“Everyone knows” that free atmospheric oxygen appeared about 2300 million years ago, thanks to the waste products of blue-green bacterial photosynthesis. At least the land surface became an oxidising environment and a progressively redder place, as Fe-2 was oxidised to Fe-3 which forms insoluble oxides and hydroxides. Paradoxically, the shallow sea floor of earlier times was redder than anything since, because of exactly the same oxygen-containing, ferric minerals. It hosted the largest build-up of any metal concentration in Earth’s history; the banded iron formations (BIFs) that have for a century or more been the source of industrial iron. A simple, and probably accurate explanation for BIFs is that iron dissolved in ocean water that lacked oxygen as Fe-2, and was supplied by sea-floor volcanism. Once blue-green bacteria began pumping out oxygen, an oxidising reaction dumped both elements as slimy red sediment where the two met. Dissolved iron consumed oxygen – just as well, because to most prokaryote life it is a poison – yet as oxygen productivity rose (and perhaps sea-floor spreading slowed) dissolved iron was increasingly removed by oxyidation from sea water. The tipping point, when air contained oxygen and sea water became starved of iron (a vital micronutrient for phytoplankton) is difficult to address since the two chemical environments are so different and interact in complicated ways. BIFs continued to form for about half a billion years after the first sign of atmospheric oxygen, then they disappear from the geological record at 1800 Ma ago. There were minor reappearences in the Neoproterozoic, at the time of “Snowball Earth” events, and that is a fascinating topic in its own right. Clearly, there was a long period of transition to what we can regard as a thoroughly modern world. Studies that use sulphur isotopes suggest that in the Mesoproterozoic the upper ocean was oxygenated while bottom waters were perpetually akin to those of the Black Sea today. Conditions in them may have been highly conducive to burial of dead organic matter – rapid drawdown of atmospheric CO2, but allowing the massive production of methane by anaerobic bacteria. Methane is a far more potent greenhouse gas than carbon dioxide, so controls over climate may have been very different from today’s. Molybdenum offers an independent and potentially useful means of testing hypotheses about ocean chemistry. It enters the sea in river water, which in post 2300 Ma times would have been oxygenated, allowing the formation of the soluble and very stable molybdate ion. In anoxic ocean floor conditions, bacteria that generate hydrogen sulphide remove molybdenum as the sulphide, which is why modern Mo concentrations remain stable – it ends up in a very small percentage of ocean floor sediments. The stable isotopes of molybdenum (97Mo and 95Mo) fractionate during precipitation of the element, the heavier one being preferentially removed during sulphide precipitation, to give high 97Mo/95Mo ratios in sediments. The opposite seems to occur if precipitation is in the oxide form, as in sea-floor manganese nodules. Geochemists from the Universities of Rochester and Missouri, USA have compared Mo isotopes from apparently anoxic Mesoproterozoic sediments with those in modern euxinic basins (Arnold, G.L. et al. 2004. Molybdenum isotope evidence for widespread anoxia in mid-Proterozoic oceans. Science v. 304, p. 87-90). The Precambrian results are isotopically much lighter than modern ones, suggesting that 97Mo did not become enriched in seawater as a result of oxide precipitation in the equivalent of modern manganese nodules. They estimate that 10 times more of the ocean floor was anoxic than today or since about 1300 Ma ago. So far no comparable work has been done of the extremely abundant black shales and schists of the Neoproterozoic, that link with “Snowball Earth” events. Whether or not “modern” redox conditions emerged 1300 Ma ago, with probably a big impact on climate controls, the oddest time climatically was between about 750 and 600 Ma ago. Not only were there several dramatic coolings and warmings, but the main indicator of organic carbon burial, d13C, went haywire. As did the BIFs, did ocean anoxic conditions once more get footholds. Molybdenum isotope data seem likely to shed some light on those strange times.
