zooks777

Neanderthals: diet, gait and ornamentation

Published on March 3, 2011 by zooks7771 Comment

Anyone who has followed British TV series featuring the survival specialist Ray Mears will be well aware of the wealth of wild foods available from plants even in cold climes: Mears is famous for persuading his camera crews to try what he eats when ‘out bush’. Surviving gatherer-hunters, such as the native people of Australia, have encyclopaedic knowledge of what is edible and how to find plant victuals, and we can surmise that such skills date back to the earliest hominins. Neanderthals have been widely regarded as being exclusive meat eaters – the Innuit of Greenland can subsist on a meat- and fish-only diet, showing that it is a perfectly wholesome strategy – but new evidence reveals that they also ate a wide variety of vegetables, and cooked them. Neanderthals suffered from plaque (calculus) and that dental biofilm preserves traces of their diet (Henry, A. G. et al. 2010. Microfossils in calculus demonstrate consumption of plants and cooked foods in Neanderthal diets (Shanidar III, Iraq; Spy I and II, Belgium), Proceedings of the National Academy of Sciences, doi/10.1073/pnas.1016868108). Teeth from the famous Neanderthal sites of Shanidar in Iraq and Spy in Belgium had substantial plaque deposits. The authors found a wide variety of starch grains and silica-bearing hard parts that are characteristic of a wide range of plants (phytoliths) embedded in the plaques. Food plants included grasses, such as wild barley and sorghum; starchy roots, such as water lily; date palm, and a wide variety of starch grains and phytoliths that proved difficult to link to specific plants. Clearly, Neanderthals were not exclusively hunters of large and small game. The exclusively hunting hypothesis arose from analysis of fossilized fecal matter preserved with Neanderthal remains in occupation sites dating to the onset of frigid conditions in Europe, and in any case only shows what their producer’s last few meals contained. We can expect a closer look at teeth of other hominins from now on, as mineralized plaque is almost as indestructible as teeth themselves.

Neanderthals definitely did hunt, and evidence is that they were able regularly to bring down enormous beasts such as elephants and rhinoceroses. The question is, did they have to chase their prey animals so that they weakened through heat exhaustion before the kill, as in the case of the San hunters of SW Africa? To do that they would have had to be endurance runners. Comparing their ankle bones with those of modern humans suggests they were not very athletic in this way. (Raichlen, S.A. et al. 2011. Calcaneus length determines running economy: Implications for endurance running performance in modern humans and Neandertals. Journal of Human Evolution, v. 60, p. 299-308). Running well and keeping it up over long distances depends to a large extent on the efficiency of the Achilles tendon, the largest in the whole body: it literally puts a ‘spring in the step’ and couples muscle power to the role of feet in running. The calcaneus bone in the ankle provides leverage from the elastic storage of power in the Achilles, so its length is a guide to running efficiency. Neanderthals had a longer calcaneus than modern humans and probably had to spend considerably more muscular energy in keeping up with prey; they would have tired more quickly. The authors put this down to an evolutionary adaptation in cold climes to the lesser chance of prey animals succumbing to heat exhaustion. That would also perhaps explain evidence from other parts of Neanderthal skeletons for severe injuries, probably caused during hunting. They probably used ambush techniques and close-quarters stabbing with spears; a very risky strategy with unexhausted big game.

Interestingly, close on the heels of the Neanderthal Achilles tendon work a newly discovered foot bone of Australopithecus afarensis (Ward, C.V. et al. 2011. Complete fourth metatarsal and arches in the foot of Australopithecus afarensis. Science, v. 331, p. 750-753) shows that, like us, it had arches whereas modern apes do not. This seems to settle a lengthy debate about how australopithecines walked – they are long acknowledged to have been at least part bipedal. The 4th metatarsal is crucial: in apes its shape gives the flexibility needed to negotiate and grip branches, whilst in Homo sp. it endows the foot with the rigidity and stability to balance, absorb shock and use the toes efficiently in walking. This is pretty fundamental stuff en route to ‘proper’ humans, yet skull morphology dominates discussion of hominin anatomical relationships: the earliest tools (~3.4 Ma; see Another big surprise in EPN of September 2010) are a million years older than the earliest human, H. habilis. But they overlap in age with and occur in the same area as Australopithecus afarensis. So, should these beings actually be renamed H. afarensis?

