Stress and the Cambrian Explosion

The opening of the Phanerozoic Eon at the base of the Cambrian is, as everyone knows, characterised by the appearance of body fossils of organisms that were preserved because they had calcium-rich hard parts. Thereafter biological diversity grew and grew, despite episodic sets back. Why calcium carbonate and phosphate skeletal parts evolved is still a mystery, although it may have had something to do with an increase in the calcium-ion concentration of seawater. Earth had not long emerged from the last of several truly deep freezes, associated with evidence for which are carbon isotope signals that may indicate repeated mass extinctions of life forms that left few tangible traces. Whatever the truth, it must have lain in some major change in global environmental conditions. Evidence for one such widespread chemical stress has emerged from black shales at the Precambrian-Cambrian boundary in the Oman and China (Wille, M. et al. 2008. Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary. Nature, v. 453, p. 767-769).

Molybdenum, like many transition metals, has several valence states, some soluble in oxidising conditions, others when conditions are reducing. Solution or precipitation when redox conditions change may cause fractionation among stable isotopes, and isotopes of Mo are a case in point. The Swiss-German-US team found that close to the base of the Cambrian the 98Mo/95Mo ratio underwent rapid changes in black shales of Oman and China. They ascribe this to a major upwelling of hydrogen sulfide-rich deep seawater, indeed it would be difficult to suggest any other mechanism that could have caused the shift. Molybdenum is soluble in oxidising waters, and the increase in Mo concentrations in the shales at the time of the isotopic anomaly must mark a shift to reducing conditions in 542 Ma surface seas, hence the link to such an upwelling. Such rises in highly toxic ‘sour gas’-rich water have been suggested as a possible cause for the mass extinctions at the ends of the Permian and Triassic (see Global warming, sour gas and mass extinctions in the January 2007 issue of EPN).

The globally abundant Ediacaran fauna of soft, bag-like and quilted organisms that lived in the late Neoproterozoic has no counterpart in the Cambrian record, even in lagerstätten. Moreover, the Cambrian shelly fauna does not simply spring into place fully formed: it developed over a protracted period and did not simply succeed or evolve from the Ediacaran. It looks like there was the last of a succession of Neoproterozoic mass extinctions at the outset of the Phanerozoic. Indeed the Mo anomalies coincide with abnormally ‘light’ carbon isotopes in the black shales, due the accumulation of massive amounts of dead organisms, and formation of large phosphatic deposits globally.

Yet another blow for creationism

The Devonian transition from fish to four-legged animals is represent by one of the best time sequences showing the development of physical features from one use to another, in their case from fins to legs. Lobe-finned fishes and the earliest amphibians show this nicely, with the missing link of Tiktaalik found in 2006 (see A fish-quadruped missing link in EPN for June 2006) seeming to gild the lily. Now, yet another member of the sequence neatly connects the limb form and function of lobe-fins to the peculiar Tiktaalik (Ahlberg, P.E. et al. 2008. Ventastega curonica and the origin of tetrapod morphology. Nature, v. 453, p. 1199-1204). But perhaps the ID school will consider it a case of the designer continually changing his or her objective.

What, pray, is the platypus?

In a mood of solemn gaiety the world greeted the publication in May 2008 of the the duck-billed platypus or Ornithorhynchus anatinus genome (Warren, W.C. and a very large number of other authors 2008. Genome analysis of the platypus reveals unique signatures of evolution. Nature, v. 453, p. 175-183). My reaction to the title of the paper was, ‘So it blooming well should’. The eponymous platypus has few rivals for oddness: it has a beak for a start; detects its prey using electrosounding; has venom-injecting spurs; females lay eggs but suckle little platypuses, despite having no nipples (the milk is exuded by belly skin when sucked); has fur like an otter; no teeth and the male ejaculates sperm that hunt in packs. It lives in Australia and has kindly eyes. The vast authorship needed considerable space to fully document this strange package of characteristics, leaving little room to expand on the novelty of its genome. In a nutshell, the platypus combines features both reptilian and mammalian: no surprise there. But it is dissimilar from ducks.

Vivipary in armoured fishes

The extinct placoderms  were formidable predators of Silurian to Devonian seas and brackish waterways; in fact they were the vertebrates of those Periods. Being covered by articulated platy armour, their heads are well represented in the fossil record, but their aft parts are not, having been naked of protection. They were anatomically advanced creatures, but succumbed to the late-Devonian mass extinction, unlike other fishes, including those that figure in the ancestry of all terrestrial vertebrates. Placoderms provide the first example of the evolution of live birthing, not to recur until the evolution of the higher mammals in the last 100 Ma. Evidence for placoderm vivipary comes from an astonishing Australian fossil that contain embryos a few centimetres long (Long, J.A. et al. 2008, Live birth in the Devonian period. Nature, v. 453, p. 650-652).

A volcanic nursery for life

Aside from Darwin’s ‘warm, little pond’, all sorts of environments have been suggested for the origin and early nurturing of life. One possibility is in the nutrient-rich cavities between pillows in ocean-floor lavas. The evocative black smokers marking hydrothermal springs on the ocean floor have long been known to host abundant live, from the microbial to the large. Yet the entire volcanic pat of the oceanic lithosphere interacts with water to source hydrothermal vents. The hydration and oxidising reactions that take place in basalts are exothermic, and so yield plenty of energy, both thermal and chemical. This retrogression has offered potential for biological chemautotrophy since mantle-derived magmas first met liquid water; arguably since 4.5 Ga ago. A study of organic infestation of glassy pillowed basalts reveals that today there are up to ten thousand times more prokaryotic cells in exposed seafloor basalts than there are in the overlying seawater (Santelli, C. M. and 7 others 2008. Abundance and diversity of microbial life in ocean crust. Nature, v.  453, p. 653-656). The study relied on RNA sequencing of organic material in the glasses, rather than microscopic examination.

Using thin sections and high-powered microscopes shows up tell-tale signs of the effects of colonisation of surfaces on fractures in oceanic basalt, backed up evidence for the cells themselves. The effects are distinctive and potentially offer a means of judging microbial colonisation of ancient crust, especially that of early Archaean age.

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