Out of Africa: a little less blurred?

DNA from the mitochondria of humans who live on all the habitable continents shows such a small variability that all of us must have had a common maternal ancestor, and she lived in Africa about 160 ka ago. Since this was first suggested by Rebecca Cann, Mark Stoneking and Allan Wilson of the University of California, Berkeley in 1987 there has been a stream of data and publications – subsequently using Y-chromosome DNA and even whole genomes – that both confirm an African origin for Homo sapiens and illuminate it. Analyses of the small differences in global human genetics also chart the routes and – using a ‘molecular clock’ technique – the timings of geographic and population branchings during migration out of Africa. As more and better quality data emerges so the patterns change and become more intricate: an illustration of the view that ‘the past is always a work in progress’. The journal Nature published four papers online in the week ending 25 September 2016 that demonstrate the ‘state of the art’.

Three of these papers add almost 800 new, high-quality genomes to the 1000 Genomes Project that saw completion in 2015. The new data cover 270 populations from around the world including those of regions that have previously been understudied for a variety of reasons: Africa, Australia and Papua-New Guinea. All three genomic contributions are critically summarized by a Nature News and Views article (Tucci, S & Akey, J.L. 2016. A map of human wanderlust. http://dx.doi.org/10.1038/nature19472). The fourth paper pieces together accurately dated fossil and archaeological findings with data on climate and sea-level changes derived mainly from isotopic analyses of marine sediments and samples from polar ice sheets (Timmermann, A & Friedrich, T. 2016. Late Pleistocene climate drivers of early human migration. Nature, doi:10.1038/nature19365). Axel Timmermann and Tobias Friedrich of the University of Hawaii have attempted to simulate the overall dispersal of humans during the last 125 ka according to how they adapted to environmental conditions; mainly the changing vegetation cover as aridity varied geographically, together with the opening of potential routes out of Africa via the Straits of Bab el Mandab and through what is now termed the Middle East or Levant. They present their results as a remarkable series of global maps that suggest both the geographic spread of human migrants and how population density may have changed geographically through the last glacial cycle. Added to this are maps of the times of arrival of human populations across the world, according to a variety of migration scenarios.

timmermann

Estimated arrival time of anatomically modern human migrants from Africa (top); estimated population densities around 60 thousand years ago (bottom). (From: Timmermann et al. 2016 doi:10.1038/nature19365)

The role of climate change and even major volcanic activity – the 74 ka explosion of Toba in Indonesia – in both allowing or forcing an exodus from African homelands and channelling the human ‘line of march’ across Eurasia has been speculated on repeatedly. Now Timmermann and Friedrich have added a sophisticated case for episodic waves of migration across Arabia and the Levant at 106-94, 89-73, 59-47 and 45-29 ka. These implicate the role of Milankovich’s 21 ka cycle of Earth’s axial precession in opening windows of opportunity for both the exodus and movement through Eurasia; effectively like opening and closing valves for the flow of human movement. The paper is critically summarised by a Nature News and Views article (de Menocal, P.B. & Stringer, C. 2016. Climate and peopling of the world. Nature, doi:10.1038/nature19471.

