Category: Geobiology, palaeontology, and evolution

Probably the greatest ecological truism is that without oxygen there would be no life forms on Earth above the level of a restricted number of prokaryotes. Since around 2.4 Ga, when free atmospheric oxygen first appeared, levels have risen to the present 21% – it was probably as high as ~30% in the Carboniferous and Cretaceous Periods. Charting the rise has been difficult and the history of oxygen is written with a very broad brush. If there had been sudden increases in the availability of oxygen in the atmosphere and oceans there ought to have been a bursts of evolutionary radiation and diversity, but often oxygen-related causality for events such as the Cambrian Explosion have been speculative, as have cases for the inverse, declines due to downturns in oxygen levels (see Oxygen depletion before P-T extinction in the November 2003 issue of EPN). Recently a proxy for the redox chemistry of the global ocean, and therefore for relative changes in atmospheric oxygen, has been developed. It is based on the abundance and isotopic composition of the element molybdenum (Mo) in sedimentary rocks: higher ⁹⁸Mo relative to ⁹⁵Mo (the d⁹⁸Mo value) signifies higher oxygen levels. Its recent use in relation to evolutionary radiations (Dahl, T.W. et al. 2010. Devonian rise in atmospheric oxygen correlated to the radiations of terrestrial plants and large predatory fish. Proceedings of the National Academy of the US, v. 107, p. 17911-17915) has produced interesting results. The US-Swedish-Danish-British team analysed the Mo in euxinic (reduced) marine black shales, which concentrate the element from seawater, in the Proterozoic and Phanerozoic Eons. Increases in δ⁹⁸Mo occur at the time of the Cambrian Explosion, as expected, and also during the Devonian. The latter correlates with increasing diversification of large fishes and among early terrestrial plants, and may have been the greatest leap in the bioavailability of oxygen in Earth’s history, stemming from the ‘greening’ of the land. So far Mo-isotope data have not been obtained from Carboniferous, Permian or Cretaceous back shales, but the ratio of Mo to organic carbon content in black shales of those ages  – a less constrained proxy –  does confirm what has been suspected: highs (greater than present levels) in the Carboniferous and Cretaceous and lows during the Permian and Triassic. However, any hopes that the approach can be calibrated to actual oxygen levels seem likely to be optimistic as the controls over dissolved molybdenum supply to the oceans and its transfer to sediments are extremely complex.

Added 14 January 2011. Some of the team feature in a related article (Gill, B.C. et al. 2011. Geochemical evidence for widespread euxinia in the Later Cambrian ocean. Nature, v. 469, p. 80-83) that ticks all the geochemical boxes for the evolutionary effects of depleted oxygen; i.e. extinctions. They use new measurements of sulfur isotopes in conjunction with published carbon-isotope  and other geochemical data from a wide range of Late Cambrian sediment types and environments in six well-known sections of that age. Spikes in the relative abundance of ³⁴S match those in ¹³C along with a decrease in Mo in one section (see above), suggesting temporary increases in carbon and sulfide burial during periods of oxygen deficiency in the Late Cambrian ocean. Massive sequestration of organic carbon may have led to the extremely cold Late Cambrian climate, as described in A chilly Late Cambrian (this issue). Combined with changes in redox conditions associated with ocean anoxia this would have especially stressed animals, even on continental shelves had oxygen depleted water risen from the depths where sulfur and carbon burial were going on.

See also: Shields-Zhou, G. 2011. Toxic Cambrian oceans. Nature, v. 469, p. 42-43.

Related Articles

• Oxygen crash led to Cambrian mass extinction (newscientist.com)