Tantalising glimpses suggesting that Neanderthals were not brutes, such as possible shell jewellery, use of pigments and scattering of flowers at burials, has been accumulating for years. The latest has been unearthed from a cave in the north of Italy, in association with Levallois tools that are distinctive of Neanderthals (Peresani, M. et al. 2011. Late Neandertals and the intentional removal of feathers as evidenced from bird bone taphonomy at Fumane Cave 44 ky B.P., Italy. Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1016212108). Wing bones of vultures, eagles, owls, crows and various other birds show grooves and scratches suggesting that the long flight feathers had been carefully removed: there isn’t much meat on a wing. Since fletched arrows are believed not to have been invented until much later times, it seems pretty certain that the feathers were aimed at personal adornment, or even clothing. The evidence is very convincing and so helps confirm earlier suspicions of feather-use from wing bones found at a variety of Neanderthal sites. Some hollow bird bones are also suspected of having been used as whistles. Given the recent genetic evidence of their sexual interaction with anatomically modern humans, gradual build-up of signs of a rich cultural life make the Neanderthals significantly more attractive than the famous view of geneticist Steve Jones in 1994 that ‘If you met an unwashed Cro Magnon dressed in a business suit on the Underground, you would probably change seats. If you met a similarly garbed Neanderthal, you would undoubtedly change trains’.

Bigging-up the Ediacaran

Published on March 2, 2011 by zooks777Leave a comment

Cropped and digitally remastered version of an... — A distinctive Ediacaran fossil. Image via Wikipedia

The biota dominated by large, indistinct and generally flabby creatures named together with the eponymous Period (635-542 Ma) from their type occurrence in late Neoproterozoic sediments of the Ediacara Hills of South Australia is made up of imprints of a strange bunch of organisms – bags; discs; donut-shapes and the enigmatic quilted organisms that likely subsisted by osmotically drawing nutrients from ocean water through their skins – together with others that have forms suggestive of extant groups – cnidarians; bilaterian embryos; mollusc-like and segmented forms. The Avalon fauna of Newfoundland, discovered after those of Charnwood Forest, UK and the Ediacara Hills, added other life forms, including the fractal-like rangeomorphs from earlier (~579 Ma) times in the Ediacaran. Recently, the oldest known (630-551 Ma) members of the Ediacaran biota were presented (Yuan, X. et al. 2011. An early Ediacaran assemblage of macroscopic and morphologically differentiated eukaryotes. Nature, v. 470 , p. 390-393). Unlike the better known organisms that were preserved against all odds in quite coarse sand- and siltstones, the host rocks in South China are a more familiar lagerstätten of black shales in which fossils take the form of carbonaceous films. These preserve considerable detail and are unlike the later Ediacaran organisms. Many resemble marine algae (seaweeds), some very like kelp, in their living positions and probably represent quite sunlit seabed habitats(the authors also suggest that some rare forms may be bilaterian worms and cnidarians). Dating of this Lantian assemblage stems from several ash beds and correlation of C-isotope anomalies with other Ediacaran sections.

From their age, the Lantian fossils are of organisms that evolved shortly after the Marinoan (635 Ma) ‘Snowball Earth’ episode, whereas the faunas of Newfoundland and Australia followed the less prominent Gaskiers glacial epoch (582 Ma). So they represent another evolutionary surge presaged by global ice cover and massive stress for all terrestrial life. If the Lantian organisms were algae, then photosynthesising eukaryotes may have been the first large multicelled organisms. All eukaryotes – autotrophs and heterotrophs – are obliged to live in oxygenated conditions, so at least shallow water after the Marinoan glacial event must have been such, although preservation of the Lantian fossils does indicate anoxic conditions during burial. The association of evolutionary bursts with two ‘Snowball Earth’ periods ought to point palaeobiologists to the sedimentary sequences that followed the earliest such event, the Sturtian (~720 Ma), which shows similar violent swings in C-isotopes that indicate surges and declines in burial of organic matter. So far only sponge-like fossils have been found from the Cryogenian Period of the Neoproterozoic that encompasses the Sturtian and Marinoan glacial episodes (Maloof, C. & 8 others 2010. Possible animal-body fossils in pre-Marinoan limestones from South Australia. Nature Geoscience, v. 3, p. 653-659).

