Category: Geobiology, palaeontology, and evolution

Amber from a mine in Myanmar generates a steady income from sales to palaeoentomologists, each bead of the lithified resin being a possible lagerstätte in its own right. Two scientists at Oregon State University and Cornell were fortunate enough to find a small, Early Cretaceous bee that is so well-preserved as to show even the leg hairs on which bees carry pollen (Poinar, G.O. & Danforth, B.N. 2006. A fossil been from Early Cretaceous Burmese amber. Science, v. 314, p. 614). Indeed, the hairs carry several grains of pollen. This is the oldest known bee by more than 35 Ma, and it coincides with the start of the explosive radiation of flowering plants.

Only around 2.2 Ga did the atmosphere contain sufficient oxygen to oxidise iron(II) to iron(III) and leave its trace in red soils and terrestrial sedimentary rocks. That opened the way for the emergence and evolution of the Eucaryan domain of organisms, most of which depend on oxygen. For their predecessors, the prokaryote Bacteria and Archaea, oxygen would have been intensely toxic, especially for those which used anoxygenic forms of metabolism. Almost certainly oxygen was released for more than a billion years before the Great Oxidation Event, by blue-green bacteria, only to be mopped up by oxidation of abundant iron(II) ions dissolved in sea water. Getting an idea of the diversity of pre-2.2 Ga life is possible by examining the organic chemicals produced when they decayed under anoxic conditions, i.e. from oil and kerogen. Unfortunately, the great age of their host rocks has resulted in many Precambrian sediments having been heated and metamorphosed, so that different biomarkers break down into less distinctive compounds. There are, however, sediments that may have remained more or less unaffected, and one sequence in the Canadian Shield has yielded astonishing results (Dutkiewicz, A. et al. 2006. Biomarkers from Huronian oil-bearing inclusions: An uncontaminated record of life before the Great Oxidation Event. Geology, v. 34, p. 437-440).

The sediments are conglomerates rich in uranium, having been deposited under reducing conditions that helps precipitate uranium from solution, and have been mined extensively in the Elliot Lake area of Ontario. Oil seems to have entered fluid inclusions in quartz that cemented the conglomerates, shortly after the conglomerates were deposited at about 2.45 Ga. The oil contains a host of complex organic compounds that have never been degraded by heating. Some can be linked to blue-green bacteria, which undoubtedly created of oxygen continuously. That they gave rise locally to favourable conditions for oxygen-using organisms is clear from other biomarkers. Those are steranes that are derived by breakdown of sterols, which in turn are only known to be created by the enzymes exclusive to Eucaryan metabolism. Steranes have been found in even older sediments, but they were back shales that could easily have been contaminated by much younger organic materials seeping through the host rock. Oil in fluid inclusion within diagenetic minerals is far less likely to have been contaminated, so the Elliot Lake samples define a minimum age for the emergence of the Eucarya far earlier than the appearance of actual microfossils that show the distinctive cell nucleus that defines the domain Eukarya.

Precambrian bonanza for palaeoembryologists

Signs of relatedness among groups of organisms often show up well during their early growth as embryos, so their fossils in very old rocks are of great use in establishing when different groups emerged (see Ancient baby penis worm hits the news in EPN February 2004 issue). A deposit containing possible embryos of deutorostomes (see Age range of early fossil treasure trove, in EPN March 2005 issue), in which the first orifice to emerge during embryonic development is the anus, is of considerable interest. Nowadays, the group contains animals with mirror symmetry (bilaterians), including the vertebrates. First reports of fossil embryos from the 580 Ma old Doushantuo Formation of southern China in 2004 drew fire from palaeontologists who preferred to believe that the smooth almost spherical objects, like the fictitious life forms in a supposedly Martian meteorite, were probably oolith-like mineral growths. Undeterred, their finders have extracted yet more from the exposures (Chen, J-Y. and 12 others 2006. Phosphatized polar lobe-forming embryos from the Precambrian of southwest China. Science, v. 312, p. 1644-1646). They demonstrate clearly that the objects do show lobes in an early stage of development that break the embryos initial symmetry so that different kinds of tissue can develop to form adults. The find matches well with evidence from the genes of modern bilaterians that the basic branching of the Animal Kingdom occurred well before the Cambrian Explosion of shelly fossils. Since more or less all modern phyla are represented by Cambrian fossils, that is not surprising.