Magnetic polarity reversals
The Earth’s magnetic field is changing all the time, in its intensity, direction and, now and again, its polarity. It’s the last that proved the key to sea-floor spreading and plate tectonics, though ocean-floor magnetic “stripes”, and which has become a key stratigraphic tool for correlation and approximate dating. Along with palaeomagnetic pole determinations, that are vital to continental reconstructions, the whole field still remains largely empirical. Although widely agreed to be connected to changes in motions in the core, exactly what happens during reversals of geomagnetic polarity remains enigmatic, despite 40 years having passed since they were first recognised. There is no doubt that they are quick events, but to judge their pace and what happens to field strength and direction during a “flip” requires high quality data that is well-calibrated to time. Most early work focussed on magnetisation in igneous rocks, where the signal is strong. Minerals such as igneous magnetite acquire a permanent magnetisation once they cool below their Curie temperature, but since accurate radiometric dating gives an age, not a range of ages, it might seem that all that is possible with lavas and intrusions is to obtain a series of points. Fine for a time series, but useless for the details of reversals. However, by modelling the cooling history of an igneous body, it is possible to calibrate different levels within it to time. With careful choice, it has proved possible to find flows in flood basalt sequences that include the brief progress of a reversal. The results seem very odd, the pole itself seeming to migrate rather than jump from north to south, and gross changes in intensity over a short time. Improved instrumentation allows a shift from strongly magnetic basalts, to sediments that preserve much weaker signals. These are due to the alignment with the field of magnetic grains as they slowly settle. Marine sediment cores can now be magnetically characterised – the principle behind magneto-stratigraphy. For geomagnetists the most recent reversals have proved especially instructive, when the sedimentary record is analysed (Clement, B.M. 2004. Dependence of the duration of geomagnetic polarity reversals on site latitude. Nature, v. 428, p. 637-640). On average, the last four “flips” took about 7000 years to complete by migration of the magnetic poles. Yet there is an oddity in the detail. Sites at low latitude show significantly shorter periods (down to 2000 years) than those at high latitude (as much as 10000 years). Clement’s explanation for the difference is the persistence of the lower intensity non-dipole field, which might suggest different core processes or a single process with several components that evolve at different rates.
Sulphur cycling and sea-level change
Sulphur is one the major prerequisites for life after carbon, hydrogen, oxygen and nitrogen, and the bulk of it is supplied by sulphate ions. After chlorine, the SO42- ion is the most abundant anion in the oceans. Not very much is added annually by river drainage, and although anaerobic bacteria remove some by reducing it to hydrogen sulphide so that it is removed from solution as a result of precipitation of insoluble iron sulphide, the sulphur cycle has been considered to be the most sluggish of all the major geochemical rhythms at the Earth’s surface. Because iron sulphide is highly reactive in oxidising conditions, should marine sulphide-rich sediments become exposed at the surface their oxidation to sulphuric acid and iron hydroxide would rapidly add sulphate ions to seawater. Studies of sulphur isotopes seem to suggest that this is not very important however. Through sulphate-sulphide reducing bacteria, sulphur is implicated in the carbon cycle because of its sheer abundance, not so much from the encouragement and burial of the bacteria, but because they induce the highly reducing conditions that help a larger proportion of dead organic matter to remain unoxidised and become buried. In a roundabout way, sulphur has a role in climate controls. In fact, two roles. Sulphate ions affect the alkalinity of seawater, and on that depends the oceans’ ability to dissolve CO2 from the atmosphere. The big question is, “Does the sulphate content of seawater ever change fast enough to have some impact on climate in the short term?”. Most studies of the S-cycle have focused on sulphur isotopes, so a new twist is bound to be interesting. Alexandra Turchyn and Daniel Schrag of Harvard University looked instead at the isotopes of oxygen within barium sulphate contained within seafloor sediments since the Late Miocene (about 10 Ma ago) (Turchyn, A.V. & Schrag, D.P. 2004. Oxygen isotope constraints on the sulfur cycle over the past 10 million years. Science, v. 303, p. 2004-2007). Up until 6 Ma, the barite d18O (measured against mean ocean water values) stayed constant at about 9.5‰, and then rose to around 12.5‰ by 3.5 Ma. Through the Late Pliocene and Pleistocene, the period of repeated glacial-interglacial cycles, it fell dramatically to its present level of 7.9‰. In that later period, the average d16O of deep water foraminifera rose significantly. The decline in “heavy” oxygen in marine sulphates can be linked to increased exposure of pyrite-bearing marine sediments during glacial sea-level falls when “light” atmospheric oxygen enters the sulphate ions that are produced. Modelling suggests sulphate ions in seawater increased by as much as 20% during the Great Ice Age. Whether that had an influence on the oceans’ take-up of carbon dioxide from the atmosphere in the last 3 Ma is yet to be evaluated. However, Turchyn and Schrag’s detection of a short term shift in the sulphur cycle, and attributing it to falling sea level, may allow a new approach to global sea-level change, which has mainly been deduced from features in stratigraphy.