This multiple-dispersal model for the spread of anatomically modern humans (AMH) finds some support from one of the genome papers (Pangani, L. and 98 others 2016. Genomic analyses inform on migration events during the peopling of Eurasia. Nature (online). http://dx.doi.org/10.1038/nature19792). A genetic signature in present-day Papuans suggests that at least 2% of their genome originates from an early and largely extinct expansion of AMH from Africa about 120 ka ago, compared with a split of all mainland Eurasians from African at around 75 ka. It appears from Pangani and co-workers’ analyses that later dispersals out of Africa contributed only a small amount of ancestry to Papuan individuals. The other two genome analyses (Mallick, S. and 79 others 2016. The Simons Genome Diversity Project: 300 genomes from 142 diverse populations. Nature (online) http://dx.doi.org/10.1038/nature18964; Malaspinas, A.-S. and 74 others 2016. A genomic history of Aboriginal Australia. Nature (online). http://dx.doi.org/10.1038/nature18299) suggest a slightly different scenario, that all present-day non-Africans branched from a single ancestral population. In the case of Malaspinas et al. an immediate separation of two waves of AMH migrants led to settlement of Australasia in one case and to the rest of Mainland Eurasia. Yet their data suggest that Australasians diverged into Papuan and Australian population between 25-40 ka ago. Now that is a surprise, because during the lead-up to the last glacial maximum at around 20 ka, sea level dropped to levels that unified the exposed surfaces of Papua and Australia, making it possible to walk from one to the other. These authors appeal to a vast hypersaline lake in the emergent plains, which might have deterred crossing the land bridge. Mallick et al. see an early separation between migrants from Africa who separately populated the west and east of Eurasia, with possible separation of Papuans and Australians from the second group.  These authors also show that the rate at which Eurasians accumulated mutations was about 5% faster than happened among Africans. Interestingly, Mallick et al. addressed the vexed issue of the origin of the spurt in cultural, particularly artistic, creativity after 50 ka that characterizes Eurasian archaeology. Although their results do not rule out linked genetic changes outside Africa, they commented as follows:

‘… however, genetics is not a creative force, and instead responds to selection pressures imposed by novel environmental conditions or lifestyles. Thus, our results provide evidence against a model in which one or a few mutations were responsible for the rapid developments in human behaviour in the last 50,000 years. Instead, changes in lifestyles due to cultural innovation or exposure to new environments are likely to have been driving forces behind the rapid transformations in human behaviour …’.

Variations in interpretation among the four papers undoubtedly stem from the very different analytical approaches to climate and genomic data sets, and variations within the individual sets of DNA samples. So it will probably be some time before theoretical studies of the drivers of migration and work on global human genomics find themselves unified. And we await with interest the pooling of results from all the different genetics labs and agreement on a common data-mining approach.

Signs of life in some of the oldest rocks

Vic McGregor and Allen Nutman examine metasedimentary strata at Isua, West Greenland
For decades the record of tangible signs of life extended back to around 3.4 billion years ago, in the form of undulose, banded biofilms of calcite known as stromatolites preserved at North Pole in the Pilbara region of Western Australia. There have been attempts to use carbon-isotope data and those of other elements from older, unfossiliferous rocks to seek chemical signs of living processes that extracted carbon from the early seas. Repeatedly, claims have been made for such signatures being extracted from the 3.7 to 3.8 Ga Isua metasediments in West Greenland. But because this famous locality shows evidence of repeated metamorphism abiogenic formation of the chemical patterns cannot be ruled out. Isua has been literally crawled over since Vic McGregor of the Greenland Geological Survey became convinced in the 1960s that the metasediments could be the oldest rocks in the world, a view confirmed eventually by Stephen Moorbath and Noel Gale of Oxford University using Rb-Sr isotopic dating. There are slightly older rocks in Canada, which just break the 4 Ga barrier, but they were metamorphose at higher pressures and temperatures and are highly deformed. The Isua suprcrustals, despite deformation and metamorphism show far more diversity that geochemically can be linked to many kinds of sedimentary and volcanic rock types.

 

Two of the Isua addicts are Allen Nutman of the University of Wollongong, Australia and Clark Friend formerly of Oxford Brookes University, UK, who have worked together on many aspects of the Isua rocks for decades. Finally, thanks to melt-back of old snow pack, they and colleagues have found stromatolites that push the origin of life as far back as it seems possible for geoscientists to reach (Nutman, A.P. et al. 2016. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature, v. 537, published online 31 August 2016, doi:10.1038/nature). The trace fossils occur in a marble, formerly a limestone that retains intricate sedimentary structures, which show it to have been deposited in shallow water. The carbon and oxygen isotopes have probably been disturbed by metamorphism, and no signs of cell material remain for the same reason, but the shape is sufficiently distinct from those produced by purely sedimentary processes to suspect that they resulted from biofilm build-up. The fact that they are made of carbonates suggests that they may have been produced by cyanobacteria as modern stromatolites are.

isua strom

Stromatolite-like structures from a metasediment in the Isua area of West Greenland (credit Allen Nutman, University of Wollongong, Australia)

The age of the structures, about 3.7 Ga, is close to the end of the Late Heavy Bombardment (4. 1to 3.8 Ga) of the Solar System by errant asteroids and comets. So, if the physical evidence is what it seems to be, life emerged either very quickly after such an energetic episode or conditions at the end of the Hadean were not inimical to living processes or the prebiotic chemistry that led to them.