Pocket sauropods

The largest animals to roam the land were vegetarian dinosaurs of the sauropod group. The biggest reached a length of more than 30 metres, and were commensurately tall. These giants permeate our perception of Mesozoic life on the continents, along with their monster predators. Now, children made nervous by such titanic creatures (and I was definitely one of them) can be reassured that there were ones that were not so crushingly big (Sander, P.M. et al. 2006. Bone histology indicates insular dwarfism in a new Late Jurassic sauropod dinosaur. Nature, v. 441, p. 739-741). A near-complete skeleton of a sauropod that was only 6 metres long turned up in Lower Saxony in Germany, along with other remains suggesting individuals as small as 1.7 m. Europasaurus was first thought to be a juvenile of a much larger species, but Sander et al. developed means of microscopic bone analysis that clearly show fully mature bone growth. In the Late Jurassic central Germany was covered by sea, except for a number of large islands. The most likely explanation for such a tiny species is that it adapted to island life in much the same way as other, more recent mammals did, such as pigmy elephants and hippos on many islands in the Mediterranean and the Indonesian archipelago.

Rich as the fossil record is, it is terribly incomplete, for the obvious reason that the chance of preservation over fragmentation and destruction of body parts is extremely small. That is especially the case for the high-energy and oxidising land and freshwater environments. Each fossil species can easily be assumed to be a one-off, appearing, thriving for a short while and then disappearing: ripe for the assumption of divine creation, as Linnaeus assumed. Very rarely indeed, specimens emerge that fill in the many gaps needed by evolutionary theory, the most celebrated being Archaeopterix that bridged the gap between dinosaurs and birds. That transition has been enriched by a whole series of older fossils from Chinese lagerstätten that show the transition in sublime detail.

The comparative anatomy of fish and land vertebrates suggests a common ancestry, and the Devonian to Early Carboniferous terrestrial record has yielded tantalising fish with lobed fins (e.g. Eusthenopteron and Panderichthys) and almost fish-like animals with four rudimentary limbs (e.g. Acanthostega and Ichthyostega). Yet a gap remained to be filled in the apparent transition from aquatic to land-dwelling vertebrates. US palaeobiologists engaged in seeking candidates from the Late Devonian of Arctic Canada have found one that reduces any uncertainty tremendously (Daeschler, E.B et al. 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature, v. 440, p. 757-763. Shubin, N.H. et al. 2006. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature, v. 440, p. 764-771). The fossil, prepared with lengthy and painstaking care, shows such amazing anatomical detail as to demonstrate clearly that the fin and shoulder girdle are indeed intermediate between fish and tetrapods, whereas previous candidates supporting a transition are either definitely fish or tetrapods. Tiktaalik slots nicely into the time gap too, about 2 Ma younger than the most tetrapod-like fish Panderichthys and slightly older than fish-like quadrupeds. The outcome of a deliberate search for an animal to fit the gap, Tiktaalik above all demonstrates the predictive capacity of palaeontology, which counters a common epithet flung by those bent on divine intervention and/or intelligent design. Based on this outstanding success, fossil hunters will be encouraged to sift on a stratigraphically finer scale for yet more steps in vertebrate evolution, including our own.

See also: Ahlberg, P.E. & Clack, J.A. 2006. A firm step from water to land. Nature, v. 440, p. 747-749.

That life plays a role in surface geological processes is self-evident. Death and the burial of dead organic matter feed back to climate by removing carbon from the atmosphere and hydrosphere, thereby reducing the ‘greenhouse’ effect and increasing the oxidation potential of the outer Earth – a discovery of the late 20^th century. James Lovelock’s Gaia hypothesis proposes that life’s influence as a means of balancing conditions for its own continuity is a primary factor behind the behaviour of our home world, although a great many geoscientists doubt that bold generalisation. It seems to many that the influence of both deep mantle processes and extraterrestrial forces not only provided the conditions for planetary evolution, both inside and at the surface, but created the conditions for life’s emergence and its survival.  Life has been pushed to the brink of complete extinction several times by both truly primary parameters. Yet Gaia is still a persuasive idea, or at least a metaphorical itch that must be scratched from time to time. Perhaps the boldest attempt at pushing Lovelock’s notions to the limit appears in a recent essay (Rosing, M.T. et al. 2006. The rise of continents – An essay on the geologic consequences of photosynthesis. Palaeogeography, Palaeoclimatology, Palaeoecology v. 232, p. 99-113).

