kump, L.R. 2009. Tipping pointedly colder. Science, v. 323, p. 1175-1176). That is a point that should not be lost at a time when anthropogenic global warming continues unabated, despite so much hype by the G20 leaders at their London meeting in early April 2009. Climatic ‘flips’ can go either way.

 See also: Documenting the Palaeogene transition from ‘hothouse’ to ‘icehouse’ in EPN for August 2005, and Magmatic link to the Palaeocene-Eocene warming in EPN for July 2007

Flirting with hand axes

Published on by zooks777Leave a comment

A biface, Acheulean hand axe is more than object of beauty produced by exquisite skill, this industrial genre was invented by African Homo ergaster around 1.6 Ma ago, became a central feature of Palaeolithic archaeology, and lasted until the last few hundred thousand years. Nobody doubts that production of these objects implies a brain that fashioned able to visualise a complex shape within a shapeless lump of rock and to devise a way of achieving it. Moreover, its longevity spanning several species of Homo to our own shows that skills were efficiently passed down through hundreds of thousand generations: possible evidence for linguistic skills in the makers and teachers. But what was it for? Experts have been at a loss to agree on a function: too heavy for hafting to a spear; more awkward for cutting than earlier Oldowan pebble fragments; produced with careful three-dimensional symmetry when a hand tool needs none; time consuming to make yet often found in great abundance and apparently hardly used. One idea is that they were in fact for throwing, in the manner of a discus, yet broken biface axes are rare. A more appealing hypothesis is that they were made for ‘show’ as an element in human sexual selection (Kohn, M. & Mithen, S. 1999. Hand axes: products of sexual selection? Antiquity, v. 73, p. 518-526). Kohn and Mithen argued that the primary function of hand axes was to advertise a maker’s “good genes”: an indicator of the knap­per’s geographic knowledge of suitable resources; his ability to execute a plan; his dexterity and patience; and his so­cial awareness. Those are all attractive qualities in a potential mate. They also suggested that the axes’ often near-pristine quality and occurrence in great numbers at some sites indicate that once their purpose was served, they were thrown away: ‘That man is so cool, he must be good at surviving’. Ten years after Kohn and Mithen first mooted the hypothesis it has come under criticism by April Nowell and Melanie Lee Chang, of the universities of Victoria, Canada and Oregon USA, respectively  (Nowell, A. & Chang M.L. 2009.The case against sexual selection as an explanation of handaxe morphology. Paleoanthropology, v. 2009, p. 77-88).

The critique begins by examining Kohn and Mithen’s interest in symmetry as an element in attractiveness, that Nowell and Chang concede, but consider to have arisen not in a sexual context but in development of vision, despite vision being an evolutionary ‘given’ vastly older than hominins. After a discussion of how fully modern human females base their sexual choices on non-physical attributes of potential mates, such as “niceness,” intelligence, sense of humour, compatibility, willingness to work hard and evidence that the partner in question is attracted to them, Nowell and Chang examine available archaeological evidence. Much of this concerns the ‘absence of evidence’. For instance, there is no evidence to suggest that females did not make hand axes and living females in gatherer-hunter societies do make tools. Other criticisms include: the absence of hand axes from Asia until migration there by H. sapiens [but the biface axe had not been invented when H. ergaster migrated there from Africa around 1.8 Ma]; not all biface axes are symmetrical [but they are nonetheless impressive]; and axes in large numbers generally occur where prey has been butchered, as at Boxgrove, and may have accumulated by hundreds of years of use and loss at such sites by seasonal hunting. The most serious criticism is that some hand axes do show minute patterns that indicate that they were used; although most axes have never been examined for wear patterns. My own conclusion is that the critique is based on absence of evidence for biface axes as ritual objects in sexual selection, but that is not evidence of absence, and I wonder if the 10 years taken to bring together contrary evidence has a bit to do with casting doubt on a not quite ‘PC’ idea. There are many intriguing facets of the fossil and archaeological records of hominins, none more so than those which may have a cultural connotation, like ochre caches (see Deeper roots of culture in EPN of March 2009) and the tear-shaped Acheulean axe. For most we may never know their true context, but can be sure that any curiosity and imagination we apply are reflections of imaginative and curious forebears.