 

Allwood, A.C. 2016. Evidence of life in Earth’s oldest rocks. Nature, v. 537, published online 31 August 2016, doi:10.1038/nature19429

Lucy: the australopithecine who fell to Earth?

The specimen of Australopithecus afarensis known far beyond the confines of palaeoanthropology as Lucy remains the iconic figure of hominin evolution, 42 years after her discovery by Donald Johanson and Tom Gray near Hadar in the Awash valley of the Afar Depression, Ethiopia. Her skeletal remains were not complete, but sufficient to recognize that they were from the oldest known upright hominin and that they were female, the pelvis having affinities to that of human women rather than other extant apes. Yet her skull was more akin to apes with a brain volume about the same as a modern chimpanzee’s. Part of the reason for her fame stems from being named after a character in a somewhat mystical song by the British pop group, the Beatles, which was played over and over in the palaeontologist’s camp – good job the find wasn’t during the 1990’s acme of gangsta rapper Apache.

Subsequently, the entombing strata were radiometrically dated at around 3.2 Ma. Lucy, in common with most fossils roughly in the human line of descent, has from the outset been the subject of controversy, even at one time being said to be misnamed because of alleged male characteristics; a view swiftly discarded. Like the treasures of Tutankhamun, Lucy’s actual remains have been exhibited far and wide, including a 6-year stay in the US. Fears of damage in transit led the Ethiopian authorities to produce casts for distribution, and Lucy is now restricted to the National Museum in Addis Ababa. As a further precaution, all the actual bones were rendered in digital 3-dimensions using a high-resolution CT scanner during her US sojourn. It is these scans that have led to a surprising development as regards her original fate. Apart from signs of a single carnivore tooth mark, her remains were not devoured by scavengers, nor did early anatomical examinations suggest any sign of disease and she was estimated to have been a young mature female when she died – the cause of death was unknown.

Model of the australopithecus Lucy in the muse...

Model of the Lucy (Australopithecus afarensis) in the museum of Barcelona (credit: Wikipedia)

Detailed forensic analysis of the CT scans (Kappelman, J. and 8 others 2016. Perimortem fractures in Lucy suggest mortality from fall out of tall tree. Nature, v. 537, published online 29 August 2016, doi:10.1038/nature19332) revealed far more than did the original bones, including evidence for numerous fractures in Lucy’s limbs, ribs and cranium, many of which are of the compressive or ‘greenstick’ kind. Those in the left ankle and leg bones (talus, tibia, fibula and femur) are compressive and suggest a severe vertical impact of the heel with enough force to smash the strongest bones in the body, driving the hip into the pelvis. Damage to the ribs, pelvis and lower spine (sacrum) is commensurate with a further horizontal, frontal impact of the torso. Arm (humerus), wrist (radius)  shoulder blade (scapula) and collar (clavical) bone fractures are typical of injuries sustained when a falling person tries to break a fall by stretching out the arms. Damage to the cranium and lower jaw (mandible) suggest this instinctive defence posture was futile. None of the fractures show signs of healing, so the multiple traumas were immediately fatal.

Forensic reconstruction of how Lucy fell to meet her end. (credit: John Kappelman et al, doi: 10.1038/nature19332)

Forensic reconstruction of how Lucy fell to meet her end. (credit: John Kappelman et al, doi: 10.1038/nature19332)

The traumatic pattern is reminiscent of someone falling onto hard ground from great height; perhaps equivalent to a four- or five-storey building (see animated reconstruction here). In Lucy’s case, the most likely scenario is from a large tree, perhaps while foraging or sleeping in a safe refuge from ground predators. Forensic analysis of newly dead victims of severe falls generally show massive soft tissue damage by penetration of bone fragments or a ‘hydraulic ram effect’ in which abdominal organs are thrust upwards to produce cardiac damage. That Lucy was found almost intact rules out dismemberment by scavengers or transport by flood water. Indeed, the preservation of even tiny slivers of fractured bone seems to suggest her burial in flood plain sediments before decomposition had set in. A question that the authors do not address is whether or not she may have been deliberately interred, which to me seems a possibility that could be drawn from the detailed evidence. I wonder what a modern coroner might conclude: probably misadventure.