Assuming that carbon-isotope evidence from the oldest sediments known (3.8 Ga, West Greenland) that life selectively took up light ¹²C is valid, there seems to be a remarkable coincidence between the origin of life on Earth and the oldest known continental rocks (4.0 Ga, northern Canada). Rosing et al. suggest that this is no coincidence, but the result of the effect of living organisms on magmatism at subduction zones, most particularly on the mineralogy of old oceanic lithosphere that descends there. Their essay starts by emphasizing that modern photosynthesis contributes three times more energy to surface processes than does heat flow from the mantle, and that energy must accomplish a commensurately significant amount of mainly geochemical work, some of which occurs in basalts of the ocean floor as they spread from constructive margins. Continental crust is widely accepted to form as a result of hydrous fluids rising above subduction zones to cause different conditions for melting of the overriding mantle wedge than those for partial melting of mantle rock beneath mid-ocean ridges and oceanic islands. Multistage fractionation processes that operate on basaltic magmas formed by this wedge melting result in separation of residual magmas that are sufficiently enriched in silica and other elements to crystallize as, broadly speaking, granitic rocks. Since they cannot be metamorphosed to a form that exceeds the density of the mantle, such rocks cannot be subducted, unless debris shed from them mixes as sediment with subducting oceanic lithosphere. So continents become more or less permanently growing edifices on the face of the Earth. The central questions that Rosing et al. focus upon are: why did continents not form from the outset of the Earth’s evolution, once tectonics and oceans had stabilized, and why the coincidence? Their answer to both is that life played a fundamental role in increasing the amount of water that ends up in old, cold oceanic crust, thereby helping the peculiarities of wedge melting to become established. Essentially they appeal to life’s ability to transform energy of different sources, for example heat from the mantle and the energy carried by electromagnetic radiation, and transmit it through biogeochemical cycles from its source to the lithosphere. Specifically, they speculate that this life-mediated energy transfer accelerated the conversion of dry minerals in basalt to water-rich clays. In turn, that had its effect on subduction-zone geochemistry.

Rosing et al.’s seems to have a willful flaw: they focus on the incorporation of solar energy into the Earth system by photosynthesis from the time when continental materials first appeared in substantial bulk, between 3.8 and 4.0 Ga. So far there is a mere shred of evidence from ambiguous carbon isotope studies that photosynthesising organisms were around before about 3.4 to 3.5 Ga. There is no trace of such shallow-water organisms as stromatolites until that time. Nor is there any significant sign of where one end product of photosynthesis, oxygen, must have been secreted away by reaction with dissolved iron(II) – banded iron formations only become prominent in the later Archaean. Whatever organic activity might alter ocean-floor basalts, it is hardly likely to have used photosynthesis, unless the early oceans were shallow enough (200-300 m) to pass light to their floor. The key to alteration of anyhydrous minerals in basalt to form clays is the availability of hydrogen ions (products of oxidation) to donate electrons through hydration reactions, and they are available from a great many processes other than living ones. Then, of course, there is the key issue of whether any influence – direct or indirect – by photosynthesis can be seen on modern ocean-floor geochemical processes. Since it doesn’t go on down there, whereas a great many oxidation reactions that produce hydrogen ions do, makes the hypothesis impossible to test. In fact it is not a hypothesis but speculation, and it has a great deal of company from other ideas to explain the missing 600-800 Ma of Earth’s evolution. Most of those centre on the mechanics of slab-pull force, the pace of sea-floor spreading and the angle of likely subduction during geothermally much hotter times. Oddly, the third author, Norman Sleep, introduced a great deal of basic theory behind these other explanations.  This is one of two articles from March 2006, whose time of publication – close to 1 April – may give a clue to its weight. It is interesting seasonal reading, and everyone should look forward to further debate.  However, like the magnificent Verneshot hypothesis (See Mass extinctions and internal catastrophes in June 2004 issue of EPN), it may die in a deafening silence.

Methane, methanogens and early climate control

Expulsion of methane from gas hydrates in shallow marine sediments has been implicated several times as the likely cause for sudden bouts of global warming, such as that at the end of the Palaeocene 55 Ma ago. The gas, produced by primitive, anaerobic prokaryotes known as methanogens, is more powerful at delaying loss of heat to space than is carbon dioxide. It is a greenhouse gas of enormous potential power, although in an oxygen-rich atmosphere it has a short life before being oxidised to CO₂ and water. Methanogens themselves, which survive only in airless places, evolved very early in the Earth’s history as witnessed by their genetic molecules being very different from those of other members of the Bacteria and Archaea domains. The ambiguities of carbon isotopes in ancient carbonaceous rocks being able to discriminate different metabolic processes, has led to considerable debate about when methanogens first made their appearance. That was probably well before the oceans were able to contain dissolved oxygen, which is highly toxic to anaerobic prokaryotes, i.e. in the Archaean. A good sign that such cells were around would be, in some way, to detect their main metabolic product, methane.  The place to look would be in fluid inclusions enclosed in minerals that were definitely produced by seafloor sedimentary processes. The best candidate would be quartz in cherts precipitated from seafloor hydrothermal vents, where such organisms would have both the energy and the fuel to thrive. A group of Japanese geochemists have systematically looked for such fluid inclusions in a variety of Archaean cherts and they found sufficient evidence to at least give a minimum age for the presence of methane-producing bugs (Ueno, Y. et al. 2006. Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era. Nature, v. 516, p. 516-519).