Though it is highly likely that burial of fossils for millions of years destroys any trace of their DNA the massive bones of large creatures can preserve cell material. A near complete 67 Ma old Tyrannosaurus rex, fondly known as ‘Big Mike’ has revealed blood cells in thin sections of its bone (Schweitzer, M.H. 2010. Blood from stone. Scientific American, v. 303 (November 2010), p. 38-45). Her article also covers traces of blood vessels, and collagen of similar antiquity. The research involved positive reaction of antibodies against proteins, thereby proving the materials to be organic and not products of biomineralisation formed during the process of fossilisation. Potentially such forensic work can tease out relationships among animal groups whose fossils preserve organic materials, in a similar way to indications of the rise of prokaryote groups by biogeochemical marker molecules in carbonaceous shales. Indeed, sequences of fossil proteins from dinosaurs closely resemble that of modern birds. One of the great surprises of the late 20^th century was the growing evidence that the stem-line for birds was dinosaurian, specifically the theropod group. This is nicely summarized by another review article (O’Donoghue, J. 2010. Flight of the living dead. New Scientist, v. 208 (11 December 2010), p. 36-40) that addresses the certainty of birds’ evolution from dinosaurs; which of the fossils is bird, which feathered dinosaur and when did they separate; and why did birds survive the end-Cretaceous mass extinction while dinosaurs famously succumbed – probably a matter of breeding; its pace, that is. The two articles together suggest a fruitful way forward for palaeobiologists.

Further material about biochemical relics in fossils and methods used to detect and analyse them can be found in Hecht, J. 2011. Waking the dead. New Scientist, v. 209 (22 January 2011 issue), p. 43-45.

The mainstay of geobiologists’ efforts to chart the timing and pace of mass extinctions and diversification since 1997 has been the monumental collation of information in fossil collections undertaken by the late Jack Sepkoski from the 1980s until shortly before his death in 1999. It was his plotting of marine fossil genera numbers against their time ranges that first quantified the ‘Big Five’ and lesser mass extinctions, and the course of re-diversification that followed in their wake. One problem that Sepkoski was unable to account for was the inherent biases in collections: under-representation of earlier genera than younger ones; different representation from different areas partly because developed-world collections are larger than those from the majority world and partly because modern diversity changes with latitude; and varying preservation of less-substantial organisms. Well aware of the shortcomings of his initial compilations, Sepkoski with others set up the Palaeobiology Database (PBDB) that now encompasses almost 100 thousand collections. Sadly, Sepkoski did not live to analyse this record with statistical methods that lessen the influence of bias, but one of his successors has done just that (Alroy, J. The shifting balance of diversity among major marine animal groups. Science, v. 329, p. 1191-1194). Alroy’s approach sets out to represent the rare with a fair weighting relative to common groups of organisms, using a complex multivariate method called ‘shareholder’ sampling, which corrects some of the artefacts in Sepkoski’s work and earlier manipulation of the PBDB.
One important feature is that Alroy’s method does not assume that all groups follow the same ‘rules’ of diversification and adaptive radiation, particularly after mass extinctions. The upshot is a history with ups and downs, but not such a prominent growth in diversity in the late-Mesozoic and Cenozoic Eras as that in Sepkoski’s original compilation, although life did become richer. For someone, like me, who has not followed the developments since Sepkoski’s original work, there is another significant difference. There are 7 or 8 significant falls in diversity rather than 5. The Triassic-Jurassic boundary no longer shows a mass extinction, but the opposite. Major extinctions show up for the mid-Carboniferous, mid- and end-Jurassic and the Oligocene, where none were especially noticeable in the original plots by Sepkoski. Finally diversity peaks in the Siluro-Devonian and the Permian figure as prominently as that of the late-Cretaceous. Clearly, rules are few and one that was almost an assumption, that diversification of marine life after mass extinctions was exponential, is no longer borne out. Whether or not this new approach will bear fruit in refining or redefining the ecological dynamics that shaped and continue to shape life on Earth remains to be seen. It is tempting to be a bit cynical: is it all punctuated chaos (Bennett, K. 2010. The chaos theory of evolution. New Scientist, v. 208 (16 October 2010), p. 28-31)?

Most researchers concerned with the origin of life acknowledge that some preparatory organic chemicals would have been required, whose origin Darwin ascribed to a ‘warm, little pool’, and Haldane and Oparin to electrical discharges in the early atmosphere; both lines having been followed-up in practice by more recent scholars. A variety of biologically useful chemical ‘building blocks’ have also been recognised in comets, some meteorites – carbonaceous chondrites – and even in interstellar dust clouds. So one school looks to their supply from outside the Earth system. One possibility has had more scanty attention – the effects of impacts, as the power involved seems overwhelming for the survival of delicate organic molecules. Nir Goldman and his colleagues at the Lawrence Livermore National Laboratory in California have had a second look at this unlikely scenario (Goldman, N. et al. 2010. Synthesis of glycine-containing complexes in impacts of comets on early Earth. Nature Chemistry, v. 2, p. 949–954). Their approach has been to examine the implications of impact shock at likely collision speeds followed by post-shock expansion on mixtures of water, ammonia, carbon monoxide and dioxide, and methanol that are almost guaranteed in the make-up of most cometary ices. Their modelling suggests that carbon-nitrogen bonds form under shock conditions in long chain compounds. In the aftermath of huge collision shock the impact products undergo rapid expansion and cooling during which the chains can break down to simpler molecules, including some akin to amino acids such as glycene. The bombardment of Earth in the Hadean Eon (4.5-3.8 Ga) involved huge masses of material, almost certainly some delivered by icy comets that would have greatly increased the amount of water and the number of CHON compounds in the early Earth’s outer parts.