Homo erectus in a cold climate

The famous Zhoukoudian Cave where Peking Man, now known to have been Homo erectus, was first found in 1929 is a lugubrious place. It seems the hominin fossil remains of at least 40 individuals were dragged there and eaten, hopefully by predators. They are by no means the oldest Asian hominins at less than 1 Ma, and their ancestors, probably African H. ergaster, migrated that far around 1.6 to 1.8 Ma ago. Until this year, decent ages from Zhoukoudian were a problem: the errors on estimates of around 500 ka were too large (the likely time lies in a ‘datability gap’ between the capabilities of Ar-Ar and 14C dating methods) to see if the hominins were living at such a high latitude (40ºN) in warm or cold conditions. The latter would be of great interest as it suggests both the use of fire and clothing, and probably adaptation to cooked tubers. In fact, even in the current interglacial episode Beijing gets mighty cold in winter. However, cosmic-ray bombardment can produce unstable isotopes that are suited to dating in that gap, provided materials have been exposed to them. The fossil-containing sediments in Zhoukoudian Cave contain quartz that was exposed at the surface and washed in at the same time as H. erectus individuals were dragged in. Decay of cosmogenic 26Al to 10Be and measurement of parent and daughter isotopes in quartz grains have yielded ages of 770±80 ka, somewhat older than earlier estimates (Shen, G. et al. 2009. Age of Zhoukoudian Homo erectus determined with 26Al/10Be dating. Nature, v. 458, p. 198-200). This age roughly correlates with layers in the western Chinese windblown loess deposits that were deposited during the dry conditions of a minor glacial episode.

See also: Ciochon, R.L. & Bettis, E.A. 2009. Asian Homo erectus converges in time. Nature, v. 458, p.153-154. Gibbo0ns, A. 2009. Ice age no barrier to ‘Peking Man’. Science, v.  323, p. 1419.

 

Walking with the ancestors

From time to time the most evocative hominin trace fossils come to light, such as the Australopithecus afarensis footprints fount by Mary Leakey at Laetoli in Tanzania. A recent one is of footprints of a probable H. ergaster dating back to 1.5 Ma near Lake Turkana in Kenya, not far from the site of the famous ‘Turkana Boy’ skeleton of the same species (Bennett, M.R. and 11 others 2009. Early hominin foot morphology based on 1.5-million-year old footprints from Ileret, Kenya. Science, v. 323, p. 1197-1201). Not only does the trackway reveal details of flesh, skin and bones of the feet, but careful analysis of 3-D scans of the prints, in the context of the mechanical properties of the material walked upon, allows the authors to show that the person who left them moved in essentially the same way as do we when walking through soft mud. They are distinctly different from the Laetoli prints, showing arches and very distinct big toes that are so necessary for ‘springiness’ and bipedal balance respectively.

See also: Crompton, R.W. & Pataky, T.C. 2009. Stepping out. Science, v. 323, p. 1174-1175.

The Great Bend of the Pacific ocean floor

Published on by zooks7771 Comment

Ocean island chains are trackways of moving lithospheric plates relative to the underlying mantle. Mantle hotspots act in a similar manner to a candle that would burn a line in a sheet of paper were one to be passed over it. The largest, most coherent and best studied ocean island chain is that of the Hawaiian Islands and the Emperor Seamounts  in the NW Pacific. The volcanoes that built the chain range in age continuously from Late Cretaceous (81 Ma) at the northern tip of the Emperor Seamounts where they touch the Kamchatka Peninsula to the present in the Big Island of Hawai’i itself. So far, so good for the hotspot-track hypothesis. But the chain is bent into a WNW segment (Hawaii) and one that trends NNW (Emperor). That might seem to be superb evidence that the direction of West Pacific sea-floor spreading underwent a sudden, 60º change around 47 Ma (the age of the Diakakuji seamount at the apex of the bend). However, measurements in 2001 of palaeomagnetic latitude in sea-floor cores along the chain revealed clear palaeomagnetic evidence that the Hawaiian hot spot has not always been fixed relative to moving lithospheric plates.  From Late Cretaceous to Late Eocene times the hotspot seems to have been was shifting southwards relative to the north magnetic pole at a rate comparable with that of sea-floor spreading, and then became stationary to explain the 60° bend in the chain (See American Geophysical Union 2001 Fall Meeting in EPN for January 2002).

Further work has been done since 2001, and a review of the huge oddity that bucks John Tuzo Wilson’s 1963 theory of hotspots fixed in space and time is timely (Tarduno, J. et al. 2009. The bent Hawaiian-Emperor hotspot track: inheriting the mantle wind. Science, v. 324, p. 50-53). Data have moved on to suggest that the hotspot is indeed the head of narrow mantle plume originating deep down, perhaps even near the core – mantle boundary (CMB). But could such a massive structure change it’s behaviour so that its head would move? Some have suggested the development of a propagating crack in the Pacific lithosphere and then its closure, but no evidence points unerringly that way. After considering a range of possible mechanisms, the authors suggest that the great bend records past changes in mantle flow beneath the West Pacific, so that the plume would itself have bent in the vertical dimension. Seismic tomography has revealed apparently low-angled zones of hot, low-velocity mantle, such as one that may (or may not) connect with the Afar plume beneath the triple junction of the East African Rift, the Red Sea and the Gulf of Aden after rising from the CMB south of Cape Town. They are tantalising results, because the resolution is simply not good enough to be sure. It needs an order of magnitude better tomographic resolution of mantle features to truly make more headway.