The Dresser Formation (3.45-3.50 Ga) in the early Archaean of Western Australia contains abundant pillow basalts and chemogenic, silica-rich sediments. These cherts seem to have been fed by fissures through which hydrothermal fluids moved, and it is quartz from these syn-sedimentary quartz-rich dykes that revealed abundant fluid inclusions that had clearly formed as the quartz crystals grew. The inclusions contain carbon dioxide with traces of methane. Most important, the carbon in the methane is highly enriched in heavy ¹³C, evidently due to cell processes drawing in the lighter isotope ¹²C; the methane is almost certainly biological in origin. So it is possible to say both that methanogens had evolved before 3.5 Ga, and that they added methane to the Archaean atmosphere. Such a highly reduced gas would become a permanent constituent of the air, because oxygen had yet to be released by other organisms so that methane would be oxidise quickly, as happens today. The discovery by Ueno et al. is important from another standpoint than the appearance of a particular kind of metabolic process.

From the time of its accretion until well into the early Precambrian, the Earth received a great deal less energy from the Sun than it does today. Solar hydrogen fusion had not then achieved the level of efficiency that it has now. Without some means of trapping heat in the atmosphere, the Earths mean surface temperature would have been well below the freezing point of water. Without a ‘greenhouse’ effect, the planet, well endowed with water, would have been inescapably locked inside a thick crust of ice. In some respects it would have resembled a large version of one of the Outer Planet’s icy moons, such as Enceladus (see Yet another weird world later). Life would have found it difficult to emerge, if at all, at such low temperatures. Like Enceladus and other distant moons, some liquid water would have been present due to heating from the mantle and magmas, but the white surface would always have reflected away most of the Sun’s heat – geothermal heat is vastly less than that of solar origin. The most recently proposed means whereby the Earth could have escaped permanent frigidity and sterility from the ‘weak, young Sun’ is that volcanic exhalation of CO₂ would eventually have developed ‘greenhouse’ conditions.  However, it would have had to reach much higher atmospheric concentrations that now, perhaps greater than some geochemists believe to be theoretically possible. Being a much more powerful ‘greenhouse’ gas, methane helps overcome such theoretical difficulties. It can only be produced in quantity by biological processes, and that poses a conundrum, despite Ueno et al.s discovery. Without an atmosphere containing gases that could trap solar warmth since shortly after planet formation, the cold trap would have taken an icy grip holding back the emergence of life, such as primitive methanogens. Does that therefore imply that such organisms emerged far earlier than the start of tangible geological history?

Mass extinctions have been the principal time markers in the Phanerozoic stratigraphic column since 19^th century palaeontologists recognised sudden changeovers in the fossil record. Two close the Palaeozoic and Mesozoic Eras, two more end Periods (Ordovician and Triassic) and others mark Stage boundaries. Greatest focus has been on the magnitudes of each extinction, greatly assisted by the statistics compiled by the late Jack Sepkoski. The adaptive radiations that filled abandoned niches and restored and, in most cases, expanded diversity are equally interesting.  Such recoveries from depleted stocks of organisms have been of immense influence over biological evolution. Resulting from chance events, as far as the Earth’s biota are concerned, the families and species that arose would not otherwise have appeared: the most powerful blow to any notion that biological advances are in any way pre-ordained.

Until recently, it seemed that each recovery was an extremely protracted affair. Over 5 to 10 million years seemed to be the case for aftermaths of the largest extinctions. To a marked extent, analysing recoveries from the fossil record is not so easy as tying the great declines in diversity to a time. It is a matter of working out the rate at which new genera arose or originated through speciation, and that is affected by geographic biases in the fossil record.  They arise from less collecting in remote areas and variations in the volume of exposed strata in others.  Correcting the biases is possible to some extent, but that still leaves the challenge of statistical analysis. From an extraordinary expansion of analytical expertise, which extends to economists’ methods of understanding stock market trends and the flair of physicists, a very different story of restocking seems about to emerge. A technique called vector autoregression applied to faunal diversification corrected for biases suggests that recoveries were very much faster than previously thought, in fact almost immediate by comparison with the time-precision of the stratigraphic column (Lu, P.J. Motohiro Yogo, M and Marshall, C.R., 2006. Phanerozoic marine biodiversity dynamics in light of the incompleteness of the fossil record. Proceedings of the National Academy of Sciences, v. 103, p. 2736-2739).

See also: Kerr, R.A. 2006.  Revised numbers quicken the pace of rebound from mass extinctions. Science, v. 311, p. 931.

Is the Cambrian Explosion real evidence for an evolutionary burst?