The mainstay of geobiologists’ efforts to chart the timing and pace of mass extinctions and diversification since 1997 has been the monumental collation of information in fossil collections undertaken by the late Jack Sepkoski from the 1980s until shortly before his death in 1999. It was his plotting of marine fossil genera numbers against their time ranges that first quantified the ‘Big Five’ and lesser mass extinctions, and the course of re-diversification that followed in their wake. One problem that Sepkoski was unable to account for was the inherent biases in collections: under-representation of earlier genera than younger ones; different representation from different areas partly because developed-world collections are larger than those from the majority world and partly because modern diversity changes with latitude; and varying preservation of less-substantial organisms. Well aware of the shortcomings of his initial compilations, Sepkoski with others set up the Palaeobiology Database (PBDB) that now encompasses almost 100 thousand collections. Sadly, Sepkoski did not live to analyse this record with statistical methods that lessen the influence of bias, but one of his successors has done just that (Alroy, J. The shifting balance of diversity among major marine animal groups. Science, v. 329, p. 1191-1194). Alroy’s approach sets out to represent the rare with a fair weighting relative to common groups of organisms, using a complex multivariate method called ‘shareholder’ sampling, which corrects some of the artefacts in Sepkoski’s work and earlier manipulation of the PBDB.

One important feature is that Alroy’s method does not assume that all groups follow the same ‘rules’ of diversification and adaptive radiation, particularly after mass extinctions. The upshot is a history with ups and downs, but not such a prominent growth in diversity in the late-Mesozoic and Cenozoic Eras as that in Sepkoski’s original compilation, although life did become richer. For someone, like me, who has not followed the developments since Sepkoski’s original work, there is another significant difference. There are 7 or 8 significant falls in diversity rather than 5. The Triassic-Jurassic boundary no longer shows a mass extinction, but the opposite. Major extinctions show up for the mid-Carboniferous, mid- and end-Jurassic and the Oligocene, where none were noticeable in the original plots by Sepkoski. Finally diversity peaks in the Siluro-Devonian and the Permian figure as prominently as that of the late-Cretaceous. Clearly, rules are few and one that was almost an assumption, that diversification of marine life after mass extinctions was exponential, is no longer borne out. Whether or not this new approach will bear fruit in refining or redefining the ecological dynamics that shaped and continue to shape life on Earth remains to be seen. It is tempting to be a bit cynical: is it all punctuated chaos?

Comet impacts’ candidature for origin of life

Most researchers concerned with the origin of life acknowledge that some preparatory organic chemicals would have been required, whose origin Darwin ascribed to a ‘warm, little pool’, and Haldane and Oparin to electrical discharges in the early atmosphere; both lines having been followed-up in practice by more recent scholars. A variety of biologically useful chemical ‘building blocks’ have also been recognised in comets, some meteorites – carbonaceous chondrites – and even in interstellar dust clouds. So one school looks to their supply from outside the Earth system. One possibility has had more scanty attention – the effects of impacts, as the power involved seems overwhelming for the survival of delicate organic molecules.  Nir Goldman and his colleagues at the Lawrence Livermore National Laboratory in California have had a second look at this unlikely scenario (Goldman, N. et al. 2010. Synthesis of glycine-containing complexes in impacts of comets on early Earth. Nature Chemistry, v. 2, p. 949–954). Their approach has been to examine the implications of impact shock at likely collision speeds followed by post-shock expansion on mixtures of water, ammonia, carbon monoxide and dioxide, and methanol that are almost guaranteed in the make-up of most cometary ices. Their modelling suggests that carbon-nitrogen bonds form under shock conditions in long chain compounds. In the aftermath of huge collision shock the impact products undergo rapid expansion and cooling during which the chains can break down to simpler molecules, including some akin to amino acids such as glycene. The bombardment of Earth in the Hadean Eon (4.5-3.8 Ga) involved huge masses of material, almost certainly some delivered by icy comets that would have greatly increased the amount of water and the number of CHON compounds in the early Earth’s outer parts.