The nearest Earth-like planet

What could be more exciting for exobiologists and planetary scientists than to discover that a nearby star is orbited by a planet approximately the same mass as the Earth that may support liquid water: a world in the ‘Goldilocks zone’? It seems that Proxima Centauri, the Sun’s closest companion star (4.2 light years distant), might have such a planet (Anglada-Escudé, G. And 30 others 2016. A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature, v. 536, p. 437-440).  It is one of 34 candidates found to date with various levels of likelihood for having the potential to produce life and support it. To fit the bill a planet first has to orbit a star at a distance where the stellar energy output is unlikely to vapourise any surface water yet is sufficient to keep it at a temperature above freezing point, i.e. the ‘Goldilocks’ or circumstellar habitable zone is closer to a cool star than to a hot one. Note that the liquid-water criterion requires that the planet also has an atmosphere with sufficient pressure to maintain liquid water. It also needs to have a mass close to that of the Earth (between 0.1 to 5 Earth masses) and a similar density, i.e. a candidate needs to be dominated by silicates so that it has a solid surface rather than being made mainly of gases and liquids.

The location of Alpha Centauri A and B, Proxim...

The location of Alpha Centauri A and B, Proxima Centauri and the Sun in the Hertzsprung-Russell (HR) diagram. (credit: Wikipedia)

Proxima Centauri b, as the planet is called, was not discovered by the Kepler space telescope using the transit method (drops in a star’s brightness as a planet transits across its disk) but by terrestrial telescopes that measure the Doppler shifts in starlight as it wobbles because of the gravitational affect of an orbiting planet. As well as being close, Proxima Centauri is much smaller than the Sun so such effects are more pronounced, especially by planets orbiting close to it. The planet that has excited great interest has an orbital period of only 11.2 Earth days so is much closer to its star and may have a surface temperature (without any greenhouse effect) of 234 K (21 degrees less than that of Earth). The wobble suggests a mass and radius are likely to be 1.3  and between 0.8 to 1.4 times those of Earth, respectively. So Proxima Centauri b is probably a silicate-rich world. But, of course, such limited information gives no guarantee whatever of the presence of liquid water and an atmosphere that can support it. Neither is it possible to suggest a day length. In fact, such a close orbit may have resulted in the planet tidally locked in synchrony with its orbit, in the manner of the Moon showing only one face to the Earth. Moreover, its star is a red dwarf and is known to produce a prodigious X-ray flux, frequent flares and probably a stream of energetic particles, from which only a planet with a magnetic field is shielded. All red dwarfs seem to have such characteristics, and the list of possible Earth-like planets show them to be the most common hosts.

It is too early to get overexcited as technologies for astronomical detection of atmospheres and surface composition are about a decade off at the earliest. Being so close makes it tempting for some space agency to plan sending tiny probes (around 1 gram) using a laser propulsion system that is under development. Anything as substantial as existing planetary probes and certainly a crewed mission is unthinkable with current propulsion systems – a one-way trip of 80 thousand years and stupendous amounts of fuel.

China’s legendary great flood did happen

The Biblical Flood is one of several legendary catastrophes that over the millennia have made their way into popular mythology. Indeed, Baron Georges Cuvier explained his stratigraphy of the Paris Basin and fossil evidence for extinctions of animals as the results of repeated inundations. His opinions and those of other scientists of the catastrophist school reflect the philosophical transition that began with the Enlightenment of the 18th century: curiosity and observation set against medieval dogma. It seems that transition is incomplete as there are still people who seek remains of Noah’s Ark and propose alien beings as the constructors of the huge geoglyphs of the Nazka Desert in Peru. On the other hand, Walter Pitman – one of the pioneers of plate tectonics – and his colleague William Ryan sought a rational explanation for the Flood, based in part on a more detailed description of a flodd in the Near East in one of the oldest written documents, the Epic of Gilgamesh (~2150-1400 BCE). In 1996 they published a hypothesis that such flood legends may have arisen from oral accounts of the flooding of the previously cut-off Black Sea basin through the Bosphorus as global sea level rose about 7600 years ago.