About 543 Ma ago, remains of organisms that secreted hard parts suddenly appear in the fossil record.  Most palaeontology has focussed on such easily fossilised organisms from the Phanerozoic Eon that began at that time. Whether or not the Cambrian Explosion was a truly significant event, bar the appearance of hard parts – that is quite a mystery in itself – is highlighted by the presence of members of almost all modern animal phyla in the Early Cambrian record. Did they all suddenly explode onto the scene at its outset, or were they around well beforehand as almost completely soft-bodied creatures? Comparative molecular biology of living animals, and the concept of molecular ‘clocks’ has for a while suggested that the origination of modern phyla was considerably earlier than the start of the Phanerozoic. Increasing the database on which such ideas can be based helps improve their precision and scope, assisted by novel methods of mathematical analysis. The 23 December 2005 issue of Science contained an analysis of more than 12 thousand amino acids involved in the genomes of members of 9 or 26 extant animal phyla (Rokas, A.. et al. 2005. Animal evolution and the molecular signature of radiations compressed in time. Science, v. 310, p. 1933-1938). Preliminary study suggests that indeed the early history of the metazoans was remarkably compressed in time, probably in the 50 million years after the ~600 Ma Snowball Earth event, and possibly within a few million years of the base of the Cambrian. However, tests of hypotheses based on such indirectly related data are notoriously difficult, and Rokas et al. have taken a bit of stick (Jermiin, L.S. et al. 2005. Is the ‘Big Bang’ in animal evolution real? Science, v. 310, p. 1910-1911). It seems yet more work on molecular biology of the remaining 17 phyla and a great deal of mathematical wrangling is yet to come.

Sequences that reveal the Permian-Triassic boundary continue to receive a great deal of attention, spurred by the seemingly cryptic nature of the conditions that caused up to 90% of all living things to die. Globally, the boundary is marked by a sudden and large fall in the proportion of ¹³C in carbonates and sedimentary organic matter.  Since the d¹³C anomaly follows the biotic decline, it is less likely to reflect any cause of the extinction, such as a massive methane release from destabilised gas hydrates and global warming, than an effect of whatever went on.  Joint research by UK, Dutch and US organic geochemists focused on the P/Tr boundary in northern Italy, where it is dominated by shallow-marine carbonates (Sephton, M.A. et al., 2005. Catastrophic soil erosion during the end-Permian biotic crisis. Geology, v. 33, p. 941-944). They analysed the organic compounds preserved in the section, and found that the extinction zone coincides with a major increase in total organic carbon, which is dominated by large amounts of compounds (polysaccharides) that typify soils and leaf litter.  They explain the anomaly as the result of a short period of rapid soil erosion from the terrestrial hinterland of the shallow Late Permian sea.  Since virtually all continental crust had stabilised in the Pangaea supercontinent, tens of millions of years beforehand, such erosion was unlikely have been a result of some sudden tectonic uplift. But it might have been triggered by sudden loss of the vegetation that retards soil erosion on the continental surface. The P/Tr extinction affected both marine and terrestrial organisms, and Sephton et al recognise that their discovery of evidence for soil stripping on a grand scale reflects that unified fate. Acid rain from the massive Siberian continental flood volcanism could well have been the trigger for ill thrift of land vegetation, or maybe removal of stratospheric ozone by release of halogen (chlorine and bromine) compounds let in destructive UV radiation.

Methane-induced warming around 55 Ma ago was one of the greatest environmental upheavals of recent geological time. Pretty quickly, all the methane belched out by destabilisation of sea floor gas hydrates would have forced up atmospheric CO_2­ concentrations.  The estimated climatic effect was astonishing: a global temperature rise of the order of 5-10°C in 10-20 thousand years. The early Eocene world would have become a steamy place, and the changes certainly tally with shifts in a range of faunas, from foraminifera to large mammals. Not many people have reported any coincident changes in plant fossils, even though a moist atmosphere charged with CO₂ would have encouraged growth enormously. A reflection of the changed conditions does come from rapidly changing leaf shapes and sizes, however. One of the key sections that does reveal floral change is in terrestrial sediments preserved in the Bighorn Basin of Wyoming, USA (Wing, S.L. et al. 2005. Transient floral change and rapid global warming at the Paleoene-Eocene boundary. Science, v. 310, p. 993-996). Tied down from a dramatic change in carbon isotopes, the boundary section not only shows the rapid dominance of leaves with extended ‘drip tips’ that allow rainwater to be shed quickly, but an influx of genera unknown from the Palaeocene below.  The invasive groups are known from sediments of that age from much further south in the US, and even from Europe at the other side of the opening Atlantic Ocean. So it seems that there was a rapid northward plant colonisation over 4 to 20 degrees of latitude. The section perhaps gives a flavour of floral changes that might occur should modern anthropgenic warming go unchecked.