Being multicellular does not necessarily qualify a fossilised organism as being a member of the eukaryote domain: such a classification is assured when there is strong evidence for many cells constituting a functional whole with specialised parts. That eukaryotes also have cells with nuclei and a variety of organelles is a prerequisite for living members, though such evidence is extremely rare and disputed for fossils, and the earliest convincing examples are from 1700 Ma sedimentary rocks. Using a molecular clock approach to the differences in genetic make-up between modern eukaryotes might seem one means of estimating when the last common ancestor of all of them lived, but the Catch-22 is having incontrovertible examples from the distant past as means of calibrating that approach. A fourth possible ‘fingerprint’ is the presence of biomarker chemicals in sedimentary rocks that are exclusive to living Eucarya, steranes derived from sterols being an example.

Since the 1970s the oldest candidate for eukaryote status has been a coiled form a few centimetres across made from a strap-like carbon film, known as Grypania that some regard as a primitive alga. Yet it could equally be a colonial bacterium. Grypania are know as far back as those found in the 1900 Ma ironstones of Michigan, USA. Thin black shales from a mixed marine and terrestrial sequence of 2100 Ma siltstones and sandstone in Gabon, West Africa now provide something far more spectacular (El Albani, A. and 21 others 2010. Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago. Nature, v. 466, p. 100-104). They are complex and look a little like an irregular discus 1-2 cm across. Being replaced by fine grained iron sulfide they preserve odd internal structures discernible using X-ray tomography – folds of their central node – signs of flexibility in the original material – and scalloped flanges with radial slits. To the authors this suggests coordinated growth rather than the amorphous characteristics of bacterial biofilms such as stromatolites. They are completely unlike any living colonial bacteria. Their host rocks have yielded steranes characteristic of eukaryote biochemistry, but contamination from groundwater cannot yet be ruled out.

Only the one and a half billion year younger Ediacara fauna comes close in terms of complexity of form to the Gabon fossils. Yet whether they are the earliest-known eukaryotes or bacterial colonies whose growth was coordinated between the cells of which they were composed by some unknown exchange of information cannot be pinned down. However, their age is appropriate for the rise of the oxygen-demanding Eucarya, a few hundred million years after the start of planetary oxygenation. Perhaps more important, the surprising find will give palaeobiologists the impetus and confidence that large body fossils can indeed be found in the Palaeoproterozoic Era.
See also: Donoghue, P.C.J & Antcliffe, J.B. 2010. Origins of multicellularity. Nature, v. 466, p. 41-42.

Ordovician lagerstätte in Morocco
It was during the Ordovician Period that multicelled life really took off (see The Great Ordovician Diversification in the September 2008 issue of EPN), but the fossil record seems to suggest that the wonderfully diverse soft-bodies fauna of the Cambrian, exemplified by that from the Burgess Shale, didn’t survive to take part. It turns out that this may be an artefact of imperfect preservation, for a Lower Ordovician equivalent of the Burgess Shale has been unearthed in Morocco (Van Roy, P. et al. 2010. Ordovician faunas of Burgess Shale type. Nature, v. 465, p 215-218). It is just as rich and even shows more organic detail, highlighted in reds, oranges and yellows because iron sulfide that mineralised soft parts has since been gently oxidised. A fascinating link with the Burgess Shale is that the fossil taxa from the Moroccan lagerstätte are related to those in that most famous Middle Cambrian rock unit.

On the subject of exceptionally preserved fossil material, one of the Burgess Shale oddities, new specimens of Nectocaris pteryx allow a detailed reconstruction. What a stunning beast! This 5 cm stem-group cephalopod had two tentacles, enormous stalked eyes and a funnel shaped device that may have been for squid-like jet propulsion. Reconstruction of its back-end suggests a cuttlefish-like means of propulsion by a flap of tissue around the main body, but with no sign of any stiffening ‘bone’.