Chinese mythology too contains graphic descriptions of catastrophic flooding in the legend of Emperor Yu, first written down at the start of the first millennium BCE. Rather than being a victim or a survivor of catastrophe, Yu is credited with relieving the aftermath of the supposed flood by instigating ingenious systems of dredging and rechanneling the responsible river, and instigating the start of Chinese civilisation and the Xia Dynasty. Such detail conveys a greater air of veracity than a substantial boat containing male and female representatives of all animal species ending up on top of a mountain once Flood waters subsided! Recent research by Quinglong Wu of the School of Archeology at Peking University, together with other Chinese and US colleagues along the Yellow River has nailed the truth of the legend to events in the headwaters of the Yellow River (Wu, Q. and 15 others 2016. Outburst flood at 1920 BCE supports historicity of China’s Great Flood and the Xia dynasty. Science, v. 353, p. 579-582).

Map of the Yellow River

Map of the Yellow River from the Qing Dynasty. (Photo credit: Wikipedia)

The team discovered evidence for a huge landslide in a terrace of the Yellow River where it flows through the Jishi Gorge. Probably dislodged by an earthquake, the slide blocked the gorge so that a large lake formed above it. The lake also left sedimentary evidence on the flanks of the gorge, which suggest that it may have been as much as 200 m deep and impounded 12 to 17 km3 of water. Downstream of the gorge sediments of the Guanting Basin contain chaotic sediments characteristic of outburst floods, probably deposited once the landslide dam was breached. 14C dates of charcoal from the outburst flood sediments give a likely age for the massive event of 1922±28 BCE. Astonishingly, remains of three children from a cave near the Yellow River are buried in the flood deposits and provided an age within error of that of the flood: they were victims. Sediments extending to the coast in the North China Plain are the repositories of much of the archaeological evidence for the evolution of Chinese culture along with signs of rates of sedimentation. The definite signs of a catastrophic flood upstream coincides with the transition from Neolithic to Bronze Age artefacts in the Yellow River flood plain.

Ancient oceanic lithosphere beneath the eastern Mediterranean

The extensive active subduction zones around the Pacific ocean are responsible for a dearth of oceanic lithosphere older than about 200 Ma that still remains where it formed. Trying to get an idea of pre-Mesozoic ocean-floor processes depends almost entirely on fragmented ophiolites thrust or obducted onto continent at destructive plate margins. Yet the characteristically striped magnetic signature above in situ oceanic lithosphere offers a good chance of spotting any old oceanic areas, provided the stripes are not imperceptible because of thick sediment cover.  One of the most intriguing areas of ocean floor is that beneath the eastern Mediterranean Sea in the 3 km deep Herodotus Basin, which has long been thought to preserve a relic of old ocean floor.  Roi Granot of Ben-Gurion University of the Negev, Israel has analysed magnetic data gathered along 7 000 km of survey lines and indeed there are vague traces of stripy geomagnetic variation that has a long wavelength, to be precise there are two bands of . Mathematical analysis of the magnetic profiles suggest that they have a source  about 13 to 17 km beneath the seabed: probably crystalline crust beneath thick Mesozoic sediments (Granot, R. 2016. Palaeozoic oceanic crust preserved beneath the eastern Mediterranean. Nature Geoscience, doi:10.1038/ngeo2784).

English: Age of oceanic lithosphere Deutsch: A...