Dinosaur dung, the Deccan Trap and grass

Yes, it has to come to a pretty pass when geologists will tramp to the very base of the Deccan continental flood basalts, dig up and then finger through dinosaur crap. The temptation of a bed consisting of little other than coprolites  deposited by sauropods, especially beneath the very lavas implicated by some in their demise, is huge. It isn’t the first time that coprophilia has struck the vertebrate palaeontological community, for a very good reason: if dinosaurs grew so darned big what did they eat? That it included grass is a surprise for palaeobotanists, but would have been a great treat for the thunder lizard, for there is nothing more toothsome to a herbivore than a hay snack; much better than a monkey puzzle leaf. Indian and Swedish geologists hit the headlines with their discovery (Prasad, V. et al. 2005. Dinosaur coprolites and the early evolution of grasses and grazers. Science, v. 310, p. 1177-1180). The lithified dung contains unmistakable traces of silica-rich phytoliths that occur only in grasses. Some possible grass pollen has been found before in Late Cretaceous sediments, but the crown-group Poaceae, that still thrive today, had been thought to have appeared later than the Early Eocene. It now seems likely that grasses appeared first in Gondwana, being transferred to Eurasia by the collision of its wandering fragment India around 50 Ma ago – India had already begun to move independently at the time of Deccan eruptions. Genetic studies of grasses points to their origin about 80 Ma ago, so it is likely that those in the dung are among the earliest. The Indian titanosaurs that ate them were not grazers, however, because the dung is also full of remains of conifers, palms and other vegetation that would have been abundant in those times. Interestingly, mammals from palaeosols within the Deccan lava sequence have cheek teeth reminiscent of the dominant grazers of later time.

Clay minerals and the origin of life

J.D. Bernal, a former student of J.B.S. Haldane, had as wide a range of interests as his mentor. Though a member of the Communist Party of Great Britain at the height of its loyalty to Stalin, during World War II he was a scientific advisor to Churchill. Among his many contributions was an idea inspired by Haldane’s conviction that life emerged from the inorganic world through simple chemical processes. Bernal thought in terms of a template sufficiently complex to shape early organic molecules, and clay minerals fitted that particular bill because they contain loosely bonded, yet complex passageways between the sheets of linked SiO₄ tetrahedra that form the bulk of their structure. A group of geochemists from Arizona State University have experimented on the organic catalytic potential of clays by simulating conditions around sea-floor vents that may have been the haven in which terrestrial life first formed (Williams, L.B. et al. 2005. Organic molecules formed in a ‘primordial womb’. Geology, v. 33, p. 913-916). Their ‘feedstock’ was dilute methanol and the clays that they chose were montmorillonite, illite and saponite, the last a member of the smectite group with high magnesium that forms by hydrothermal alteration of olivine and pyroxene in basalts. More complex hydrocarbons, with up to 20 carbon atoms per molecule, did indeed form in their experiments. The results suggest that smectite clays protect such unstable hydrocarbons from thermal decay, but no distinct life-forming molecules, such as amino acids, showed up. The products were polycyclic aromatic hydrocarbons, but it is possible that they would have formed a diverse feedstock for other processes once the hydrothermal clays were deposited in cooler conditions.

In Neoproterozoic sedimentary sequences evidence for low latitude glaciation crops up at two and probably several other times; so-called ‘Snowball Earth’ events.  Opinion is divided on several aspects of these events: whether or not they truly coated the Earth in glacial ice; their influence on biological evolution; the processes that started and terminated them.  From a biological standpoint, a completely ice-bound surface – both land and oceans – would have stressed organisms to the extreme.  Marine life (all that there was in those times) may only have survived in a few refuges from the ice, perhaps around submarine hydrothermal vents or in ephemeral sea-ice leads and polynya. If that were so, then these frigid episodes would have created important evolutionary ‘bottlenecks’, from which sprang several adaptive radiations: ‘Snowball’ epochs may have determined the forms and genetic diversity of all later life, especially among the Eucarya, of which we are a part. Probable deep-ocean anoxia would have been particularly stressful for organisms that depend on oxygen.

The key to establishing whether or not Neoproterozoic frigid episodes did bring eucaryan life to the verge of extinction lies in the diversity of life during those periods.  That is not an easy task as all life until just before the Cambrian Explosion was both soft-bodied and minute.  One means of assessing diversity is to study biochemical remnants of cell processes preserved in reduced ocean sediments (Olcott, A.N. et al. 2005. Biomarker evidence for photosynthesis during Neoproterozoic glaciation. Science, v. 310, p. 471-474). Olcott and colleagues studied black shales from Brazil whose age is within that of a frigid episode (740-700 Ma), and which contain textural evidence for abundant sea ice and low temperatures. Recovered biochemical compounds indicate considerable diversity, with a mixture of photosynthetic blue-green bacteria and eucaryan algae, with anaerobic bacteria of several types.  The results indicate open water to allow photosynthesis – although it is possible for light to penetrate several metres of sea ice – together with deeper anoxic waters.  Since the samples span a section almost 100 m thick, it seems this diversity persisted for a long period.  However, the most that it can establish with certainty is that thin sea ice or open water did persist at the low palaeolatitude of late-Precambrian Brazil.  The Neoproterozoic record has abundant, widespread black shales, and quite possibly there are others associated with evidence for glacial events.  The importance of the paper lies in showing that biomarkers can be used as effectively in the Precambrian as in the Phanerozoic, and an expansion of this approach can be expected.