Oldest fossils of large seaweeds, possible animals tell story about oxygen in an ancient ocean (eurekalert.org)
Narbonne, G.M. 2011. When life got big. Nature, v. 470, p. 339-340

Feeding habits of ammonites

Published on March 1, 2011 by zooks777Leave a comment

Emerging in the Upper Palaeozoic and rapidly diversifying through the Mesozoic, thereby surviving over a period of 340 Ma, ammonites proved to be a stratigrapher’s dream organism as well as being the most widely collected fossils. As well as their rapid evolution of form, they were able to spread widely though the oceans in larval form, through the jet propulsion they shared with other cephalopods and because they floated when dead and drifted with currents. Much of ammonite taxonomy has centred for almost two centuries on their external for: ribs, keels, knobbles, intricacy of the sutures separating each body chamber and the previous one and whether or not their growing shell coils hid earlier parts or developed into an open spiral. These characteristics enables such a wealth of easily recognised genera and species that as zone fossils ammonites have been used to finely divide Mesozoic sediments; Jurassic ammonites locally divide the Jurassic (199 – 145 Ma) into time slices each of which represent a few hundred thousand years.

What is least familiar to non-specialists is the feeding apparatus of ammonites and what they actually ate. Thanks to the use of high energy X-ray images it turns out that, unlike squid, octopuses and the similar looking modern Nautilus, some Cretaceous ammonites would not have been able to rip apart large prey (Kruta, I et al. 2011. The role of ammonites in the Mesozoic marine food web revealed by jaw preservation. Science, v. 331, p. 70-72). Instead of a large beak-like process the ammonites studied sported a rasp-like radula, similar to that used on lettuce by the slug. The radula is armed with tiny but quite fearsome looking barbs, suitable for grating but not gnawing. The analysed ammonites may probably have eaten plankton. Indeed, one specimen turned out to have fragments of its last meal lodged in its radula; an isopod and a small gastropod. That diet tallies with the likely habitat of some ammonites; they were probably able to change their buoyancy by manipulating the gas and water content of their abandoned earlier body chambers to move up and down in the upper ocean. However, such was the stratigraphic duration, global spread and diversification of the ammonites, further studies of this kind would be needed to verify general plankton feeding. However, such a diet may well explain the conundrum of the total extinction of ammonites at the end of the Cretaceous while the superficially similar nautiloids survived and live today. The Cretaceous-Palaeocene (K-P formerly K-T) mass extinction devastated plankton, while larger marine organisms lived on to serve as nautiloids prey.

Ammonites’ last meal: New light on past marine food chains (eurekalert.org)

Gold, magma and groundwater in Nevada

Published on March 1, 2011 by zooks777Leave a comment

Twin Creeks gold mine, Nevada, USA; Carlin-sty... — Twin Creeks gold mine, Nevada, USA; Carlin-style mineralisation. Image via Wikipedia

With the price of gold having climbed to above $1400 oz^-1 while social revolutions develop in North Africa and the Middle East, articles on how gold deposits form will get a wider readership than they would have during its doldrum years in the late 20^th century – the price has increased 7-fold since 2000. The bulk of gold nowadays can be mined profitably from ores in which it cannot be seen, except using a microscope, at grades well below 1 g t^-1 (1 part per million) thanks to cheap heap-leaching with sodium cyanide of lightly milled ore. The epitome of low-grade gold is that produced at huge open-pit mines in Nevada from sedimentary host rocks. The gold is far too fine grained to have been deposited as placers, and also occurs dissolved in pyrite (Fe₂S), so most experts regard it as having been introduced by hydrothermal fluids. Yet that covers two possibilities: by deep penetration of groundwater or from magmatic waters, and it is hard to decide which, again because the mineralisation is too fine grained to allow conclusive studies of fluid inclusions and stable isotopes. Also, such evidence as there is suggests low temperature fluids (~200° C) with low salinity; ambiguous data.

By using a synergy of ore-mineral chemistry, experimental data and ages of magmatism and mineralisation, Nevadan geologists have developed a convincing model for these ‘Carlin-type’ deposits (Muntean, J.L. et al. 2011. Magmatic-hydrothermal origin of Nevada’s Carlin-type gold deposits. Nature Geoscience, v. 4, p. 122-127). First, the mineralisation is of Eocene age and was introduced in Lower Palaeozoic sediments. The Eocene in the western USA saw the end of a period of compressional tectonics related to subduction since the Jurassic, fluids from which gave rise to partial melting of the overlying mantle wedge. This was succeeded by extensional tectonics and further intrusive magmatism dated between 40 to 36 Ma. This provided thermal energy and passageways for fluid migration. The second line of evidence is that hydrogen- and oxygen isotopes from fluid inclusions in hydrothermal gangue minerals show evidence that both mantle-derived and meteoric water mixed in the ore-forming fluids, and sulfur isotopes are similarly evidence of dual origin. Thirdly, the authors reasonably postulate from experimental data that basaltic back-arc magmas of Jurassic to Eocene age may have repeatedly added metals, including gold, to the mantle wedge that underpinned Nevada during subduction over a 175 Ma period. Thus later extension-related magmatism sourced in the wedge would itself have become metal enriched from this ‘fertile’ source. Moreover, conditions would have been ripe for highly oxidised conditions in the magmas and high concentrations of water in their fractionated descendants. Under such conditions gold and other metals favour entry into hydrothermal fluids. Given the extensional tectonic conditions such fluids could rise efficiently. Initially highly saline and very hot, rapid rise of the fluids would eventually result in them cooling adiabatically and separating into a dense salty liquid or brine and remaining vapour. That would force down the chlorine content in the vapour, favour some metals (Fe, Ag, Pb, Zn and Mn) ending up in the brine, while others (Au, Cu, As and Sb together with S) would remain in the vapour phase together with dissolved CO₂ in large amounts, making the vapour acidic. Able to pass into the fractured Palaeozoic cover, the fluids widened fractures in the carbonate sediments and facilitated their own precipitation of minerals, the foremost being gold-bearing pyrite. Nevada is probably unique, but my goodness it is a big gold province; >6000 t of gold in tham thar hills.