‘Clean’ coal and soda pop

Published on by zooks777Leave a comment

An option much touted as a means of having our cake (power stations fired by fossil fuels, especially coal) and eating it (escaping runaway global warming while enjoying a high-energy lifestyle) is extracting carbon dioxide from flue gases, or even the atmosphere itself, and safely disposing of it in long-term storage. Carbon capture and storage (CCS) is not a well-tried technology. Yet some authorities claim it is at the least a means of ‘tiding-over’ an economy that depends to such a degree on fossil carbon burning as an energy source that it seems unlikely that alternative, carbon-neutral sources can be deployed in time to stave off increasingly awful and plausible climate and thereby social scenarios. There are others who are convinced that CCS is merely an excuse to continue with ‘business as usual’, and therefore fraught with dangers. Whichever, there are elements of CCS that do concern geoscientists, such as where should it be stored and in what form. Leaving aside some of the geological issues of storage, such as depleted natural petroleum fields or deep aquifers, what happens to CO2 at depth? There are five possibilities: it remains as a gas; under high pressure it may take on liquid form (CO2 can exist only as gas or ‘dry ice’ at atmospheric pressure); it reacts with the rock itself to form some kind of carbonate; under moderate pressure and low temperature it may combine with water to form a gas-hydrate ‘ice’, as does methane; or it may dissolve in water under high pressure.

The ideal form for long-term storage would be in the form of solid carbonate, but that demands bicarbonate ions combining with calcium, magnesium or perhaps sodium ions. One possibility is through dissolution in highly saline groundwater. The chemical reactions are not complex, but depend on the solubility of carbonates being exceeded because of massive increases in bicarbonate concentrations. However, experiments have had little success. Another means of solid storage is by the combination of atmospheric CO2 with calcium hydroxide to form calcium carbonate, which is what happens when lime plaster slowly ‘cures’. The downside is that the only means of making Ca(OH)2 is by kilning limestone: no free lunch there. To cut a long story short, a view is emerging that CO2 pumped, in whatever form, into wet rock will end up dissolving in groundwater, to form vast quantities of ‘sparkling’ water, or ‘soda pop’ (Gilfillan, S.M.V. and 10 others 2009. Solubility trapping in formation water as dominant CO2 sink in natural gas fields. Nature, v. 458, p. 614-618). The British, Canadian, US and Chinese team investigated nine natural gas fields in which CO2 is present as well as petroleum gas, using noble gases and carbon isotopes as tracers of the chemical fate of the natural CO2 as the reservoir rocks filled with oil and natural gas during maturation. They discovered that the bulk of CO2 ended up dissolving to form a weakly acidic water under pressure. This is a recipe for filling huge analogies of soda siphons. They did discover that some CO2 ended up as solid carbonate, but no more than 15%. As those who add Perrier or Volvic to their Scotch should know, carbonated springs are not unknown. Consequently, CCS that uses confined aquifers poses the danger of eventual leakage, whether CO2 is stored as gas, liquid or in solution. Petroleum geologists often claim that no trap is leak proof, and extensive areas of gas leakage are known over most oil fields; they are an important sign for explorationists, if they can be detected. The other issue is that fans of CCS set much store in re-use of depleted commercial oil and gas fields for sequestration. Such fields have already been depressurised, and nobody knows whether or not they were leaky to gas and water.

See also: Aeschbach-Hertig, W. 2009. Clean coal and sparkling water. Nature, v. 458, p. 583-4.

When the Mediterranean evaporated

Published on by zooks777Leave a comment

Much to geologists’ surprise seismic surveys and drilling of the Mediterranean basin revealed that it is floored by an immense thickness of evaporite salts, laid down during the Late Miocene about 6 Ma ago (Messinian Stage).  The event has been dubbed the Messinian salinity crisis, and ascribed to the cutting off from the Atlantic of the Mediterranean Sea causing lowering of sea level by evaporation. The formation of the evaporite sequence has been overshadowed by what happened to restore the Mediterranean: a humongous waterfall at the Straits of Gibraltar. New modelling of the salt-forming event has had a technically surprising outcome (Govers, R. Choking the Mediterranean to dehydration: The Messinian salinity crisis. Geology, v. 37, p. 167-170). It suggests that most of the salt body formed before sea level fell. Sea level lowering reduced the load on the sea floor and allowed isostatic uplift to develop a flow barrier at the Straits of Gibraltar, further cutting off resupply of Atlantic water. The other factor seems to have been the effect of sluggish eastward subduction of a lithospheric slab that eventually resulted in subsidence so that the Atlantic could re-flood the Mediterranean basin.