Possible abiotic mechanisms for DNA splitting and cell membranes
The central feature of the DNA helix is its ability to ‘unzip’ and recombine as part of the replication that is essential for all known living things. In doing this, DNA copies itself. It is one thing to deduce this from DNA’s structure and the meiotic aspect of reproduction, but quite another to figure out how it might have arisen. An experiment that mimics conditions in porous sea-floor lava – a temperature gradient and small-scale convection inside a capillary tube – shows that this lengthways splitting does occur on the hot side of the gradient. On the cool arm of the convection the halved ribbons of DNA reassemble (Mast, C.B. & Braun, D. 2010. Thermal trap for DNA replication. Physical Review Letters, v. 104, p. 188102-188105). This is a long way from life’s origin and even that of DNA as an isolated entity without a cell, but perhaps one step towards a better understanding of both. It seems pretty certain from a range of evidence – e.g. heavy metal centred proteins and heat-shock proteins – that life sprang from physical and chemical processes around hot vents on the ocean floor. What’s next on the experimental agenda: membranes to bag-up genetic material such as DNA as a precursor to the cell? It’s been done (Budin, I. et al. 2009. Formation of protocell-like vesicles in a thermal diffusion column) using fatty acids that are relatively easy to generate abiotically. Some can transform into flexible membranes that curve in on themselves – amphiphiles – and vesicles of these formed in Budin et al.’s capillary tubes.
See also: McAlpine, K. 2010. Life cooked up in undersea cauldrons. New Scientist, v. 206 (29 May 2010 issue), p. 14.

Apart from the change in name from the K-T (Cretaceous-Tertiary) to the K-Pg (Cretaceous-Palaeogene) Event, following the abolition by the International Commission on Stratigraphy of the name Tertiary – given by Giovanni Arduino to the penultimate geological Era, in favour of Cenozoic (Palaeogene + Neogene + Quaternary) the eponymous mass extinction has steadily become a less regular news item. Views had settled in to three camps: driven by an impact; by Deccan volcanism or by the two conspiring together. Yet a host of geoscientists, from institutions whose addresses take up 8 column inches in Science, have been beavering away to settle the issue one way or another (Schulte, P. and 40 others 2010. The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary. Science, v. 327, p. 1214-1218).  The main biotic changes and geochemical signatures of the K-Pg Event all coincide at 65.5 Ma with the world-wide Chicxulub ejecta layer, after two thirds of the Deccan Traps had been erupted. In an extensive and readable summary of all the evidence the authors conclude that the Chicxulub impact did trigger the massive die-off. Despite global change associated with volcanism, life went on ‘down to the wire’ (a wire once marked the finish line in horseracing). The authors rule out the Deccan volcanism as a causative factor on account of little more than a 2º C warming effect while it lasted, set against the likely near-instantaneous release of at least 100-500 billion tons of SO₂ by an impact into massive sulfate-rich sediments around the Chicxulub site (the release by Deccan volcanism has been estimated at 0.05 to 0.5 Gt per year throughout its million-year duration). Such a release along with dust and water vapour flung into the atmosphere are modelled to have reduced global temperatures by up to 10º C – a reduction greater than that reached by the last glacial maximum. The re-entry of such a mass in rainfall within a few years would have acidified large areas of surface ocean water: a 3-4 orders of magnitude larger effect than that of slow release by volcanism. The authors conclude that the most important remaining work is to delve deeper into the impact site itself to quantify likely chemical emissions, and then to develop models of the actual deadly processes that ensued.