A possible earthquake prediction tool?

Published on March 1, 2011 by zooks7771 Comment

Christchurch earthquake damage - 22 Feb 2011 — Damage in Christchurch, New Zealand. Image by Shazster via Flickr

Every time seismic disaster strikes, as it did in Christchurch New Zealand on 22 February 2011 to kill at least 160 people and destroying a third of the city’s buildings, people long for some means to be forewarned of pending earthquakes early enough to escape collapsing buildings. Many approaches have been suggested over the years, such as changing water levels in wells, increased emission of radon and even the behaviour of animals in advance of major events. Ideally, seismic early warning tools should be generally applicable, easily implemented and possible to telemeter immediately to local and national authorities. Probably the best place to seek such a method is in the field of seismology itself, and one candidate recently emerged (Bouchon, M. et al. 2011. Extended nucleation of the 1999 M_w 7.6 Izmit earthquake. Science, v. 331, p. 877-880). This examines foreshocks of the tragic events at around midnight 16 August 2009 in NW Turkey that ripped along 150 km of the North Anatolian Fault to kill around 17 000 people. The seismological records of the Izmit earthquake are not good quality, but Michel Bouchon and his French and Turkish colleagues, experts on the event, were able mine the ‘blurred’ data using new techniques. What they found was a sequence of 18 small earthquakes up to 45 minutes before the main one, each of which showed remarkably similar seismogram traces. From them they were able to show that most of the foreshocks arose from the same place on the fault and involved the same kind of deformation; by slippage in a patch or nucleus only about a few hundred metres wide at 15 km depth on the main fault. At each successive foreshock the rate of slip can be shown to have speeded up, and in the final 2 minutes before the main earthquake the localised acceleration was at its fastest. Also the low-frequency ‘rumble’ associated with each shock steadily got more powerful. These features define a similar shape for each seismogram record in the foreshock sequence.

The Izmit data tally well with a theoretical scenario for the initiation of movement along a fault. As tectonic stress builds up it begins to be dissipated by slow creep that can focus on a small part of the fault. Since this weakens that patch, subsequent creep is likely to favour the same place which becomes a nucleus for later events. If the stress loading is large enough to presage an eventual rip along a greater section of the fault such a major event will probably propagate sideways from the nucleus weakened by creep. Given sensitive seismometers suitably placed along threatening faults zones linked by telemetry to a central unit, as might seem sensible anyway, automated analysis of foreshock records with the signature of spatially restricted creep that begin to show an accelerating sequence might give the 5 to 10 minutes of warning that are the minimum to reduce fatalities in major earthquakes. However, analysis of better data from some other earthquakes does not reveal the same features, but it is early days and similar patterns may emerge from yet others: fault systems behave in a range of ways depending on their tectonic settings. The other issue is the cost of installations and facilities and their maintenance over long periods – how could somewhere like Haiti find the resources. And sadly, some earthquakes, like that beneath Christchurch occur on faults that show no sign at the surface.

New Zealand earthquake caused by new fault line (independent.co.uk)

Coal and the end-Permian mass extinction

Published on March 1, 2011 by zooks777Leave a comment

It is hardly contested these days that the massive Siberian Traps – the largest known continental flood basalt province – had something to do with the mass extinction at the Permian/Triassic boundary. Yet what actually produced sufficient, planet-wide environmental stress to slaughter up to 90% of all previously living species has not been pinned down. It was probably a combination of direct and indirect outcomes of the volcanic outpourings and several mechanisms have been suggested, such as: acid rain produced by SO₂ emissions from the magma; global warming as a result of volcanic CO₂having accumulated in the atmosphere; a marked fall in the oxygen content of the atmosphere (see New twist for end-Permian extinctions in the May 2005 issue of EPN); increased phosphate fertilization of the oceans leading to anoxia and release of hydrogen sulfide gas.