Oxygen and mammalian evolution

So much in the geological history of surface processes depends on either the dearth or the superabundance of oxygen. That is no surprise for a host of reasons, one being that it is the most reactive common element when free of bonds, and another is that the most powerful means of releasing oxygen is the capture of energetic solar photons by the pigments residing at the heart of photosynthesis. To grossly paraphrase James Lovelock, the principal reason for not sending people to Mars to search for life is that the planet’s atmosphere tells us that even if was there, it wouldn’t be very exciting.  Oxygen gas is at vanishing low levels on the Red Planet, even if there is lots locked up in its iron-oxide rich surface.

The greatest event in the history of terrestrial life, apart from its emergence, was exploitation of the means of breaking hydrogen-oxygen bonding in water, which is what common photosynthesis is all about.  It opened the entire planet to life from the restricted, though diverse habitats of most Bacteria and Archaea in the earlier anoxic world.  First, oxygen-excreting cyanobacteria were able to colonise the entire ocean surface, depending on available nutrients. In doing so and generating free oxygen they threatened every other organism that used metabolisms based on other kinds of chemistry: oxygen is highly toxic because of it propensity to grab free electrons.  Balanced by its oxidation of iron in early oceans, severe oxygen stress did not emerge until halfway through Earth’s history.  Once it did become able to accumulate in air and water, all ecosystems faced havoc.  Dominant prokaryotes slunk to rare places of refuge, while others seem to have combined in resisting oxidation. Their creation of the Eucarya that depend completely on available oxygen led, through the emergence of algae and then plants, to an accelerated stoking up of oxygen generation.

Once vegetation began to cloak the land, an extra 30% of the planet’s surface opened new vistas for animals and increased oxygen production and complementary burial of carbon.  Indeed, explosive growth of atmospheric oxygen during the Carboniferous resulted in animal expansion to the air, through ominously huge insects.  The first clearly traced ancestors of mammals seem to have appeared in the Permian, though their descendants only got the chance to dominate once reptiles, especially dinosaurians, lost their grip as a result of the K-T extinction. At the time of a far greater loss of living diversity, at the end of the Permian, it is now clear that in a relatively short time oxygen levels had fallen from their highest to one of the lowest in the Phanerozoic record (see New twist for end-Permian extinctions in the May 2005 issue of EPN).

Anoxic oceans were a regular feature of the Mesozoic and early Cenozoic. It is their preservation of abundant buried carbon that holds a key to, in an anthropocentric sense, the greatest of evolutionary leaps; the rise of large mammals and ourselves.  A large team of US scientists has used the now abundant records of carbon isotopes in both buried organic matter and marine carbonates to reconstruct changes in atmospheric oxygen content (Falkowski, P.G. and 8 others 2005.  The rise of oxygen over the past 205 million years and the evolution of large placental mammals. Science, v. 309, p. 2202-2204). Their modelling suggests that at the start of the Jurassic, atmospheric oxygen stood at around only 10%.  Through that period it rose dramatically to 16%, fell equally abruptly and then rose again to about 18%, thereby creating the conditions for some of the largest sources of petroleum.  Cretaceous times saw a slow rise, until around the time of the global warming at the Palaeocene-Eocene boundary (55 Ma).  The middle of the Cenozoic was a further period of dramatic increase in oxygen levels, to their highest (~23% in the Oligocene) since the peak during the Carboniferous. Latterly atmospheric oxygen has waned to around 21% today.

Falkowski et al. compare their new atmospheric oxygen curve with evolutionary spurts among mammals, of which the simplest to understand is the parallel rise of mammalian average size.  The metabolism of all mammals, like birds, has 3 to 6 times the oxygen demand of reptiles.  Not only were Mesozoic mammals challenged in stature by the air they breathed, reptiles were easily able to grow to monstrous proportions because of their less demanding physiological processes.  The first signs of the placental nurturing of mammalian foetuses, which requires a high oxygen level, coincides roughly with the Mesozoic maximum (100-65 Ma).  The end-Cretaceous extinction of the dominant dinosaurian reptiles removed the main competition against the subtle advantages of placental mammals, and was followed by further increase in oxygen.  The Cenozoic permitted terrestrial mammals to reach sizes almost comparable with dinosaurs, and to go beyond them among whales.  Moreover, it saw explosive diversification, one branch of which, the primates, leads to ourselves.