The finding of Tiktaalik, a supposed ‘missing link’ between bony fishes and amphibians (see A fish-quadruped missing link in EPN issue for May 2006) seemed to resolve the descent of tetrapods nicely. As is common, if inconvenient, nature has thrown a spanner in the works through a remarkable find in Polish rocks much older than those containing Tiktaalik and more evolved tetrapods (Niedźwiedski, G. et al. 2010. Tetrapod trackways from the early Middle Devonian period of Poland. Nature, v. 463, p. 43-48). Quarrymen unearthed extensive tracks appeared during excavation of intertidal limestones of the Middle Devonian Eifelian Stage (392-398 Ma). The bedding surface also shows raindrop pits and desiccation cracks, so the tracks were made by creatures able to survive out of water. The prints (up to 26 cm wide) are three times bigger than the paws of later amphibians that left fossil remains, but like them they show signs of more than 5 toes. The maker of one trackway was a good walker, having left no trace of dragging its belly through the mud, and it either had no tail or carried it aloft since there is no trail left by a tail either. Another, smaller animal left a separate trackway showing a very different gait. There seems little doubt that these animals were well advanced towards completely terrestrial lifestyles. Tiktaalik from 380 Ma sediments in Arctic Canada obviously cannot have been ancestral to them, and nor are there any fossils from the Middle Devonian that look like candidates. The hunt is on for fossilised remains of whatever walked the walk, and may emerge in the not-too-distant future from subtidal sediments of the same formation.

See also: Janvier, P. & Clément, G. 2010. Muddy tetrapod origins. Nature, v. 463, p. 40-41.

‘Roger, I think that triffid just moved’

The nasty surprise awaiting the bulk of human population blinded by radiation from a meteor shower in John Wyndham’s Day of the Triffids was that the genetically engineered, oil-yielding triffid plants could not only deal out deadly stings but they walked and ate dead meat. So it is that palaeontologists have found with the flabby, quilted bag-like organisms of the late Neoproterozoic Ediacaran fauna. They were animals of some kind, but hitherto considered to be completely sessile, except in larval form. They seem not to have been able to bite or gnaw, but probably absorbed victuals through their skins. Imagine the shock when palaeontologists from Oxford and Memorial University of Newfoundland found trackways in the famous biome of Mistaken Point in Newfoundland (Liu, A.G. et al. 2010. First evidencee for locomotion in the Ediacaran biota from the 565 Ma Mistaken Point Formation, Newfoundland. Geology, v. 38, p. 123-126). This throws an entirely new light on the very first sizeable animals: some of them were muscular. But not very adventurous, for the trails are only up to 17.2 cm long. Several of the traces show curved ridges, much like though far smaller than those left in wet sand by a buttock-shuffling baby, but ascribed by the authors to use of an ‘inflatable pedal disk’ in the manner of some cnidarians today – they ‘blurted’ along no doubt. The darned things must have had a purpose in moving, and chasing down prey springs easily to mind, only to be swiftly rejected. Alarmingly, at least for their totally torpid companions, some of the trackways clearly end in a depression: did they lie in wait? Yet not a one shows the telltale three-fold pedestal symmetry of Wyndham’s triffids…

Believable Archaean fossils

Some years back a major spat broke out over the reality of microscopic features purported to be evidence for bacterial life in 3.5 Ga rocks from Western Australia (See Doubt cast on earliest bacterial fossils in April 2002 issue of EPN), which has rumbled on ever since among highly regarded groups of palaeontologists. Those who refuted those finds as merely mineralogical structures that just seem to look biogenic have more work pending. Much more convincing evidence has been found in 3.2 Ga cherty rocks from South Africa (Javaux, E.J. et al. 2010. Organic-walled microfossils in 3.2-billion-year-old shallow marine siliciclastic deposits. Nature, v. 463, p. 934-938).  They are big, by microfossil standards, 3-dimensional structures up to a third of a millimetre across, and clearly resemble cells. Some have even been separated from their matrices by dissolving away silica with hydrofluoric acid, so are not merely figments of the authors’ imagination. They are carbonaceous with very negative δ¹³C values typical of organically processed carbon and show abundant evidence of intricate structures found in living cells. Raman infrared spectroscopy also shows that they have been metamorphosed at the same grade as the rock that host them, so they cannot be later contaminants. In all these respects the little spherules are a cut above previously described structures reckoned to have been early Archaean life forms, convincingly taking concrete evidence for the existence of living things back a remarkable billion years: the previous oldest true fossils are about 2.2 billion years old.

In one respect the find may be truly breath taking. Spherules this size cannot be from the life-domain Archaea, and at the very least they are particularly large cells of Bacteria. Yet, bacterial cells contain little that could produce such robust little objects, which resemble single-celled eukaryotes known as acritarchs. The earliest definite acritarchs data back to 1.8 Ga. Geochemical evidence for eukaryotes was not sought in the spherules, but there has been speculation that some Archaean rocks have yielded chemical biomarkers that point to the presence of the ancestors of multicelled life at an astonishingly early date in Earth’s history. Clearly Javaux and colleagues work is a precursor of a lot more, now that we have hard-to-refute evidence for 3.2 Ga life.