Interestingly, the part of Siberia where the basalt floods took place is rich in coal measures and carbon-rich shales. Their thermal metamorphism by an overlying pile of lavas could conceivably have added huge amounts of CO₂ and methane to the atmosphere, creating strong greenhouse conditions: gas release from combustion and baking would have been almost instantaneous as each major flow came into contact with carbonaceous sediments. Yet direct evidence of widespread carbon combustion at the P/Tr boundary has not yet been demonstrated, although there are abundant gas-release structures in Siberia of around that age (Svenson, H. et al. 2009. Siberian gas venting and the end-Permian environmental crisis. Earth and Planetary Science Letters, v. 277, p. 490-500).

From a study of a near-continuous section of deep water marine sediments, whose ages range from Late Carboniferous to Cretaceous, something surprising has emerged. Silica-rich shales that span the P/Tr boundary show a major shift in d¹³C that matches the C-isotopic signature of the boundary elsewhere, and two lesser anomalies before the boundary event. At each C-isotope anomaly the shales also contain fly ash (Grasby, S.E. et al. 2011. Catastrophic dispersion of coal fly ash into oceans during the latest Permian extinction. Nature Geoscience, v. 4, p. 104-107), which forms today only in the rapid high-temperature combustion of coal in thermal power stations. It does not form from natural fires in underground and exposed coal seams that are caused by spontaneous combustion, usually ignited by rapid oxidation of pyrite in coal. The ash particles are smaller than 50 µm, and like similar sized, but denser, volcanic ash could easily be carried large distances. The Canadian team suggests that the fly ash formed when Siberian Trap basalts burned coals and organic-rich sediments, explosive release or explosive phases of the volcanism injecting them high in the atmosphere. Coal fly ash is not identifiable by normal microscopy, and its absence from the geological record may reflect that fact. Using organic petrography routinely on rocks from occurrences of the P/Tr and other boundary sequences should settle the matter.

Coal-fired trigger of mass extinction (nature.com)

The post-glacial North Atlantic

Published on February 28, 2011 by zooks777Leave a comment

One of the main controls over Earth’s climate is the way that water in the North Atlantic convects. At present it is behaving like a liquid conveyor belt that links the tropics and well to the north of the Arctic Circle. Warm salty water that reaches boreal latitudes cools and also becomes saltier as sea ice freezes out fresh water. It therefore gets denser and sinks to the ocean floor in the Denmark Strait between Iceland and Greenland, and between Iceland and the Faeroe Isles. This downwelling drags surface water polewards from the tropics to replenish the system, thereby creating the Gulf Stream and North Atlantic Drift that warms coastal north-western Europe as far as the northern tip of Scandinavia. It was not always this way; evidence has accumulated to indicate that the North Atlantic ‘conveyor’ shut down periodically during the run-up to the last glacial period and in the climatic hiccup of the Younger Dryas (12.6-11.5 ka). The best supported hypothesis as to why it may do that is through massive influx of freshwater to lower the density of surface water in the northernmost North Atlantic. The progressive summer retreat of sea-ice in the Arctic Ocean and the likelihood of ice-free summers there in the near future raises fears that such a shut-down may occur once again, because of freshening of surface water by ice meltwater, with devastating climatic results for Europe at least. The circulation also transports carbon dioxide dissolved in cold descending surface water to abyssal depths helping buffer its atmospheric concentration: a shut-down would allow greenhouse gas emitted by society to build up in the air.