Ages of oceanic lithosphere (credit: Wikipedia)

The shape of the anomalies cannot be matched with those of younger magnetic stripes, but can be modelled to fit with a sequence of normal-reverse-normal magnetic polarity preserved in continental sequences of early Carboniferous age, about 340 Ma ago. At that age, the lithosphere would by now be old, cold and dense enough to subside to the observed depth, but the fact that it escaped subduction during amalgamation of Pangaea in the Upper Palaeozoic or when Africa collided with Eurasia in the early Cenozoic is a puzzle. Granot reckons that it most likely formed in Pangaea’s great eastern ocean embayment, known as Palaeotethys. An interesting view, but one that does not seem likely to lead any further, simply because of the great depth to which the oceanic material is buried. The deepest yet to be achieved is only 12 km in the onshore Kola Superdeep Borehole in Russia. So the changes of getting samples are slim, even if the overlying sedimentary pile proves to have hydrocarbon potential.

English: Pangea animation

Pangea break-up animation ( credit: Wikipedia)

Oceans of magma, Moon formation and Earth’s ‘Year Zero’

That the Moon formed and Earth’s geochemistry was reset by our planet’s collision with another, now vanished world, has become pretty much part of the geoscientific canon. It was but one of some unimaginably catastrophic events that possibly characterised the early Solar System and those around other stars. Since the mantle geochemistry of the Earth’s precursor was fundamentally transformed to that which underpinned all later geological events, notwithstanding the formation of the protoEarth about 4.57 Ga ago, I now think of the Moon-forming event as our homeworld’s ‘Year Zero’. It was the ‘beginning’ of which James Hutton reckoned there was ‘no vestige’. Any modern geochemist might comment, ‘Well, there must be some kind of signature!’, but what that might be and when it happened are elusive, to say the least. One way of looking for answers is, as with so many thorny issues these days, to make a mathematical model. James Connelly and Martin Bizzarro of the University of Copenhagen, Denmark, have designed one based on the fact that one of the volatile elements that must have been partially ‘blown off’ by such a collision is lead and, of course, that is an element with several isotopes that are daughters of long-term decay of radioactive uranium and thorium (Connelly, J.N. & Bizzarro, M. 2016. Lead isotope evidence for a young formation age of the Earth–Moon system. Earth and Planetary Science Letters, v. 452, p. 36-43. doi:10.1016/j.epsl.2016.07.010).

Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed.

Artist’s impression of the impact of a roughly Mars-size planet with the proto-Earth to form an incandescent cloud, from part of which the Moon formed. A NASA animation of lunar history can be viewed here.

Loss of volatile daughter isotopes of Pb produced by the decay schemes of highly refractory isotopes of U and Th would have reset the U-Pb and Th-Pb isotopic systems and therefore the radiogenic ‘clocks’ that depend on them in the same way as melting or high-temperature metamorphism resets the simpler 87Rb-87Sr decay scheme. Each radioactive U isotope has a different decay rate that produces a different Pb isotope daughter (235U Þ 207Pb; 238U Þ 206Pb, so it is possible to devise means of using present-day values of ratios between Pb isotopes, such as 207Pb/206Pb, 206Pb/204Pb and 207Pb/204Pb, to work back to such a ‘closure’ time. In short, that is the approach used by Connelly and Bizzarro. The most complicated bit of that geochemical ruse is estimating values of the ratios for the Earth’s modern mantle and for the Solar system in general – a procedure based on what we can actually measure: lots of mantle-derived basalts and lots of meteorites. Cutting out some important caveats, the result of their model is quite a surprise: ‘Year Zero’ on their account was between 4426 and 4417 Ma years ago, which is astonishingly precise. And it is pretty close to the measured age of the of lunar Highland anorthosites – products of fractional crystallisation of the Moon’s early magma ocean – and also to that of the oldest zircons on Earth. But is also about 60 Ma later than previous estimates

The Connelly and Bizzarro paper follows hard on the heels of another with much the same objective  (Snape, J.F. and 8 others 2016. Lunar basalt chronology, mantle differentiation and implications for  determining the age of the Moon. Earth and Planetary Science Letters, v. 451, p. 149-158. doi.org/10.1016/j.epsl.2016.07.026). Once again omitting a great deal of argument, Snape and colleagues end up with an age for the isotopic resetting of the lunar mantle of 4376 Ma to the nearest 18 Ma; i.e. an age significantly different from that arrived at by Connelly and Bizzarro. So the answer to the question, ‘When was there a vestige of a beginning?’ is, ‘It depends on the model’… Thankfully, neither estimate for ‘Year Zero’ has much bearing on the big, practical questions, such as, ‘When did life form?’, ‘Has there always been plate tectonics?’