A ginger dinosaur

The Early Cretaceous of SE China has become justifiably famous by providing a regular supply of superbly preserved small dinosaurs and early birds believed to have had a dinosaurian ancestry in the Jurassic. We have become accustomed to seeing computer generated graphics of brightly coloured dinosaurs since the BBC series Walking with Dinosaurs, first broadcast in 1999, but they owe more to imaginative assumptions based on strongly patterned living lizards than to fossil evidence. That is set to change, with the discovery of actual colouring agents in a Chinese find (Zhang et al. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature, v. 463, p. 1075-1078). The melanosomes are in exquisitely preserved feathers that adorned and probably warmed small dinosaurs as well as the famous bird fossils from the same sedimentary rocks. One specimen of Sinornithosaurus may have sported a coat patterned in black and russet, while Sinosauropteryx seems to have had a tail and back crest striped in shades of red-brown. Could this be for camouflage, display or some aspect of regulating heat? The big leap follows some 6 months on from the discovery of melanosomes in bird feathers from Eocene oil shales in Germany, that may have given them a starling- or hummingbird-like iridescent sheen (Vinther, J. et al. 2009. Structural coloration in a fossil feather. Biology Letters, v. 6, p. 128-131). The huge diversity of modern coloration among birds, from feathers and in the skins of lizards is widely believed to function primarily as a species-dependent means of display, with some influence from camouflage and thermal properties. Whichever, it must have been an integral aspect of speciation for a very long time indeed, yet even the best fossils cannot yield full ornament information, and reconstructions will rely on artistic licence, but now with a little more confidence that creatures didn’t just come in one colour, like Model-T Fords.

To spice up the stereotypical view that ginger = bad-tempered it seems that as well as being mottled with that hue Sinornithosaurus may have been venomous (Gong, E. et al. 2010. The birdlike raptor Sinornithosaurus was venomous. Proceedings of the National Academy of Science, v. 107 p. 766-768). Its skull shows grooved teeth, the grooves leading to a pocket at the base of the teeth. It may also have evolved to feed on birds…

Coming up with a theory for the origin of something so complex and ancient as life on Earth might seem to be at the pinnacle of hubris, yet such ideas are not uncommon. A novel slant on the ‘Big Question’ centres on how cells get their energy, rather than on trying to put together all manner of chemical prerequisites (Lane, N. 2009. The cradle of life. New Scientist v. 204 (17 October 2009) p. 38-42). Mike Russell began his career as a geochemist looking at hydrothermal mineral deposits and the intricacies of their formation, while at the University of Strathclyde, Scotland. He now works at NASA-JPL in Pasadena, California inspired by the views of a self-funded eccentric Cornish farmer, Peter Mitchell. Cell energetics, according to Mitchell, are about pumping protons through cell membranes to effect the oxidation and reduction fundamentals of metabolism; in short electrochemical gradients. That is now recognised by every cell biologist, though once it was considered absurd. Russell’s take on that novel truism is that the environment of life’s origin must have involved similar processes taking place in the absence of living cells, which inherited proton pumping. His choice is mineralised pinnacles full of foam-like voids that can act as minute chemical factories: not the famous sulfidic black smokers of ocean ridge systems, but cooler features formed of carbonates precipitated from alkaline sea-floor hydrothermal vents. The carbonate foam in ancient examples, well-known to Russell from their mineralisation, contains bubbles lined with iron sulfides. Sulfides are known to have catalytic properties; proteins in living cells that convert CO₂ to sugars have Fe-S bonds at the core of their structure; alkaline hydrothermal vents emit hydrogen released by alteration of olivine in ocean-floor basalt to serpentine minerals; bubbles in carbonate foam look very like potential precursors to cells. To produce the first living cells, these features together in one enclosed space need 10 steps of quite simple chemistry. Except, that is, for nucleic acid production…