See also: Derry, L.A. & Murray, R.W. 2004. Continental margins and the sulfur cycle. Science, v. 303, p. 1981-1982
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Impacts’ effects
Algorithms that model the physical effects of extraterrestrial impacts from the Lunar and Planetary Laboratory of the University of Arizona, headed by Jay Melosh, have been assembled into a handy on-line calculator, with notes on the processes involved. If you want to find out if you will be fried, buried or blown to smithereens (probably all three if our luck is really out), and the chances of being harmed by alien lumps of rock or ice, you can find the calculator at http://www.lpl.arizona.edu/impacteffects/ . It is not recommended for estate agents, because, unlike many other disastrous events, impacts can be anticipated anywhere.
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Early humans of Beijing
One of the most remarkable achievements of early humans (Homo ergaster aka H. erectus) was not their tools, but their migration out of Africa around 1.8 Ma, to reach as far as Indonesia and China. There is no evidence for that feat having occurred again until fully modern humans arrived in east Asia about 70 ka ago. The toolkit of Asian “Action Man” is unimpressive, in the sense that it resembles the slightly reshaped broken pebbles of the Oldowan culture, that first appears in the African archaeological record about 2.4 Ma ago. Development in Africa of the enigmatic and beautiful bi-face or Acheulean axe was after the first Asians had departed, around 1.5 Ma. So what were these early wanderers like; what did they want? The decade-long work in China by Noel Boaz, an anatomist from the Ross School of Medicine in New Jersey and anthropologist Russell Ciochon of the University of Iowa will soon appear in their book Dragon Bone Hill, an Ice-Age Saga of Homo Erectus (Oxford University Press), which they preview in the 17 April 2004 issue of New Scientist (p. 32-35). Boaz and Ciochon have worked mainly in Zhoukoudian near Beijing, a major resource for human remains whose different levels extend back to about 800 thousand years. Another site in China, Longouppo, contains disputed remains as old as 1.8 Ma, as are Dubois’ famous discoveries of the type specimens of H. erectus by the Solo River in Java. From the time when Zhoukoudian became famous among Chinese apothecaries as a source of “dragon’s bones” (a mixture of human and other animal remains) there has always been an air of myth about the findings there – a permanent dwelling for hundreds of thousand years, protected from glacial temperature falls by the consistent use of fire. In essence, the publicised view is that “Peking Man” led a cosy hearthside existence for a very long time indeed. Boaz and Ciochon tell a different, and more mundane story. Most bones in the deposit are those of a great variety of other animals, with disproportionately few of human origin, and those are highly fragmented. The dominant species is a giant hyena, and many of the bones, including humans, are well gnawed, which is what hyenas do especially well. There are occasional signs of human occupation and use of fire. The human remains are encased in layered carbonate flowstone,. Records of fluctuating d18O from that matrix, matched against the global time series of climate change, show that occupation was only during interglacials – the site was abandoned or unvisited during the depth of glacial periods. Some animal bones show cut marks made by stone tools, and it is more likely that H. erectus raided to get remnants of other beasts’ kills, perhaps using fire, rather than being top of the predatory order. The great surprise throughout Asia is the complete lack of development of stone tools from the primitive culture that arrived there, until as late as 20 to 30 thousand years ago, when Asian H. erectus vanished. Apart from the stunning breakthrough to the bi-face axe, African erects also had a million-year long cultural stasis – resting on laurels with a vengeance. Finally, from a number of skulls at Zhoukodian, Boaz and Ciochon have shown signs of trauma. These are depression fractures, probably not necessarily fatal, but indicate sharp blows to the head with blunt instruments. Their interpretation is that the Chinese erects settled disputes by bashing heads; so that aspect of culture has not changed a lot since. Their story is not “politically correct”, but with publication of their book, other palaeoanthropologists can judge it on the basis of the evidence from Dragon Bone Hill.