One means of investigating the mechanisms that underlie ‘on’ and ‘off’ switching in ocean convection is to use sea-floor sediment data from the18 ka long period since the last glacial maximum (Thornalley. D.J.R. et al. 2011. The deglacial evolution of North Atlantic convection. Science, v. 331, p. 202-205). The British-US consortium used oxygen isotope data from the planktonic (near-surface) foraminifera Neogloboquadrina pachyderma preserved in sea-floor sediment cores from south of Iceland, close to where surface water descends today, to assess sea-surface temperature variations. Because of the continual exchange of CO₂ between surface water and the atmosphere, the ocean surface contains the same radioactive ¹⁴C content in carbon as does the atmosphere, at whose top the isotope is produced. When water descends this connection is cut and the proportion of ¹⁴C in it decays so that it is theoretically possible to work out the time at which deep water began to descend – its ‘ventilation age’. In practice this is done by measuring the ‘age’ of carbon preserved in planktonic and benthonic (deep- and bottom-water) foram shells, the planktonic age being the actual age used to assess the age difference between deep and surface waters. In the case of a complete shut-down of the convection the ventilation age should be high and constant; exactly the case during the last glacial maximum (19-22 ka) and most of Heinrich Stadial 1 (16.5-19 ka). When the ‘conveyor’ is functioning the ventilation age should be low, in fact from about 16-11.5 ka the ventilation age fluctuates to show 3 major and 2 lesser low to high episodes during the Bølling-Allerød and Younger Dryas, suggesting that indeed there was repeated turning-on and turning-off of the conveyor, probably triggered by pulses of fresh water into the northern North Atlantic from glacial melting. The resolution of these data is of the order of 350 years, so there may be finer detail of great interest as regards future climate.

See also: Sarnthein, M. Northern meltwater pulses, CO_2, and changes in Atlantic convection. Science, v. 331, p. 156-158.

Explosion of the exoplanets

Published on February 28, 2011 by zooks777Leave a comment

There is little doubt that it can be done, but what is so compelling about the search for worlds that orbit other stars?

By the end of the 21^st century’s first decade 500 such exoplanets had been discovered, ranging from super gas giants almost 10 thousand times the mass of the Earth to a few that are comparable in size to our home world. At present the records of size and orbital radius are biased by the relative ease

of detecting large bodies over that of Earth-sized objects. Another bias is the greater chance of observing the change in luminosity of a star as one of its planets passes between us and the star – a transit – if the planet’s orbital period is short, being close to the star. The majority of known exoplanets are less than about 8 times the Earth’s orbital radius (1 astronomical unit or AU) away from their star, although some truly huge bodies have been spotted that are up to a thousand times more remote from a star than ours is.

Labeled illustration of the Kepler spacecraft — Kepler spacecraft. Image via Wikipedia

The rate of discovery is set to burgeon now that data from NASA’s Kepler exoplanet-finding mission, launched in 2009, is producing data (Reich, E.S. 2011. Beyond the stars. Nature, v. 470, p. 24-26). The 0.95 m Kepler space telescope gazes continually at a patch of sky containing 150 thousand Milky Way stars, many of which are like the Sun. It uses the transit method, and because it is fixed on only one star field it can potentially pick up the variation of stars’ luminosity due to transiting planets that are about the size of the Earth and larger. The computations are, unsurprisingly, massive and any dips in the light curves for pixels that represent individual stars have to be confirmed by other methods or by Kepler detecting repeats of the fluctuation. One drawback is that the transit method only provides the radius of a planet and its orbital period. Mass is needed to work out an exoplanet’s density and that requires another method using the red-shift of a star due to the gravitational effect of a planet causing it to wobble; a technique fraught with difficulties and best applied to dwarf red stars. The density is important for discriminating silicate-rich exoplanets from gas-liquid bodies. The main aim of planet finders is to find those around the same size and mass as the Earth that orbit a star at a distance where they would be warm enough for liquid water to exist but not so warm that it existed only as a vapour: in the so-called ‘Goldilocks zone’.

There was an initial flurry of excitement in the press in 2010 when a scientist on the Kepler programme was misinterpreted while giving a conference presentation that resulted in headlines that hundreds of distant Earths had already been discovered in the experiment’s first year. So far Kepler has only 15 confirmed planets to its credit that range from 800 times to twice the Earth’s radius all with orbits less than that of the Earth around the Sun. Nonetheless, a couple orbit within their star’s Goldilocks zone. So there is a way to go before real excitement is justified, but Kepler data will undoubtedly be used to seek funds for other planet-dedicated programmes that can fill in the gaps and perhaps confirm the existence of distant worlds that bear some resemblance to ours. Out of Kepler’s 1235 candidate detections since launch, 68 would be Earth-sized if confirmed (Shiga, D. 2011. What’s an alien solar system like? New Scientist, v. 209 (26 February 2011 issue) p. 6-7). For such remote detection to suggest an exoplanet on which life has evolved demands that atmospheric composition can be deduced from spectra of electromagnetic radiation from the body itself: a far more difficult undertaking that finding and weighing. Free atmospheric oxygen, so far unique to the Earth, is an obvious target. However, its absence would not rule out life that did not use photosynthesis to split water molecules in making living matter, and there are plenty of life forms here that do that.