Climatic conditions for early hominin evolution

Until about 1.8 Ma ago, when early Homo erectus and perhaps other archaic hominins strode into Eurasia, our forerunners lived and evolved on only one continent – Africa. The physical and environmental conditions underlying the steps from a common ancestor with modern chimpanzees through a growing number of upright species are not well charted by the Pliocene and early Pleistocene terrestrial evidence. All that is know of this formative period is that global climate cooled in an oscillating fashion that culminated in the onset of Northern Hemisphere glaciations in the late Pliocene (~3 Ma) and a shift to drier conditions in East Africa around 2.8 Ma suggested by pollen records off the east coast. Marine sediments of the Indian Ocean, Red Sea and Gulf of Aden still offer the most convenient means of charting environmental change in detail for this crucial episode in human history. As well as oxygen-isotope and pollen-type variations, modern core analysis offers a growing number of wind-blown proxies for onshore vegetation. These include organic geochemistry plus carbon and hydrogen isotopes from trace amounts of leaf waxes. During the May to September East African Monsoon, high speed winds in the upper atmosphere drag dusty continental air from the East African Rift System over the Gulf of Aden, making sea-floor sediment an important target for tracking variations in the proxies (Liddy, H.M. et al. 2016.  Cooling and drying in northeast Africa across the Pliocene.  Earth and Planetary Science Letters, v. 449, p. 430-438. doi:10.1016/j.epsl.2016.05.005). Hannah Liddy and colleagues from the Universities of Southern California and Arizona, USA, applied these techniques to a Gulf of Aden core from offshore Somaliland to open a window on this crucial period.

Early history of hominin evolution and evidence for climate change in East Africa. Based on a diagram at the handprint.com website

Early history of hominin evolution and evidence for climate change in East Africa. Based on a diagram at the handprint.com website and in Stepping Stones Chapter 22

Early Pliocene East Africa (5.3 to 4.3 Ma), the time of Ardepithecus ramidus, was characterized by evidence for a climate wet enough to sustain grasses and riverine woodlands. Yet around 4.3 Ma conditions had shifted to ecosystems more dominated by shrubby plants able to thrive in more arid conditions. At about that time the earliest australopithecines appear in the fossil record, with A. anamensis. Yet the later Pliocene was not devoid of grasses or herbivores. There is ample carbon-isotope evidence from the teeth of hominins that shows that after 3.4 Ma the diet of A. afarensis and A. africanus included increasing amounts whose carbon derived from grasses, when. This apparent paradox can be explained by a major turn to eating meat from herbivores as vegetable foods declined with increasing aridity. This is all very interesting, especially the detailed record of δ13C in plant waxes, but there is little to indicate that steps in hominin speciation or extinction had much direct connection with fluctuations in climatic conditions. Environmental change may have formed a background to other influences that may have been wholly down to early hominin’s social and technological behaviour.

Salt and Earth’s atmosphere

It is widely known that glacial ice contains a record of Earth’s changing atmospheric composition in the form of bubbles trapped when the ice formed. That is fine for investigations going back about a million years, in particular those that deal with past climate change. Obviously going back to the composition of air tens or hundreds of million years ago cannot use such a handy, direct source of data, but has relied on a range of indirect proxies. These include the number of pores or stomata on fossil plant leaves for CO2, variations in sulfur isotopes for oxygen content and so on. Variation over time of the atmosphere’s content of oxygen has vexed geoscientists a great deal, partly because it has probably been tied to biological evolution: forming by some kind of oxygenic photosynthesis and being essential for the rise to dominance of eukaryotic animals such as ourselves. Its presence or absence also has had a large bearing on weathering and the associated dissolution or precipitation of a variety of elements, predominantly iron. Despite progressively more clever proxies to indicate the presence of oxygen, and intricate geochemical theory through which its former concentration can be modelled, the lack of an opportunity to calibrate any of the models has been a source of deep frustration and acrimony among researchers.

