Something large moved 2 billion years ago

More than 50 years ago a group of schoolchildren discovered a fronded fossil (Charnia) in the Precambrian rocks of Charnwood Forest in the English Midlands. Since then it has been clear that multicellular life originated before the Cambrian Period, when the first tangible life had previously been considered to have emerged. Discovery of the rich Ediacaran fauna of quilted, baglike and disc-like animals in 635 Ma old Neoproterozoic sediments in South Australia, and many other occurrences re-established the start of the ‘carnival of animals’ in the Ediacaran Period (635 to 541 Ma). It happened to follow the climatic and environmental turmoil of at least two Snowball Earth episodes during the preceding Cryogenian Period (850 to 635 Ma), which has led to a flurry of suggestions for the transition from protozoan to metazoan life. Yet, applying a ‘molecular-clock’ approach to the genetic differences between living metazoan organisms seems to suggest a considerable earlier evolutionary event that started ‘life as we know it’. That may have been confirmed by a discovery in much older sediments in Gabon, West Africa.

A sequence of shallow-marine sediments in the Francevillian Series in Gabon was laid down at a time of fluctuating sea level around 2100 Ma ago, when the upper oceans had become oxygenated. In them are black shales that preserve an abundance of intricate sedimentary features. Among them are curious stringy structures rich in crystalline pyrite (Fe2S). They are infilled wiggly tubes that lie in the shale bedding. CT scans reveal that the bedding has been flattened around the tubules as it became lithified. So the tubes formed while the sediment was wet and soft (El Albani, A. and 22 others 2019. Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago. Proceedings of the National Academy of Sciences, online preprint; DOI: 10.1073/pnas.1815721116). They look very like burrows. Up to 5 mm across, they can be considered large by comparison with almost all organisms known from that time. The exception comes from the same stratigraphic Series in Gabon. In 2010, El Albani and colleagues published an account of fossils preserved by pyrite that look like fried eggs, 1 to 2 cm across, with scalloped edges. Internal structures revealed by CT scanning include radial slits in the ‘whites’ and folding within the central ‘yolk’. That paper reported the geochemical presence in the host shales of steranes, which are breakdown products of steroids that are unique to eukaryotes. Could these organisms and the wiggly tube-like trace fossils indicate the presence of the earliest metazoans in the Francevillian Series?

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Palaeoproterozoic fossils from the Francevillian Series in Gabon. Top: greytone photographs of burrow-like trace fossils (Credit: El Albani et al. 2019; Fig.1). Bottom: colour photograph and 3 CT scans of discoidal fossil (Credit: El Albani et al. 2010; Fig. 4).

Until the discoveries in Gabon, the oldest organic structure that had been suggested to be a metazoan was the rare Grypania, a spiral, strap-like fossil found in a variety of strata ranging in age from 1870 to 650 Ma. Being made of a structureless ribbon of graphite, Grypania seems most likely to have been made by colonial bacteria. The two Gabon life forms cannot be disposed of quite so easily. The discoids have organised structures rivalling those in Ediacaran animals, while the wiggly tubes clearly seem to indicate something capable of movement. In both cases preservation is by iron sulfide, which suggests the presence at some stage of chemo-autotrophic bacteria that reduce sulfate ions to sulfide. Could these not have formed mats taking up irregular discs and plates? The burrows may have been formed by unicellular eukaryotes, one type of which – the slime moulds – is capable of aggregating together to form multi-celled reproductive structures as well as living freely as single amoeba. Some form slug-like masses that are capable of movement; not metazoans, but perhaps their precursors.