Naughty school kids once used to hurl glass vials that launched the most pervading smell of rotten eggs when they smashed.  Stink bombs produce hydrogen sulfide.  Interestingly, if you can smell it you are more or less safe – though not from flying glass shards.  When H₂S is more concentrated, it becomes an odourless and stealthy killer, as ‘sour gas’ emitted from oil drilling rigs.  A group of anaerobic bacteria generate the gas when there are abundant sulfate ions in oxygen-starved conditions.  They use these ions as electron acceptors in their metabolism, thereby reducing sulfate to sulfide ions; a common phenomenon in stagnant swamps, and especially prevalent at depth in the Black Sea.

Several times during the Phanerozoic global ocean depths became anoxic, when thermohaline circulation shut down.  The consequences show up in black mudrocks, rich in partially broken down hydrocarbons and iron sulfide.  Some of these are major source rocks for petroleum.  Unstirred by deep current flow, bottom waters pervaded by H₂S are covered by oxygenated water, so it might seem that there is little threat to surface dwellers and air breathers, although any animal unwarily entering toxic bottom water would instantly die.  That is why black mudrocks are repositories of exquisite fossils.  Should H₂S build up in deep water, however, there might be chemical instability that would result in large-scale emissions to the upper ocean and to the atmosphere.  Geochemists from the universities of Pennsylvania and Colorado have made some simple chemical calculations to see if such a potentially catastrophic leakage is within the bounds of possibility (Kump, L.R. et al. 2005.  Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia.  Geology, v. 33, p. 397-400).  Theoretically it is, once a threshold concentration of around 1 mmol kg^-1 of H₂S dissolved in deep water is exceeded.  There would be sulfidic upwellings involving emissions of the order of teratonnes of sulfide per year to the atmosphere; more than 2000 times that today from volcanoes, with the added risk that it would also permeate upper-ocean water.

As well as witnessing mass extinctions, the Late Devonian, end-Permian and Middle Cretaceous were characterized by widespread anoxia.  Leakage of H₂S would not only have killed directly, but would have destroyed the ozone layer that protects from UV radiation.  Inevitably, methane produced by other anaerobic bacteria would also have been released in the same way to force global warming.  Rather than being the result of dramatic impacts or monstrous flood basalt effusions, mass extinctions at these times would have been quiet, but efficient nonetheless

Lichens are not individual species, although they are given Linnaean names, but symbiotic associations of two or more species.  In the lichens the mutual relationship is between entirely different organisms: fungi with either algae or blue-green bacteria.  Although lichen form one of the plagues set to try geologists, their fossil record is extremely sparse.  Once again, Chinese lagerstätten in the Doushantuo Formation establish a first, in this case preserved in phosphorites (Yuan, X. et al. 2005.  Lichen-like symbiosis 600 million years ago.  Science, v. 308, p. 1017-1020).  The fossils show exquisite detail, sufficient to reveal both fungus-like hyphae and cells that resemble those of cyanobacteria.  They are from the late Neoproterozoic, Ediacaran period, when all manner of evolutionary developments were taking place.  One question that is unanswered is whether or not these fossils were marine or subaerial.  Modern lichens are intolerant of salt water.

Methuselah

Since the 1960s claims have been made for the oldest living organism being found in brine inclusions from salt deposits, and most have been dismissed as modern contaminants.  In 2000 that easy avoidance was ruled out by super-sterile culturing of the contents of a fluid inclusion in a Permian halite crystal from New Mexico (Vreeland, R.H. et al. 2000.  Isolation of a 250 million-year-old halotolerant bacterium from a primary salt crystal.  Nature, v. 407, p. 897-900).  The research produced a culture of a salt-tolerant bacterium that was dubbed Virgilbacillus.  However, the odd nature of the crystal could have formed much later than the deposition of the salt beds.  Confirming a Permian age for a fluid inclusion is not easy.  One approach is by comparing the composition and formation temperature of the bacterium-hosting fluid with that from other, more usual inclusions in the same deposit and from fluids that form when salt deposits are exposed to air (“weeps”), as might be included when salt deposits recrystallise long after their formation (Satterfield, C.L. et al. 2005.  New evidence for 250 Ma age of halotolerant bacterium from a Permian salt crystal.  Geology, v. 33, p. 265-268).  The study found that the inclusion fluids along with others from halite at the same level in the salt deposit have significantly different compositions from “weeps”.  The latter reflect the composition of the salts in the deposit which formed by precipitation of the less soluble components of seawater.  The inclusions have compositions more like sea water that has been concentrated by evaporation, albeit different from that of modern halite inclusions.  So it does indeed seem as if Virgilbacillus is a Permian creature.  Yet to emerge are DNA analyses that can be compared with modern salt-tolerant bacteria.