Yet as is often said, there are more ways of getting rid of cats than drowning them in butter. The search has been on for materials that trap air in much the same way as does ice, and one popular, if elusive target has been the bubbles in crystals of evaporite minerals. The trouble is that most halite deposits formed by precipitation of NaCl from highly concentrated brines in evaporating lakes or restricted marine inlets. As a result the bubbles contain liquids that do a grand job of preserving aqueous geochemistry but leave a lot of doubt as regards the provenance of gases trapped within them. For that to be a sample of air rather than gases once dissolved in trapped liquid, the salt needs to have crystallized above the water surface. That may be possible if salt forms from brines so dense that crystals are able to float, or perhaps where minerals such as gypsum form as soil moisture is drawn upwards by capillary action to form ‘desert roses’. A multinational team, led by Nigel Blamey of Brock University in Canada, has published results from Neoproterozoic halite whose chevron-like crystals suggest subaerial formation (Blamey, N.J.F. and 7 others, 2016. Paradigm shift in determining Neoproterozoic atmospheric oxygen. Geology, v. 44, p. 651-654). Multiple analyses of five halite samples from an ~815 Ma-old horizon in a drill core from the Neoproterozoic Canning Basin of Western Australia contained about 11% by volume of oxygen, compared with 25% from Cretaceous salt from China, 20% of late-Miocene age from Italy, and 19 to 22% from samples modern salt of the same type.

Salar de Atacama salt flat in the Chilean puna

Evaporite salts in the Salar de Atacama Chile (credit: Wikipedia)

Although the Neoproterozoic result is only about half that present in modern air, it contradicts results that stem from proxy approaches, which suggest a significant rise in atmospheric oxygenation from 2 to about 18% during the younger Cryogenian and Ediacaran Periods of the Neoproterozoic, when marine animal life made explosive developments at the time of repeated Snowball Earth events. Whether or not this approach can be extended back to the Great Oxygenation Event at around 2.3 Ga ago and before depends on finding evaporite minerals that fit stringent criteria for having formed at the surface: older deposits are known even from the Archaean.

Stepping Stones relaunch on-line

In 2000 I was approached by Ian Francis, then a commissioning editor at Blackwell Science if I would like to write a series of news items on advances in Earth Science for the publishers’ new website Earth-Pages. The invitation stemmed from his having read my recently published book Stepping Stones: The Making of Our Home World, which threaded a similar path through developments in the science that I helped to teach through the Open University. Ian’s initiative led to my learning a great deal by sifting through leading scientific journals, which became a weekly discipline. Much of what I commented on covered the eclectic spread of Stepping Stones, but I did not think of authoring a revised edition of the book until just a few years before I retired from the Open University in 2011. As they say; ‘what with one thing or another’ it took me another 7 or 8 years to galvanise myself for such a task. If you would like to have a look at the revised edition, it is now on-line at https://earthstep.wordpress.com/.

The famous 3.6 Ma old hominin footsteps at Laetoli in Tanzania – Stepping Stones emblematic image. (Credit: Mary Leakey)

The famous 3.6 Ma old hominin footsteps at Laetoli in Tanzania – Stepping Stones emblematic image. (Credit: Mary Leakey)

Deciding to produce it in electronic form it occurred to me to make it a possible means of geoscience self-teaching by various devices, such as suggesting key words and phrases to find more in-depth material through a web browser and, equally important, to find useful images. Fifteen years of working on over 800 posts for Earth-Pages and the publications that they were about made revising Stepping Stones a quicker task than I had anticipated. Then it dawned on me that I had written a lot more on various topics for Earth-Pages than I had in the new project. So the Earth-Pages archive is a possibly valuable learning resource, if you can navigate through it, which is not always easy. Being the source for most of the new additions to the book’s Further Reading in, inserting links from each reference to the appropriate post in the Earth-Pages archive was easy.

Oh, and another thing, so few published science authors gain satisfaction from royalties, I decided Stepping Stones v.2.0 should be free!