End-Permian crisis not so bad for ammonites

The greatest known mass extinction at the end of the Permian Period snuffed out 85% of fossil marine species. It is widely understood to have taken at least five million years for ecosystems to begin recovering, and some animal groups remained depressed for longer still, especially those living at or near the sea floor. Yet one group of cephalopods, the ceratidid ammonites, almost immediately began to thrive, despite the ammonoid sub-Class having been among the hardest hit groups (Brayard, A. et al. 2009. Good genes and good luck: ammonoid diversity and the end-Permian mass extinction. Science, v. 325, p. 1118-1121). Only three genera of ceratidids survived the cataclysm, but within 1-2 Ma there were almost 100 representatives. A similar swift recovery is shown by the completely unrelated conodont animals (now-extinct eel-like vertebrates whose teeth are generally the only parts to be fossilised). For such a success story to emerge by pure chance seems intuitively unlikely: for cephalopod  equivalents of Lazarus to go forth and multiply so nicely requires genes well-suited to the conditions that followed the mass extinction.

Most geologists would answer, ‘The continents after the start of the Silurian Period’, and from now on they could be wrong. Evidence for an earlier ‘greening’ of the land comes from a detailed analysis of thousands of oxygen- and carbon-isotope measurements in Neoproterozoic carbonate rocks (Knauth, L.P. & Kennedy, M.J. 2009. The Neoproterozoic greening of the Earth. Nature, v. 460, p. 728-732). An important consideration in understanding the geochemistry of limestones is that however they originally formed as wet sediments at some later stage their constituents were largely transformed into crystalline aggregates by lithification through the intermediary of pore fluids. During lithification chemistry is equilibrated between crystals and the pore fluids, so if pore fluids are chemically (in this case isotopically) different from the sediment the resulting rock will have been changed isotopically. Studies of Cenozoic carbonates strongly suggest that the place where carbonate sediments are lithified most quickly is in coastal areas where terrestrial groundwater mixes with marine formation water in sediments. Since colonisation of the land by photosynthesising organisms groundwater C- and O-isotopes evolves in equilibrium with those organisms. The terrestrial biomass fixes ¹²C preferentially thereby depleting their proportion of ¹³C by up to 20‰. Groundwater, having originated as water vapour evaporated from the oceans that acts preferentially on ¹²O is also depleted in ¹⁸O. Consequently, low δ¹³C and δ¹⁸O signatures are passed on to groundwater and thence to carbonate rocks when groundwater participates in lithification.

Neoproterozoic carbonates plot in the same δ¹³C vs δ¹⁸O fields as those from the Phanerozoic. Earlier Precambrian carbonate data plotted in the same way show depletion in δ¹⁸O but not in δ¹³C, which signifies no terrestrial life, but normal preferential evaporation of ¹⁶O from the ocean surface to form rain and then groundwater. Knauth and Kennedy’s results suggest a strong likelihood that carbonates of the late Precambrian were lithified by groundwater from a land surface where photosynthetic organisms were well-established and abundant. There is likely to be a sceptical backlash to this remarkable conclusion, largely because it seems that the terrestrial biomass in the Neoproterozoic would have needed to be of the same order as that in later times. Yet molecular evidence from modern fungi, lichens, liverworts and mosses suggests that they evolved in the Neoproterozoic and Chinese scientists have found traces of what look remarkably like lichens in the 600 Ma Doushantuo lagerstätte – fungus-like hyphae and cells that resemble those of cyanobacteria (see The earliest lichens in May 2005 issue of EPN). In an earlier paper, Martin Kennedy had noted that around 700 Ma, the record of marine limestones show increasing ⁸⁷Sr/⁸⁶Sr ratios, suggesting an increase in the chemical weathering of ancient continental rocks. That may have coincided with biological agencies helping break down bare rock chemically to swelling clays that show a surge in Neoproterozoic sedimentary sequences (see Clays and the rise of an oxygenated atmosphere in March 2006 issue of EPN). The same paper pointed out that such clays increase the chances of preservation of buried organic matter, thereby boosting build-up of atmospheric and dissolved oxygen, as would terrestrial photosynthesisers. The feedback of increased oxygen to other eukaryotes that had evolved as heterotrophic animals would have enabled them to increase in size. Interestingly the earliest fossil animals occur in the same Chinese lagerstätte as the putative terrestrial photosynthesisers.

See also: Arthur, M.A. 2009. Carbonate rocks deconstructed. Nature, v. 460, p.698-699; Hand, E. 2009. When Earth greened over. Nature, v. 460, p.161.