Faster development of Neanderthals
Go to any horse sale and you will see bidders closely studying the teeth of their prospective purchases; the origin of the saying, “Never look a gift horse in the mouth”. Teeth show growth ridges, and in grazing animals they are prominent, so that it is possible to judge the age of a horse easily and accurately. Human teeth are different only in the less obvious signs of growth. Microscopic examination reveals such records, down to the daily level, although the most prominent features are curious disturbances in their deposition that form approximately weekly. They appear as ridges on the crowns of teeth. The variable spacing of these perikymata provides a record of the pace at which adult teeth develop. In modern humans the spacing becomes very much closer in the later growth history (towards the tooth’s cutting edge) than in its early stages, and reflects the slow development to full adult dentition. In a painstaking study of hundreds of teeth from Cro Magnon and Neanderthal teeth, Fernando Rozzi of the University of Paris and José Bermudez de Castro of the Spanish National Museum of Natural Sciences have discovered an odd difference in the development rates of Neanderthals (Rozzi, F.V.R & Bermudez de Castro, J.M. 2004. Surprisingly rapid growth in Neanderthals. Nature, v. 428, p. 936-939). The late perikymata of Neanderthals are more widely spaced than in Cro Magnon and modern humans, strongly suggesting that Neanderthals developed to adulthood by about the age of 15, three to five years earlier than us and our immediate ancestors. As well as confirming that they are a separate species, the results suggest that Neanderthals, while acquiring brains as large, and in some cases even larger than ours, had evolved more rapid maturation and probably a genetically determined shorter adult life. This would have had some effect on transfer of culture, which in human societies is often the most important value of elderly folk. The fewer samples of teeth of earlier human species (H. heidelbergensis and H. antecessor) reveal an even greater surprise. They are more like modern human teeth (albeit with signs of somewhat faster growth), which suggests that evolution of the Neanderthals involved a regression. The authors suggest that the combination of a backward step to faster development with rapid brain growth to large size might reflect a very-high calorie diet together with adverse environmental conditions.
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River incision and anticlines
In many areas of active deformation, landforms that suggest that uplift and river down-cutting keep pace are very common. Stream courses cross zones of uplift, rather than being diverted or ponded up to form lakes. Traditionally, geomorphologists have described such drainages as “antecedent”, i.e. rivers that were present before uplift began. They can be seen on all scales up to examples such as the Indus and Brahmaputra rivers that carve their way across the actively rising Himalaya. The most common are anticlines through which streams flow in canyons perpendicular to the fold axes. A curious and common feature is that the canyons are not haphazard, but often cut the fold where its amplitude is greatest and its axis plunges away from the site of incision. The stupendous rates at which crustal rocks are eroded and transported away in the courses of the Indus and Brahmaputra, and in lesser drainages on the flanks of major extensional orogens, such as the Red Sea, clearly removes load from the crust. Consequently there is an isostatic component to the uplift involved in the two cases at a grand scale. Peter Molnar and Phillip England suggested an erosional role in large-scale uplift over a decade ago. Intervening ridges rise higher than they would if erosion was slower or non-existent. In major rift systems, the highest peaks are often within the escarpments rather than at the lip of uplift, sometimes more than 500 m higher. Bearing this well-known process in mind, Guy Simpson of ETH Zurich, has sought evidence that it functions on much smaller scales (Simpson, G. 2004. Role of river incision in enhancing deformation. Geology, v. 32, p. 341-344). That comes from the surprising symmetry of doubly plunging anticlines that are cut by rivers at their highest point. His modelling suggests that the phenomenon can occur when the crust deforms plastically, allowing isostatic response to erosion on even minor scales during compression. When deformation is by brittle means, any uplift of rigid crust is flexural and has long wavelengths, so that rivers bear no relation to local structures
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Water on Mars; almost official
Two lines of evidence from the current robotic explorations of Mars add to less tenuous ones that the planet is really wet – icy to be precise. One is mineralogical. Spectroscopy of the surface being slowly trundled across by a NASA rover, shows abundant signs of the hydrated, iron-potassium sulphate jarosite, which probably can only form under wet conditions. When it was precipitated is not known with certainty, but it occurs in layered sediments that contain structures that clearly point to transport in and deposition from surface water. The time when liquid water could exist at the surface probably goes back to the earliest events on Mars, tied to the famous canyons and more recently discovered dendritic drainage patterns. The other evidence stems from even more remote sensing, that captures short-wavelength infrared radiation emitted by the Sun and reflected from the Martian surface. Ices of water and carbon dioxide have distinct and unique reflected spectra, because of the different ways in which they absorb a small proportion of solar radiation. Results from the OMEGA instrument aboard the European Space Agency’s Mars Express satellite show that the south polar region contains as much as 15% water ice mixed with solid CO2 (Bibring, J-P et al. 2004. Perennial water ice identified in the south polar cap of Mars. Nature, v. 428, p. 627-630).
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Devonian broad-shouldered fish
How, when and under what circumstances vertebrates got limbs to take them charging across the forested land of the late Palaeozoic form a central issue in our own evolution, as well as that of the other four-footed land animals. By negative analogy with the functional though rather rudimentary enlarged fins of various modern fish that flop from pond to pond during dry seasons, many vertebrate palaeontologists have considered limbs as evolutionary adaptations in air-breathing fish once they made this a habit. As so often, the fossil record has not given up enough evidence for that to be certain. Well, an upper foreleg bone (humerus) has turned up in Late Devonian rocks from Pennsylvania at a time and in a context that strongly suggests it was carried by a fish (Shubin, N.H. et al. 2004. The early evolution of the tetrapod humerus. Science, v. 304, p. 90-93). While not able to ride a bicycle, the advanced fish probably used what became limbs to hold itself motionless while lying in ambush for its prey. That would provide a plausible point of departure from which walking might develop.
Early biomarkers in South African pillow lavas
It is now established that various kinds of bacteria infest rocks down to depths of 2 km or more, one particularly favourable habitat being in sea-floor basalts though which hydrothermal fluids travel. Although the majority probably inhabits cracks and joints, some seem to work actively to corrode rock, especially volcanic glass, thereby obtaining mineral nutrients. Signs of this microbial corrosion in modern volcanic glasses are radiating tubes on a scale of a few micrometres, that show up in micrographs, and many may have been overlooked by petrographers in all kinds of rock. That they are definitely formed by organic activity is demonstrated by the presence of nucleic acids, carbon and nitrogen in the tubules. Carbon isotopes from them show the strong depletion in 13C that is the hallmark of organic fractionation of natural carbon. A team of geoscientists, from Norway, Canada and the USA, who have steadily accumulated evidence for biological rotting in modern oceanic basalts, turned their focus to the oldest, well- preserved pillow lavas in the 3.5 billion-year old Barberton greenstone belt of north-eastern South Africa (Furnes, H. et al. 2004. Early life recorded in Archean pillow lavas. Science, v. 304, p. 578-581). Virtually identical microtubules seem common in them too, particularly in hydrated glasses that are now tinged with the low-grade metamorphic mineral chlorite. Indeed, chlorite seems to have grown preferentially from clusters of the holes, which suggests that they formed before metamorphism of the basalts. Micro-geochemical studies confirm the presence of hydrocarbons with low d13C. The bulk of the tubules occur in the inter-pillow debris, that probably formed as glassy rinds as magma protruded on the Archaean sea floor. As well as adding to evidence for ancient terrestrial life, the find has inevitably opened up the search for such signs in meteorites reckoned to have come from Mars. In two, olivine grains show similar structures, although why the olivine hadn’t broken down in the presence of water that is essential for life makes such observations worth taking with a pinch of salt. A number of studies have stymied claims for early bacterial fossils (see Artificial Archaean “fossils” and Doubt cast on earliest bacterial fossils, April 2002 and December 2003 issues of EPN) and inorganic processes conceivably might create structures that can be mistaken for ones formed by biological action. The Fischer- Tropsch process is capable of producing hydrocarbons, and produces depletion in 13C abiogenically. In the on-line April edition of Science Express (www.sciencexpress.org) experiments are reported that highlight the possible influence of chromium-bearing mineral catalysts in hydrothermal generation of hydrocarbons from inorganic carbon dioxide(Foustoukos, D.I. & Seyfried, W.E. 2004. Hydrocarbons in hydrothermal vent fluids: the role of chrome-bearing catalysts. Science Express, April 2004). The Barberton greenstone belt is well known for ultramafic lavas rich in chromium, as are most early volcanic sequences.
See also: Kerr, R.A. 2004. New biomarker proposed for earliest life on Earth. Science, v. 304, p. 503.
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Australian surface not so old
One of the most widely quoted bits of geological information that appear in non-specialist literature is that the oldest land surface on Earth is that of interior Australia. Vast tracts are Precambrian capped by horizontal Permian glaciogenic rocks in places, but for the most part by relics of lateritic palaeosols that give it is famous red appearance. The oldest outlying platform sediments are 1100 Ma old, so the actual surface does date back at least as far, but has it been exposed at the surface for that long? Dating the present surface has not been easy. New methods involving the creation of unstable isotopes by cosmic-ray bombardment offer a solution (see Measuring erosion rates, February 2002 issue of EPN), combined with apatite fission-track dating (Belton, D.X. et al. 2004. Quantitative resolution of the debate over antiquity of the central Australian landscape: implications for the tectonic and geomorphic stability of cratonic interiors. Earth and Planetary Science Letters, v. 219, p. 21-34). The results suggest that Australian landscape antiquity is a myth. Erosion rates since the Cambrian varied over most of the Red Centre from 0.4 to 4.0 metres per million years, and reached as high as 17 m per Ma on occasion. They suggest a common or garden history, comparable with those of most continental interiors. Again and again it has been buried by sediments, albeit on a flat surface, and equally it has been exhumed several times by erosion. Only at the outset of the Cenozoic did much of it sit unchanged for long, which enabled its red surface to develop. The present surface is covered with what is termed regolith by Australians, but much of that is reworked material from the Palaeocene laterites that sits in a network of shallow drainage systems, including huge ephemeral lakes. It might seem that recourse to Hutton’s “the present is the key to the past” should long ago have staved off the myth of the gnarled old place of which Australians have become inordinately proud.
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Weak jaws allow bigger brains
There is no topic in the geosciences that is more interdisciplinary than that of human origins. Geologists, anthropologists (social as well as physical), archaeologists, geochemists, linguists, geneticists, dentists, specialists in nutrition and even novelists (for example Jean M. Auel) contribute. Everyone is interested, and so everyone not only wants to have a say, but somehow to be involved. Again and again in the pages, it becomes clear that bones and artefacts can no longer make major breaks through. The Out of Africa hypothesis, although suggested by Charles Darwin and many palaeoanthropologists since, became widely accepted (though not completely) after the evidence for relatedness emerged from comparisons of mitochondrial DNA from women throughout the world. That showed clear signs of a last common ancestor for all human groups around 200 thousand years ago, to whom modern Africans were most related. At the end of March 2004 geneticists have again come up with something startling, but this time not guessed at before.
The first beings to whom the generic name Homo seems appropriate appear in the hominid fossil record about 2.0 million years ago. Apart from evidence for bipedality and their association with rudimentary, but nonetheless deliberately made stone tools, the earliest humans are marked by the fragility and roundness of their skulls. Many specialists have argued that “gracile” crania are an evolutionary pre-requisite for the growth of brain capacity – they can expand for a long period during development, before becoming completely ossified in adulthood. The predecessors of these early humans (australopithecines) and their close companions in the African savannahs (paranthropoids) had smaller brain capacity and also very bony heads. In the case of the paranthropoids, undoubtedly as closely related to earlier hominids as the first tool-making humans were, they survived as a group for another million years but never expanded their brains, nor presumably their intellects. Bone-headed hominids had one feature in common with all earlier apes, and with the genera that survive today; powerful jaws and muscles that drive them. To some degree or other they all have crests on top of their skulls, which provide the seats for these big jaw muscles. Wielding awesome biting power requires skull strength, and therefore bulky bone. That encumbers any possibility for expansion of the internal brain cavity, and also drives their bearing species into tight feeding habits.
A team of geneticists, anatomists, developmental biologists and plastic surgeons from the University of Pennsylvania and the Children’s’ Hospital of Philadelphia have studied one gene sequence of several that encode for a type of protein (myosin heavy chain) associated with the powerhouse muscles that are attached directly to bone, such as those which drive jaws (Stedman, H.H. and 9 others 2004. Myosin gene mutation correlates with anatomical changes in the human lineage. Nature, v. 428, p. 415-418). Their investigation began with an interest in muscular dystrophy and possible underlying factors. Specifically, the most interesting gene (MYH16) is expressed in primate jaw muscles. The human gene contains a mutation that prevents the accumulation of the protein in our jaw muscles, so they cannot be as strong as those of other primates and mammals in general, in which the gene functions as it should. By analysing MYH16 and related gene sequences in humans from widely separated populations, the researchers showed that the mutation in MYH16 diverged earlier than those in other MYH-related genes. To estimate the time of that divergence involved detailed analysis of the mutations in other living species – dogs, macaque monkeys, oran-utans and chimpanzees. This showed that MYH16 evolved under Darwinian selection, conferring fitness advantage, in the ancestral lineages leading to each species, whereas in humans there was no selective constraint. Under the second condition, it can be assumed that any evolutionarily neutral changes took place at a constant rate. Calculations suggest that in the human lineage, the mutation appeared 2.4±0.3 Ma ago. That coincides with the earliest appearance of tools and a little earlier than the first remains of early Homo fossils. The conclusion could be one of several: lost of biting power created conditions for expansion of a lighter skull; a changed diet to include more meat reduced the need for strong jaws, so that the mutation did not have a deleterious effect; or hands freed by walking upright did a lot of the work that other primates can only accomplish with their mouths. Whichever, once established without decreasing fitness, the road to enlarged brains and fuller consciousness was opened by a chance event.
See also: Ananthaswami, A. 2004. less bite, more brain. New Scientist, 27 March 2004, p. 7; Currie, P. 2004. Muscling in on hominid evolution. Nature, v. 428, p. 373-374
Dental records of earliest hominids
Conditions on land are not as conducive to preservation of fossil remains as those on the sea floor. When an animal dies it is generally eaten, what is left rots and is gnawed, and the action of wind and water breaks up the skeleton and transports it, and only this debris is preserved if it is buried by sediment. The best chance of preservation is if the animal falls in a lake or bog, or in the case of fully modern humans if it is deliberately buried. The so-called Turkana Boy (H. erectus) is an almost complete skeleton, because he did end up, uneaten, in a swamp. Sturdy, large animals and those small and light enough to be quickly washed to burial stand the best chance of appearing as complete fossils. Primates are medium-sized and lightweight, and that presents palaeoanthropologists with their single biggest problem, incompleteness of most fossils that they find. In the depths of the Afar Depression of Ethiopia and Eritrea, which is the most productive area for hominid specialists, conditions from the early Miocene were not the best for preservation. While the depression developed by extensional tectonics, its flanks rose to form the mighty Ethiopian escarpment from which torrents flowed seasonally. High-energy streams clearly will break up any articulated skeleton and batter what is left before they end up in gravels and sands on the floor of the depression. So it is a credit to the patience, experience and sheer visual acuity of those who work there that they can piece together the earliest parts of the human story. Yohannes Haile-Selassie, Gen Suwa and Tim White have pushed back and detailed our record further than any other group, thanks in part to the richness of the Miocene to Recent Middle Awash sedimentary and volcanic sequence with which they work. In 2001 Haile-Selassie discovered the earliest Afar hominid so far (see Taking stock of hominid evolution, March 2002 issue of EPN), Ardepithecus ramidus kadabba dated between 5.2 and 5.8 Ma. In age it roughly matches Sahelanthropus and Orrorin from Chad and Kenya. Only a leg bone from Orrorin gives some indication that it was bipedal, but all show cranial features that mark them out as probable hominids. Of all the body parts of any animal, the teeth are the most likely to survive with little change. Because our closest living relative are chimps, comparing early teeth with theirs, as well as with those of later hominids, is about the best that can be done to seek relatedness. The three notable workers on Awash hominds have now reported their results (Haile-Selassie, J. et al. 2004. Late Miocene teeth from Middle Awash, Ethiopia, and early hominid dental evolution. Science, v. 303, p. 1503-1505), which suggest the earlier find is a distinct species A. kadabba. Putting together upper and lower canines and adjacent premolars shows a close resemblance to those of modern chimps. However, it requires detailed measurements of the tooth shapes to check if the resemblance is more than superficial, and it is not. All extinct and modern apes show signs of automatic honing of their canines, whereas hominids do not. Not only A. kadabba but Orrorin and Sahelanthropus too, show no sign of canine honing. That points to early members of human evolution. Yet, the three show such close similarity that it is hard to support the idea that they are from anatomically different genera, despite their occurrence thousands of kilometres apart. It is that close resemblance (and in other features as well) that re-opens the long debate between a complex, messy “bush” of human descent made up of many contemporary, different creatures, and one of a single line of descent. Dental features are not enough to decide between the two.
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New take on end-Palaeocene warming
Six years ago vast areas of Indonesia caught fire after an unusually dry phase in the El Niño – Southern Oscillation (ENSO). Burning forest and peat deposits swathed a vast area in smoke, but another alarming aspect was the greatest addition of carbon dioxide to the atmosphere in half a century. Such a wildfire on a global scale is thought to have marked the end of the Mesozoic, perhaps triggered by the K-T impact event and encouraged by higher oxygen content in the atmosphere. Present oxygen levels seem to be at a balance that staves off spontaneous combustion of green vegetation, but only a few percent more would render vegetation much more prone to bursting into flame. The end of the Palaeocene involved a sudden global warming that coincides with a decrease in the proportion of 13C in marine carbonates. Since photoynthesis, at the base of the trophic pyramid, favours light 12C, such a negative d13C “spike” is generally ascribed to an unusually high release of organic carbon to the environment. The end-Palaeocene warming may have resulted from a massive release of methane from gas-hydrate buried in shallow seafloor sediments (See Methane hydrate – more evidence for the ‘greenhouse’ time bomb and Plankton and the end of the Palaeocene-Eocene global warming August and October 2000 issues of EPN). However, massive burning of living biomass could also produce the carbon-isotope signal. Telling the two mechanisms apart requires information from other organic-related cycles. One key is comparing the carbon- and sulphur-isotopic records that enables the place in which carbon had been stored geologically. For marine burial, the effect of aerobic bacteria that completely oxidises hydrocarbons back to carbon dioxide and water needs to have been suppressed. Periods of massive marine carbon burial coincide with oceanic anoxia episodes, when anaerobic bacteria beneath the seafloor reduce dissolved sulphate ions to sulphides, thereby depositing lots of iron sulphide (pyrite) in black organic mudrocks. This sequesters sulphur that is depleted in 32S into marine sediments, so that the marine carbon- and sulphur-isotope records fluctuate in a clearly related way. During the Palaeocene this relationship is absent, while overall the carbon isotopes do signify progressive burial of organic carbon. The decoupling of the two cycles points to carbon burial on the continents, forming peat and eventually coal deposits.
Playing games on Snowball Earth
For as long as anyone can remember there has been a parade of geoscientific bandwagons in town. Three of the floats today carry banners saying, “Snowball Earth”, “Climate models” and “continental erosion and CO2 drawdown”. Of course there is serious science aboard each, but they are getting overcrowded, especially as separate bands try to jump from one to another. When it sometimes seems, as now, that the “next Big Thing” is some way off, we get the unseemly spectacle of some bands trying to straddle two or even several of the wagons. Three is quite a feat, yet the 18 March 2004 issue of Nature contains perhaps not a vast human pyramid, but at least a tetrahedron of the genre (Donnadieu, Y. et al. 2004. A “snowball Earth” climate triggered by continental break-up through change in runoff. Nature, v. 428, p. 303-306). From about 1100 to 750 Ma ago, the bulk of continental lithosphere was gathered in a supercontinent known as Rodinia (from the Russian for “Mother Earth”). By analogy with modern Eurasia, and the stratigraphic record from the Phanerozoic Pangaea supercontinent, the centre of Rodinia would almost certainly have been dry, being so far from the ocean. Break-up of that continental mass would also probably have allowed moist maritime air to penetrate over a larger proportion of the fragments. The hypothesis that Donnadieu and colleagues try to test using linked geochemical and climate models is that such a tectonic change would increase continental weathering and reduce the “greenhouse” effect. The weak acid formed by solution of carbon dioxide in rain water can provide hydrogen ions to break down silicate minerals. The reactions contribute bicarbonate and soluble metal ions to surface and subsurface water. Ultimately, both reach the oceans and contribute to its chemistry. If conditions are suitable, calcium ions in particular combine with bicarbonate to precipitate calcium carbonate on the ocean floor, either through the action of organisms or inorganically. The two chemical equilibria involved result in a net burial of one carbon atom out of the two involved in the weathering, thereby drawing down carbon dioxide from the atmosphere. The climate model used in their cyber-experiment resolves the Neoproterozoic Earth into cells that are 10 x 10 degrees (about 100 thousand km2) and considers Rodinia at 800 Ma and the result of its break-up at 750 Ma, the time of the first good evidence for extensive low-latitude glaciation. The results, after some tinkering, suggest that increased continental weathering could have reduced CO2 levels to 250 parts per million. Taking account of a 6% less energetic Sun at the time, this would have produced sufficient cooling for ice caps to exist to sea level at the equator. So, taken at face value, the hypothesis seems plausible. However, there are major snags. First, in a mere 50 million years their model sees continental dispersion on a scale that has not yet happened to Pangaea in about 200 Ma of Phanerozoic time. Second, since continental area remains constant, the proportion of rainfall, and therefore weathering and runoff, involving continental crust also stays fixed. Third, continental weathering refers to the crystalline part of its crust, in which there are unstable minerals, such as feldspars, that can do the chemical trick. We have little idea how much of the continents at that time was veneered by sediments that are the products of earlier chemical weathering, and contribute nothing to the process. Exposing such deep crust depends to a large extent on mountain building, which continental extension does not encourage. Fourth, carbon dioxide is not the only source of hydrogen ions that are involved in weathering, especially as much of it goes on in groundwater – bacterial action and oxidation of iron sulphides create much more acid conditions that rainwater. Fifth, and most important, where is the complementary geochemical evidence? Feldspars of the continental crust, on which the hypothesis mainly rests, have high contents of rubidium compared with their oceanic counterparts, and they are old. Much of Rodinia was underpinned by crust formed as far back as 4 billion years ago. Prolonged decay of 87Rb to radiogenic 87Sr makes the strontium isotopes of continental material very different from those of the ocean floor – it has a much higher 87Sr/86Sr ratio. Since soluble strontium would be released to runoff by continental weathering, that signature makes its way to the ocean and should pop up in marine carbonates. Although the ocean strontium isotopes in the Neoproterozoic did rise a little, it did not peak until the very end. In fact, the details show that the periods around supposed “snowball” conditions involved downturns in radiogenic strontium supply to the oceans. Whatever the model suggests, all that it amounts to is the equivalent of a table-top train set
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