Video: Kepler’s Exoplanets vs. the Solar System (wired.com)
The New Kepler exoplanet data – NASA videos and pictures (nextbigfuture.com)

Comet water in lunar rocks

Published on February 28, 2011 by zooks777Leave a comment

Halebopp031197 — Comet Hale-Bopp.Image via Wikipedia

There are two main hypotheses about the origin of Earth’s oceans: that they are filled with water that was locked in the meteoritic matter that initially accreted to form the Earth, or ocean water was delivered by massive comet bombardment in the first half billion years of the Earth’s history. It hasn’t yet been possible to decide whether one of these, or both were involved, but the Moon might give a clue, even though until very recently it was regarded as being bone dry (see Moon rocks turn out to be wetter and stranger in May 2010 issue of EPN). The ratio between deuterium and hydrogen (D/H) gives a clue to the origin of water, in which both hydrogen isotopes occur (Greenwood, J.P. et al. 2011. Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon. Nature Geoscience, v. 4, p. 79-82). Using an ion microprobe to analyse the water in apatite, its dominant host in lunar rock samples, the authors were able to report two things. First, there is water in magmatic rocks of all ages found on the Moon: the earliest anorthosites of the lunar highlands and the younger basalts that fill the dark maria. Secondly, the water has D/H ratios significantly outside the terrestrial range. In detail, apatites with the greatest enrichment of deuterium relative to hydrogen are found in the maria basalts which fill enormous basins thought to have formed around 4 Ga ago as a result of cometary impacts. The D/H ratios are lower in apatites from the lunar highland anorthosites, which probably formed through flotation of low density calcium-rich feldspar as the Moon’s initially molten mantle crystallized not long after its formation through the impact of a small planet with the Earth. The highland D/H values are not wildly dissimilar from those found on Earth, yet those found in the mare basalts match the admittedly less well-constrained levels determined from comets hale-Bopp, Hyakutake and Halley. Because the Earth’s mass would ensure that it would corral 15 times more incoming extraterrestrial matter than would the Moon, the argument goes that if the Moon captured cometary water then Earth did so in trumps. The difference is that the Earths greater gravitational pull and thick atmosphere allowed it to retain gaseous and liquid water, while the Moon’s lower escape velocity let them leak away so that only mineralogically bound water could be retained.

Caltech Team Finds Evidence of Water in Moon Minerals (media.caltech.edu)

Top story and most flawed hype of 2010

Published on January 13, 2011 by zooks777Leave a comment

EPN might seem to include a disproportionate number of items on hominin evolution, including several on genetic evidence. An outcome of the Earth System’s 4.5 billion-year evolution increasingly depending on physical resources, we lie at the focus of our own curiosity studying the past primarily for ourselves. That is why the discovery from the partial genome of Neanderthal remains that all humans outside those who live in Africa carry in our DNA the ‘fruits’ of intimate relations with Neanderthals is surely the most explosive development of the 21^st century so far (see Yes, it seems that they did…in May 2010 issue of EPN). It is deepened by the publication in late 2010 (Reich, D and 27 others 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature, v. 468, p. 1053-1060) of genetic findings from remains of a third distinct hominin group that inhabited central Siberia 30 to 50 ka ago (see Other rich hominin pickings in the May 2010 issue of EPN). [Thanks go to Dr Bill Deller, legal historian, for alerting me to this.] The DNA from a tooth and a finger bone show that the individual female was genetically neither a fully modern human nor a Neanderthal in a statistical sense, but parts of the sequence, as with the Neanderthal genome, pop up in the genomes of living people. The ‘Denisovan’ signature – the authors do not assign the female to a new species – contributes 4 to 6 % of the genomes of present-day inhabitants of Papua-New Guinea and other Melanesian people of the Pacific north of Australia, but appears in no others. Since Melanesians carry some Neanderthal genetic material the new finding can be interpreted as the result of similar interfertile mating between the ‘Denisovans’ and a limited group of early fully human travellers who crossed central Asia and eventually moved through Indonesia to cross the West Pacific to Papua-New Guinea and Melanesia about 45 ka ago. For up to a twentieth of the genetic outcome of such liaisons to survive to the present suggests no idle dalliance, but proportionately common relationships.