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A unifying idea for the origin of life

The nickel in stainless steel, the platinum in catalytic converters and the gold in jewellery, electronic circuits and Fort Knox should all be much harder to find in the Earth’s crust. Had the early Earth formed only by accretion and then the massive chemical resetting mechanism of the collision that produced the Moon all three would lie far beyond reach. Both formation events would have led to an extremely hot young Earth; indeed the second is believed to have left the outer Earth and Moon completely molten. All three are siderophile metals and have such a strong affinity for metallic iron that they would mostly have been dragged down to each body’s core as it formed in the early few hundred million years of the Earth-Moon system, leaving very much less in the mantle than rock analyses show. This emerged as a central theme at the Origin of Life Conference held in Atlanta GA, USA in October 2018. The idea stemmed from two papers published in 2015 that reported excessive amounts in basaltic material from both Earth and Moon of a tungsten isotope (182W) that forms when a radioactive isotope of hafnium (182Hf), another strongly siderophile metal, decays. Hafnium too must have been strongly depleted in the outer parts of both bodies when their cores formed. The excesses are explained by substantial accretion of material rich in metallic iron to their outer layers shortly after Moon-formation, some being in large metallic asteroids able to penetrate to hundreds of kilometres. Hot iron is capable of removing oxygen from water vapour and other gases containing oxygen, thereby being oxidised. The counterpart would have been the release of massive amounts of hydrogen, carbon and other elements that form gases when combined with oxygen. The Earth’s atmosphere would have become highly reducing.

Had the atmosphere started out as an oxidising environment, as thought for many decades, it would have posed considerable difficulties for the generation at the surface of hydrocarbon compounds that are the sine qua non for the origin of life. That is why theories about abiogenesis (life formed from inorganic matter) hitherto have focussed on highly reducing environments such as deep-sea hydrothermal vents where hydrogen is produced by alteration of mantle minerals. The new idea revitalises Darwin’s original idea of life having originated in ‘a warm little pond’. How it has changed the game as regards the first step in life, the so-called ‘RNA World’ can be found in a detailed summary of the seemingly almost frenzied Origin of Life Conference (Service, R.F. 2019. Seeing the dawn. Science, v. 363, p. 116-119; DOI: 10.1126/science.363.6423.116).

Isotope geochemistry has also entered the mix in other regards, particularly that gleaned from tiny grains of the mineral zircon that survived intact from as little as 70 Ma after the Moon-forming and late-accretion events to end up (3 billion years ago) in the now famous Mount Narryer Quartzite of Western Australia. The oldest of these zircons (4.4 Ga) suggest that granitic rocks had formed the earliest vestiges of continental crust far back in the Hadean Eon: Only silica-rich magmas contain enough zirconium for zircon (ZrSiO4) to crystallise. Oxygen isotope studies of them suggest that at that very early date they had come into contact with liquid water, presumably at the Earth’s surface. That suggests that perhaps there were isolated islands of early continental materials; now vanished from the geological record. A 4.1 Ga zircon population revealed something more surprising: graphite flakes with carbon isotopes enriched in 12C that suggests the zircons may have incorporated carbon from living organisms.

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A possible timeline for the origin of life during the Hadean Eon (Credit: Service, R.F. 2019, Science)

Such a suite of evidence has given organic chemists more environmental leeway to suggest a wealth of complex reactions at the Hadean surface that may have generated the early organic compounds needed as building blocks for RNA, such as aldehydes and sugars (specifically ribose that is part of both RNA and DNA), and the amino acids forming the A-C-G-U ‘letters’ of RNA, some catalysed by the now abundant siderophile metal nickel. One author seems gleefully to have resurrected Darwin’s ‘warm little pond’ by suggesting periodic exposure above sea level of abiogenic precursors to volcanic sulfur dioxide that could hasten some key reactions and create large masses of such precursors which rain would have channelled into ‘puddles and lakes’. The upshot is that the RNA World precursor to the self-replication conferred on subsequent life by DNA is speculated to have been around 4.35 Ga, 50 Ma after the Earth had cooled sufficiently to have surface water dotted with specks of continental material.

There are caveats in Robert Services summary, but the Atlanta conferences seems set to form a turning point in experimental palaeobiology studies.

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