Denisovans shared a common ancestor with Neanderthals and ourselves, but seem to have followed a separate evolutionary path. Analysis of their DNA suggests that they diverged from Neanderthals around 640 ka and from modern Africans around 800 ka. Although these ‘molecular clock’ dates show considerable uncertainty, they extend back to a period when fossil evidence suggests the presence in Europe and Africa of Homo heidelbergensis and H. erectus respectively. The molar tooth has a morphology similar to African H. erectus and to even earlier hominins, but distinct from the teeth of Neanderthals and fully modern humans. Could the ‘Denisovans’ represent a distinct wave of emigrants from Africa? Some hominin fossils from China are dissimilar to Neanderthals and Asian H. erectus and efforts will certainly be made to establish their genetic make-up. For the moment, these findings deny any simple linear explanation for the ‘Out-of-Africa’ movement of people. Equally important, and the reason why the researchers refuse to assign the ‘Denisovans’ to a new species, is that interfertility is generally accepted as the sign of mating between members of the same species. To some extent this harks back to the ideas of the ecologist Jonathan Kingdon (Kingdon, J. 1993, Self-made Man and His Undoing. Simon & Schuster: London) that humans are a line that did not speciate over the last couple of million years, but show morphological differences that arose within the growing protection from selection pressures conferred by the use and development of tools. Kingdon’s parsimonious approach to human evolution found little favour with palaeoanthropologists, perhaps because of the kudos associated with finding and naming new species.

See also: Callaway, E. 2010. Fossil genome reveals ancestral link. Nature, v. 468, p. 1012; Bustamante, C.D. & Henn, B.M. 2010. Shadows of early migrations. Nature, v. 468, p. 1044-1045.

Perhaps it is a generational thing, stemming from popular science fiction and scientists’ speculation in the 1970s and 80s, that has encouraged the growth of exo-, xeno- and astrobiology as subdisciplines. There is a certain sadness in that all practitioners can do at present is examine the organic diversity offered by our home world and speculate about alien life forms based on that terrestrial evidence. The Earth offers plenty of scope for studying the biologically odd and awesome, especially among prokaryotes, as there are extremophiles of all kinds: the hot, the cold and the deep biospheres. But all are based on the nucleic acids shared by all life on Earth; traces of familiar amino acids occur far and wide in the cosmos, but none whatsoever of anything more complicated that could source self-replication and evolution. So it was in a mood of solemn gaiety that EPN greeted the hint of truly alien life forms among us by NASA press officers in November. It turned out to presage a paper concerning bacteria peculiar to Mono Lake in California (Wolfe-Simon, F. And 11 others 2010. A bacterium that can grow by using arsenic instead of phosphorus. Science Express, DOI:10.1126/science.1197258). The paper hinted at arsenic being used to substitute for phosphorus in the structure of nucleic acids in the bacterium when it lived in low-phosphate environments. The paper’s substance was culturing the bacterium in vitro in increasingly P-deficient water that also contained arsenic. If replicable the notion of arsenic-DNA would seem to be pretty startling, but the paper faced a storm of adverse comment.

Biomineralised columns at Mono Lake, California. Image via Wikipedia

A crucial feature of the DNA molecule is the bond between the sugar of one nucleotide and the phosphate group of another. As any geochemist knows, it is possible for elements to substitute for one another if they have similar atomic properties. Arsenic, being adjacent to phosphorus in the same group of the periodic table, is a potential substitute – arsenate for phosphate, although the former is far less stable than phosphate. Wolfe-Simon’s team is not claiming the peculiar bacterium as a candidate for alien life forms, but that is the spin widely being put on their work. All they suggest is that some bacteria can survive high-As low-P conditions and may be found in chemically highly toxic environments elsewhere. Since the cosmic abundance of arsenic is about three orders of magnitude less than that of phosphorus it is unlikely that alien genetic material somehow uses arsenic in its architecture. Besides, why should DNA be the sole basis for self-replication, the essence of truly living beings?

Added 14 January 2011: Science gave Felisa Wolfe-Simon the opportunity to reply to critics (Pennisi, E. 2010. Discoverer asks for time, patience over arsenic bacteria controversy. Science, v. 330, p. 1734-1735). Personally, I sympathise with the beleaguered team: on the launch of Stepping Stones in 1999 I was pestered continuously by journalists from both low- and highbrow newspapers. In that case I had made a joke that perhaps the human coughing reflex had stemmed from ancestral reptiles that survived the Permian-Triassic mass extinction and the emissions of the Siberian Traps: the journalists actually believed it

See also: Pennisi, E. 2010. What Poison? Bacterium uses arsenic to build DNA and other molecules. Science, v. 330, p. 1302.

Ancient humans, dubbed ‘Denisovans’, interbred with us (bbc.co.uk)

Share this:

Related Articles

Share this:

Related Articles

Share this:

Share this:

Related Articles

Share this:

Related Articles

Share this:

Share this:

Related Articles

Share this:

Related Articles

Share this:

Related Articles

Share this: