Chapter 12: What life is all about

Sections

Diversity built on chemistry

A hidden empire

Cohabitation and the eukarya

Life’s fundamental division is twofold, into prokaryotes and eukaryotes. Prokaryotes are exclusively single-celled and consist of just a bag that encloses a watery fluid (the cytoplasm). In the fluid are millions of organic molecules (fats, proteins and carbohydrates), thousands of RNA molecules in ribosomes and a single large DNA molecule, usually in a convoluted loop (Figure 12.1a). A prokaryote can do only two things, but it does them well. It uses genetic coding in its DNA to replicate and form two smaller copies of itself, which then grow to reach the original size and it reproduces by cloning. The cell manufactures everything that it needs for replication, growth and repair inside itself, drawing energy and raw materials from its surroundings through its enveloping membrane.

Figure 12.1 Basic architecture of cells: (a) the prokaryote design; (b) a simple eukaryote animal cell. Plants also contain organelles called chloroplasts

The basic eukaryote cell (Figure 12.1b) is a barrel of fun by comparison with a prokaryote. It is still just a bag full of watery cytoplasm containing simple organic compounds and a little RNA, and it also draws in energy and raw materials through the cell wall. There the similarity ends. Eukaryote cells envelop a number of smaller bodies or organelles (‘little tools’, from the Greek). The most prominent of them is the nucleus surrounded by a double membrane, and therein lie DNA molecules bound together in chromosomes, whose antics form much of the basis both for sex and for the more familiar aspects of heredity and evolution. We return to those topics a little further on. Apart from the nucleus, a eukaryote cell may also contain such delights as the vacuole, two kinds of endoplasmic reticulum and the ominously named Golgi apparatus. On these we shall not dwell. Two other kinds of organelle are more important; the chloroplast (plants only) and the mitochondrion, which are enveloped, like the nucleus, by a double membrane.

Almost all eukaryotes have mitochondria in their cells, and it is these organelles that perform most of the conversion of raw materials to organic compounds on which the  entire organism depends. The cells generate waste materials and can only function if oxygen is available. That is why virtually all eukaryotes, both plants and animals, are aerobic. There are a few that can only survive in oxygen-free environments such as animal guts and they lack mitochondria. One is Giardia and makes the world fall out of your bottom if you are unlucky enough to drink some types of polluted water. Only plant cells contain chloroplasts, and they perform photosynthesis, the simplest of all the kinds of metabolism among the eukaryotes. Chloroplasts use light as an energy source in assembling organic compounds from water and CO₂, producing oxygen as a waste product. Both mitochondria and chloroplasts have their own complement of DNA, but not in a form that participates in eukaryote sex and the inheritance that stems from it. Such non-nuclear DNA is passed on to offspring, but the vast bulk of it comes from the female without mixing in half shares of DNA from the male, as happens to nuclear DNA during reproduction. So mitochondrial DNA (mtDNA) is passed on from ‘mother’ to offspring in identical copies, except for random mutation. This makes mtDNA in animals like ourselves very useful in studying maternal relationships.

The vast diversity in form and ecological function of the eukaryotes, the most obvious members of the biosphere today, boils down to only two basic ways of life. On the other hand, the simple form of prokaryotes masks a far wider diversity of opportunities for living. Both eukaryotes and prokaryotes exploit laws of chemical combination and dissociation; and employ various sources of energy to do the work. The most familiar metabolism to us, simply because it is fundamentally what we do, is to build cells from complex organic molecules using them as a source of energy to do this. In other words we consume ‘food’ already made by other organisms. Such metabolisers are heterotrophs (‘nourishers from others’ in Greek).  The other basic lifestyle is altogether more simple, by building organic molecules from scratch. The essential CHON and other elements are available in air, water and soils. Energy to combine them comes from inorganic sources too, principally the Sun, but also from simple chemical reactions. Metabolisers following this route are autotrophs (‘self-nourishers’). The most familiar autotrophs, and those that comprise most of the mass of the modern biosphere are green, photosynthesizing plants. They are at the base of the food chain of eukaryotes like us, who are heterotrophs whether we are vegans or not.

Bar a few loathsome forms, such as Giardia, all eukaryotes, hetero- or autotrophic, employ only one basic style of metabolism. They are aerobic, and depend on oxygen. The plant base of the eukaryote food chain uses sunlight as an energy source; it is photo-autotrophic. Here we temporarily leave the eukaryotes for a look at a much more basic chemistry of living, and at the amazing range of ways in which the lowly prokaryotes exploit quite simple chemistry.

Diversity built on chemistry

At the centre of the energy flows in metabolism are sugars (carbohydrates) of which glucose is the most universal. Sugars are simple molecules based on short chains of carbon bonded to hydrogen and oxygen – the three most common reactive elements in the cosmos. The simplest sugar molecule of all has one carbon, one oxygen and two hydrogens; it isn’t even a chain. Formaldehyde, or formalin, is not at all sweet and has a lugubrious use in storing cadavers. Put a spoonful of common salt in water containing formaldehyde, and lo and behold, out of corruption comes forth sweetness (NOT TO BE TRIED!). The HCOH (CH₂O) formaldehyde molecules spontaneously link to form sugar chains, because alkali metal ions, such as sodium in the salt, act as catalysts for this polymerisation. The links can involve 2, 3, 4, 5 and 6 carbon atoms giving C₂, C₃, to C₆ sugar chains. The C₅ sugars or pentoses, ribose in particular, are vital building blocks in nucleic acids. Glucose and other better known sugars are of the C₆ variety. Glucose is broken down to CO₂ and water with the release of energy through the agency of a much more complex molecule, adenosine triphosphate or ATP. This is the most important store of useable energy for living things and it is worth examining what happens in more detail.

Two ATP molecules yield a phosphate ion to glucose and thus change to adenosine diphosphate ADP. These next combine to yield ATP and adenosine monophosphate AMP. The last is a nucleotide otherwise named adenine-deoxyribose-phosphate. Meanwhile, the phosphate-endowed glucose breaks down energetically to CO₂ and water, releasing phosphoric acid, which combines with AMP to make ATP again. In this energy-releasing cycle ATP continually reforms. In the simplest sense possible, ATP is involved in self-replication, and it is lifelike in that respect, but that alone. One stage in the process, AMP, is linked to the structure of RNA and DNA, so this observation is not trivial.

The foregoing begs the question of how sugars are formed in living things. Sugars are the intermediate step between the inorganic world of CO₂ and H₂O and that universe of complex CHONP molecules at the core of life. Fixing inorganic C, H, O, N and P etc. in life molecules is what eukaryote autotrophs do. There are several pathways, of which the most common today involves first the production of C₃ chain molecules in photosynthesis. This is the Calvin (or C₃) cycle (never to be confused with the Kelvin cycle involved in mechanical energy conversion) usually shortened to the. As Figure 12.2 shows, the C₃ pathway depends on the intervention of ATP and two other arcane compounds, NADPH and Rubisco, the last not to be confused with a professorial tea-biscuit. Oh dear, NADPH is nicotinamide adenine dinucleotide which has a phosphoric addition and is hydrogenated. Rubisco is short for ribulose bisphosphate carboxylase/oxygenase (who would have guessed it?), an enzyme that happens to be the most abundant protein on this planet. Rubisco acts as the catalyst that enables pentose sugars to combine with CO₂ thereby forming a C₃ acid (left side of Figure 12.2), when ATP and NADPH intervene to form a C₃ sugar (top) that partly polymerises to form the C₆ sugar glucose (right).

Figure 12.2  Simplified model of how CO₂ is fixed and glucose is produced by the Calvin or C₃ cycle

We can break this bewildering complexity down to energy fundamentals of metabolism that involve very little chemistry other than those summarised by the mnemonic OILRIG (‘oxidation is loss, reduction is gain’ in terms of electrons). Electrons at a high energy level, which are associated with a chemically active molecule, atom or ion that readily donates electrons (a reducing agent), form the energy source for autotrophs. The transfer, in a series of steps, of electrons to molecules, atoms or ions that readily accept electrons (oxidizing agents), releases energy to power metabolism. Part of this energy drives or pumps hydrogen ions (H⁺ or protons) across the cell wall and membrane, thereby creating an electrochemical gradient or proton motive force (PMF). The PMF is constantly discharged back across the membrane by a reverse flow of protons that participate in the replication of ATP from ADP; a sort of proton pump. Donating and accepting electrons, and thereby generating a PMF can be accomplished by a wide variety of simple reactions some of which are employed by autotroph cells.

The most familiar type of autotrophy is that using light – photo-autotrophy or photosynthesis. Electrons are excited, or raised to a higher energy shell in an element or ion, by specific wavelengths of light according to Erwin Schrödinger’s wave theory (Chapter 3). In achieving this the compounds that generate the PMF selectively absorb these wavelengths, converting electromagnetic energy in the photons or quanta to that held in the new, excited electron shell. Those wavelengths that are not absorbed impart the colour to pigments employed in photo-autotrophy. Chlorophyll, which pigments green plants (eukaryotes, remember), uses blue and red light leaving only the green part of the solar spectrum to be reflected and perceived by our eyes as colour. There are other photosynthetic pigments coloured red and purple among the prokaryotes. Electrons excited in this way confer reducing properties on the pigment compounds – it is easier for them to be donated – and they are transferred to electron carriers that have oxidizing properties. The charge balance on the pigment must therefore be restored by a return supply of electrons. This is possible through two basic schemes, both of which are employed in prokaryote photosynthesis (Figure 12.3).

Figure 12.3  Two schemes for electrons energised by photo-autotrophy: (a) electrons transferred by carriers back to the light-absorbing pigment; (b) electrons lost to an internal acceptor are replaced in the pigment by external electron donors. In both cases a proton-motive force ensues

In one scheme electrons return to the pigment molecule in a less excited state, through simple recycling after pumping protons (Figure 12.3a). In the other they are accepted by an intermediary agent NAD⁺ that then attracts protons to become NADH, itself a source of reducing power, as is its phophorylated form NADPH (Figure 12.3b). As you saw earlier, this compound is implicated in the C₃ fixation cycle, and many reactions that synthesise organic compounds require reducing power. Either a donor compound outside the cell or internal recycling resupplies electrons to the pigment.

Photo-autotrophy depends for reducing power only on a light source, a pigment capable of becoming a reducing agent, and sometimes an external electron donor. However, simpler reducing agents are common in the inorganic chemical world. They include: hydrogen; sulfur; sulfides of metals and hydrogen sulfide; ferrous (Fe²⁺) iron and ammonium (NH₄⁺) ions. The prokaryote world includes a whole range of simple organisms, for which these reducing agents provide the potential for generating energy flows. These are chem-autotrophs. Figure 12.4 shows one such scheme. Hydrogen sulfide gas is the electron donor, although a metal sulfide can serve just as well. Having donated an electron to carriers in the cell membrane, H₂S is oxidised to sulfur, and pumped protons participate in the ADP to ATP conversion. At the low-energy end of the electron flow oxygen is reduced to water, so the overall chemistry used by such a prokaryote chemo-autotroph is summarised by the equilibrium:

H₂S + ¹/₂O₂ = H₂O + S

In the same scheme electrons are driven in an energetically ‘uphill’ direction (left side of Figure 12.4) to reduce NAD⁺ or NADP⁺ to NADH or NADPH, and thereby ensure the fixing of CO₂ in organic compounds.

Figure 12.4  Electron flow and proton pumping in chemo-autotrophy based on hydrogen sulfide

Chemo-autotrophy provides prokaryote life a rich diversity of opportunity of which most of us eukaryotes are barely aware, for it occurs in places where we could not survive; generally where there is neither oxygen nor light. There are so many chemical reducing and oxidizing opportunities that, at first glance, it seems hard to believe that life on the planet is not completely overwhelmed by prokaryote ecosystems, instead of its mastery by the eukaryotes. There is one fundamental reason why this is not so today. Oxygen, the most common oxidizing agent, abundant in the air, the seas and the soil, wipes out the free availability of reducing agents in the largest environments, and with it much of the potential for chemo-autotrophy. As you saw in Part 2, this was not always the case, and free oxygen has become more abundant as geological time has passed by. There is a fascinating link between oxygen, and various lifeforms that have come into being, flourished, died out or evolved. That linkage forms a central theme to much of Part 4, crops up in later Parts, and, as you will discover, has an influence that extends to the very core of the planet.

Autotrophs, the primary producers in any ecosystem, are consumed in some way by heterotrophs. The metabolism of heterotrophs centres on the breaking of bonds in complex organic molecules, and that needs energy. As in autotrophy, energy is generated by electron transport and proton pumping. Likewise, heterotroph metabolism involves electron donors (reducing agents) and acceptors (oxidizing agents). Today oxygen is a widely available electron acceptor and all eukaryote heterotrophs use it in aerobic respiration. So too do some prokaryotes, but as in the case of their autotrophic food items, other kinds exploit many more chemical opportunities. Lots of prokaryote heterotrophs thrive in oxygen-poor conditions. Some use SO₄²⁺ (sulfate) ions dissolved in water as electron acceptors and reduce them to H₂S. You, and more particularly those close to you, may become acutely aware of some that live in your gut, if you drink water containing abundant sulfate ions!  A wide variety reduce nitrate (NO₃^–) to nitrite (NO₂^–) ions, and are the source of nitrite pollution from over-manured fields. Other electron acceptors exploited by prokaryotes include: ferric (Fe³⁺) iron; organic molecules, such as acetates (accounting for the bacteria that ‘eat’ plastics); and even CO₂ that is reduced to methane (another prokaryote product of our gut). A few prokaryote heterotrophs avoid the need for proton pumping and thereby free themselves from the need for an electron acceptor. These are fermenting bacteria, whose metabolism involves only the transfer of phosphate from ATP to glucose to form ADP then AMP, which takes up phosphate again from the final breakdown of glucose to return to ATP. Fermenting is an inefficient style of heterotrophy, to which some prokaryotes can turn when their normal electron acceptor is in short supply. Exclusive fermenters are suspected of being among the most primitive life forms still in existence.

The foregoing is a most abbreviated account of the great complexity and variety of primary carbon fixation by autotrophs and respiration by heterotrophs. It is sufficient in our context to conclude that all metabolic pathways involve the movement of electrons and protons. As a result, all are dependent on the fundamental chemical processes of oxidation and reduction, together with the variation in hydrogen-ion concentration that governs whether an environment is acid or alkaline. These governing factors are expressed by the redox potential (Eh) that measures an environment’s ability to donate or accept electrons, and its pH, respectively. The simplified equilibrium:

CO₂ + H₂O + energy = HCOH + O₂

may proceed to the right or to the left (Equations 3.1 and 3.2 in Chapter 3, where C-fixation and its role in the carbon cycle were introduced) are reducing and oxidizing reactions, respectively. How organisms function biochemically can be divided into a number of basic metabolic processes that are conditioned by environments with different Eh and pH. To some extent the concentration of various simple ions that result from the solution in water of inorganic salts, such as NaCl and CaSO₄, also play a role. Such ions in the environment have an important bearing on how protons are pumped across cell membranes, at the root of all metabolic biochemistry, except that involved in fermentation. Fortunately, as outlined in Chapter 8, the rock record preserves features that signify in a general way the Eh and pH, and water composition of past environments. These help geologists to arrive at some sensible conclusions about interplays between the inorganic part of environments and the life forms that occupied them, and about their co-evolution.

A hidden empire

Having taken a brief trip into some chemistry at the cell level, we can now go on to look at some of the living prokaryotes. Until the late 1970s, prokaryotes posed great problems to taxonomists. In 1980 molecular biologists showed that the genetics of three groups of them  differed so fundamentally from all other prokaryotes that the superficially simple organisms previously called Bacteria needed to be split into two domains standing above the level of Linnaeus’s kingdom. The Archaebacteria and the Eubacteria, now called the archaea and bacteria, have equal taxonomic status to all the eukaryotes, now the eukarya

The archaea include two main groups, plus some oddities (Figure 12.5). Some exist only in very hot water in springs around volcanoes and in hydrothermal vents on the sea floor. Most of these extreme thermophiles (heat lovers) are heterotrophs that use sulfur as an electron acceptor through SO₄^– to H₂S reduction. They are important in encouraging the precipitation of metal sulfides in ‘black smokers’ when metal ions in the hot water encounter biogenic H₂S. Sulpholobus uses H₂S as an electron donor in acidic, almost boiling hot springs to fix CO₂ in a unique way. It is a chemo-autotroph. These are the Crenarchaeota (formerly Sulfobacteria), but such is the novelty of dividing up prokaryotes along genetic lines that another chemo-autotroph is, for the moment, lumped with them – Pyrodictium, which uses the reducing properties of hydrogen gas as an energy source.

Figure 12.5 Division of the archaea based on the dissimilarity of their ribosomal RNA (rRNA), with a few example genera. Groups with affinities for hot water show as bold lines. Note nomenclature of the archaea is still fluid, so old group names are used.

The Methanogens mainly inhabit oxygen-free swamps, airless soil layers and the guts of  animals. Oxygen is deadly for Methanogens. Like Pyrodictium, they are chemo-autotrophs using hydrogen gas as an electron donor and energy source to fix carbon from CO₂, though not through the Calvin cycle. Their waste products are methane and water, and account for the eery ‘Will o’the wisp’ flames occasionally seen in swampland. Included with the Methanogens (in the Euryarchaeota) because of genetic similarities are prokayotes that were formerly known as Halobacteria that thrive in very saline environments, such as evaporating lakes and even salt pork. Perversely, most Halobacteria are aerobic heterotrophs, and contain their own oddities. One has patches of a purple pigment that enable it to be a photosynthetic autotroph. This pigment is chemically very similar to rhodopsin found in the rod photoreceptors of the human retina that enable vision in low light levels. Other Halobacteria are anaerobic fermenters. One, Thermoplasma, has no cell wall, and contains DNA with proteins like those that bind nucleic acid in the nuclei of eukaryote cells. It lives in burning coal heaps. Thermoplasma depends on highly acid conditions brought about by the breakdown to sulfuric acid of iron sulfide (pyrite or ‘fool’s gold’) in the coal. Distanced genetically from the two main groups of archaea are a few that appear to be transitional between them; the domain archaea is perpetually in a taxonomic state of flux

Diverse as the archaea seem, in terms of their ecological niches they pale into insignificance compared with the bacteria (Figure 12.6). They include groups of heterotrophs that can be aerobic or anaerobic, fermenters, chemo-autotrophs, photo-autotrophs, some combining several basic metabolic processes in twos or threes, and even parasitic forms that use ATP acquired from their hosts. One of the last is Chlamydia. It causes non-specific urethritis, a sexually transmitted disease.

Figure 12.6 Simplified genetic relatedness and lifestyles of some bacteria including two thermophilic genera (bold lines).

Five groups of bacteria function as photo-autotrophs, four of which use one or other of the processes shown in Figure 12.3, infest anaerobic environments and produce no oxygen. Indeed, oxygen is lethal to them. The fifth group, Cyanobacteria (blue-green bacteria) are very different. They once assumed supreme importance in the history of the Earth’s biosphere, atmosphere and lithopshere  So much so that we must dwell a while on the basic chemistry of their metabolism. Figure 12.7 shows the photosynthetic electron flow in Cyanobacteria cells. It is very different from the two photosynthetic schemes shown in Figure 12.3 and, instead of one, uses two reactions to light in tandem that excite electrons in two pigments (different forms of chlorophyll, which is built on a magnesium ‘core’). Both absorb part of red light, so giving the distinctive blue-green or cyan colour to the pigment. The lower energy excitation requires simply water as an external electron donor and generates free oxygen in the process. That requires a very powerful oxidizing agent involving the metal manganese (Mn), deployed by blue-greens in one of their chlorophyll pigments. No other photo-autotrophs can do that. But to reduce NADP⁺ to NADPH and thereby fix CO₂, as all autotrophs must do somehow, demands an equally powerful set of reducing condition to produce sufficiently energetic electrons. Blue-greens achieve this with the other chlorophyll pigment using an iron-sulfur Rieske protein as an electron acceptor. In this respect blue greens seem to combine chemical aspects of the processes found in two other groups of photo-autotrophic bacteria, the green-sulfur bacteria and purple bacteria. Like the latter, blue-greens also use the Calvin cycle to reduce CO₂ to carbohydrates. In short, blue-greens use a mix of anaerobic ‘chemical engineering’, produce oxygen that is fatal to anaerobes, yet somehow survive!  The key to their success lies in their unique ability among the prokaryotes to use the most plentiful commodity on the Earth’s surface, water, as an electron donor. They also need a supply of soluble ferrous (Fe²⁺) iron, once abundant in the oceans before the Great Oxidation Event (Chapter 8) and now a micro-nutrient in surface seawater. Other external electron donors are only plentiful in relatively restricted geological sites. However and whenever they evolved, the blue-green Cyanobacteria found themselves with a huge potential living space, the oceans, provided the water was lit by the Sun. The oceans would be a large sink for the blue-greens’ toxic waste product, oxygen, thereby giving their ancestors a measure of protection from their own excreted oxygen.

Figure 12.7 Electron flow in blue-greens (Cyanobacteria) involves two forms of the pigment chlorophyll  to give a double photosynthetic boost to the energy of electrons. One breaks down water, the other employs an Fe-S based Rieske protein. Both light wavelengths are reddish, so their absorption gives Cyanobacteria their distinctive blue-green colour. Symbols as in Figure 12.3.

Continuing the prokaryote story from the standpoint of comparisons between the molecular biology of modern representatives and ideas of evolutionary linkages must await the next chapter. We have neglected our kin for too long, so return to the eukaryote cell immediately.

Cohabitation and the eukarya

The preceding superficial tour of prokaryote diversity and their metabolic chemistry allows us to review the architecture of eukaryote cells from a new standpoint. When the American biologist Lynn Margulis did this in 1967, she developed an idea that had languished in the ‘haunted wing’ of cell biology since the early 20^th century. Her resurrection with supporting evidence from microbiology, initially startled her colleagues. The various  organelles within eukaryote cells (Figure 12.1), particularly mitochondria and chloroplasts, looked to her like prokaryote cells themselves. Both contain DNA, but not in a form that can participate in the sexual division that characterises eukaryotes. Both have a double membrane separating them from the eukaryote cytoplasm, which may be their own cell membrane plus an addition from the eukaryote cell. In fact, both have almost independent functions. Chloroplasts perform aerobic photosynthesis in plants, and mitochondria are the main chemical factories in all eukaryote cells bar a few oddities. Margulis’s idea was that both originated as independent prokaryotes. Indeed, chloroplasts are very like blue-green bacteria and mitochondria are similar to  a number of living aerobic, heterotrophic bacteria. Leaving aside the eukaryote nucleus, other organelles and the common tail in single-celled eukaryotes, the rest of the cell is more or less a prokaryote itself. So how might such a hypothesised ‘club’ of prokaryotes have got together?

The simplest guess would be that they represent a sort of mutual association, a product of symbiosis. There are countless analogies, from the brave little fish that pick flesh from the teeth of Great White Sharks, the lobster’s lip creature, a whole army of prokaryotes and eukaryotes in and upon our own bodies, to single-celled heterotrophic eukaryotes that are stuffed with green algae cells that are protected by and provide food for their host. The last is endosymbiosis, and this is the core of Margulis’ theory. Figure 12.8 outlines two variants of her original  scheme.

Figure 12.8 Lynn Margulis’s two endosymbiotic routes to the basic eukaryote ‘animal’ cell from a prokaryote ancestor. One engulfs aerobic bacteria to serve as mitochondria, to be joined later by whatever formed the nucleus and the flagellum that confers mobility. The other route delays mitochondria until after formation of the nucleus and flagellum. Incorporating cyanobacteria-like prokaryotes forms chloroplasts within the basic architecture and the ancestor of the plant kingdom.

The original host may have been an anaerobic prokaryote fermenter surrounded by a flexible membrane rather than a cell wall; arguably a primitive characteristic. Smaller, oxygen-respiring bacteria entered the host, probably to use some of the end products of fermentation, but in turn supplying their host with chemicals that they produced. Exchange of genetic material between host and ‘guests’ – a characteristic of some prokaryotes – cemented the relationship. Much the same process would account for chloroplasts, except the bacteria that became endosymbionts must have been aerobic photo-autotrophs, of which blue-greens are the most obvious candidate. Because both animals and plants contain mitochondria, it seems reasonable to assume that chloroplasts developed later to source the plant kingdom. The self-moving ability of single-celled eukaryotes, using their little flagellum, seems superficially easy to address in the endosymbiont theory. There are whip-like bacteria called spirochaetes, but equally such tails may have been produced by the prototype eukaryote itself. There are selection advantages in being able to move, and far more strange things have been evolved by the eukaryotes than a whip-like tail. The thorniest problem is the nucleus and its DNA-based chromosomes; the real hallmark of eukaryotes, and, as you will see in Chapter 14, the source of their later success.

Since no living prokaryote has chromosomes, an endosymbiotic origin for the nucleus seems highly unlikely. One suggestion is that it arose by infolding of the host’s cell membrane to enclose the loops of DNA that typify prokaryote genetic material. The loops – which may have been the host’s genetic material or that of symbionts – somehow became transformed to chromosomes, with all the potential that they confer.

Like many potent ideas, the Margulis endosymbiotic theory for the origin of the eukaryote cell from successive incorporation of earlier prokaryote cells offers an explanation for the previously inexplicable. It may prove to be completely wrong, but many evolutionary biologists support it today, in one form or another. If nothing else, it gives us a framework within which to assemble information in a more or less manageable way. It also gives a simple order to events, yet to be tested, that eukaryotes evolved from prokaryotes. So if we are looking for the earliest forms of life, or rather the least-evolved descendants from the first life on Earth, the focus has to be within the prokaryotes. In Chapter 14 we look at some of the genetic evidence from modern organisms that helps clarify these roots. That takes us back to some idea of primitive life. The next big topic approaches the issue from another direction, by examining ideas about how truly living things could have been produced by completely inorganic processes.

Check out Further Reading for Part IV to see source material and links to articles

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Chapter 8 Life, rock and air

Sections

Backtracking the composition of air

A preoccupation with soil

The enigma of Precambrian iron ores

Search in sedimentary rocks younger than 541 Ma and ultimately you will find fossils. With a keen eye, considerable patience and luck you will be overwhelmed by their number and diversity. In rocks older than that you will be lucky to find the most meagre trace of a long-dead organism. The inconsistency of fossil preservation and evolution’s production of durable organic remains only late in its history conspire to make analysing tangible evidence for life’s course highly uncertain. For the Phanerozoic Eon, despite its signifying ‘abundant life’, we can never be sure how many living things existed at any one time, nor how diverse that life really was, although enough preserved material makes possible educated guesses at the latter. For earlier times eyeballs-out geology gives barely a hint of either, although there is sufficient in the way of carbon-bearing sediment to whet the imagination. Chapter 3 showed just how important life on this planet has been for the development of its surface conditions, unique in the Solar System. Since we and our global environment represent the latest stage in that co-evolution, trying to reconstruct its full course is as important as seeking our own geographic roots and family history on the vanishingly tiny scale of human events.

Much detail and many surprises come from a purely biological study of ancient remains and even the genetic material of modern organisms, but here I want to introduce the gross course of events in the whole system of the outer Earth; the broadest context for our origins. Doing that centres on how the atmosphere has changed over geological time. An extremely abstract approach, you might think, and as we seek out evidence it might seem to become even more obscure. It shouldn’t, for the simple reason that life on Earth at its material base has always boiled down to the interlinking of two simple compounds – carbon dioxide and water – through the intermediary of the general building blocks of living cells – carbohydrates such as sugars (Chapter 3). As you will see in Part IV many different ways of life’s self-assembly and decay still live on in the three fundamental divisions of modern life, the archaea, bacteria and eukarya. The emergence of one special metabolism – photosynthesis – that uses the Sun’s light as an energy source brought in a third compound with which the vast bulk of life today is intimately bound. Photosynthetic autotrophs generate carbohydrates with molecular oxygen as a byproduct. We depend on an atmosphere in which that byproduct has become 21% of the volume of air. We and most animals suffocate when the oxygen level drops below about 10%, yet oxygen is one of the most reactive and dangerous common substances in the environment. Uncombined as the O₂ molecule, but with an additional electron, oxygen is deeply implicated in damaging all kinds of cell – one of the notorious ‘free radicals’. In our metabolism we have to be protected from oxygen’s powerful reactivity by its transport, not as dissolved gas, but safely bound in the iron-rich molecule haemoglobin that colours red blood cells.

Oxygen reacts with many elements and compounds, because its atomic structure has a thirst for electrons. Its name is assigned to one side of the general transfer of electrons from donors to acceptors that underpins all chemical reactions – oxidation or the transfer of electrons away from an atom, ion or compound. Its counterpart, the gain of electrons by atoms, ions or compounds, is called reduction. Note that a useful acronym for the difference is OILRIG; as regards electrons, oxidation is loss, reduction is gain. Both can be expressed in terms of two other very common components of chemical reactions; movement of a hydrogen ion (a hydrogen atom stripped of its single electron, and signified as H⁺). Oxidation can also involve the removal of hydrogen ions, and reduction is again the opposite; production of H⁺; oxidation is also gain of oxygen, whereas reduction is its loss  You will find in Part IV that moving hydrogen ions and electrons forms the basis for the chemistry of living cells. So it is no real surprise that the powerful and most common oxidizing agent, oxygen, at some stage became interwoven with that chemistry. That it did not intervene from the start of life is due to its very reactivity. Except in stars and interstellar dust clouds oxygen is very rare in its uncombined form. All of it in planets is either in mineral molecules, such as SiO₂ or quartz, or bonded with hydrogen in water; that is, if they are lifeless. Its presence as a free element is so unlikely outside of some kind of photosynthesis that oxygen’s easily detected abundance in the Earth’s atmosphere is a focus for astronomers seeking life elsewhere in the cosmos. If there be oxygen, there be photosynthetic life, or some inorganic chemistry that is extremely odd indeed. That does not demand that all life involves the release of oxygen, but such is the cosmic abundance of oxygen (the third most common element in the universe, after hydrogen and helium) that its eventual involvement as life evolves is inevitable.

‘Water is a civilised society’s second provision after law’ (an ancient Sumerian saying) , which says something interesting about the priorities of what is deemed civilised behaviour. As far as our planet goes, there has never been a shortage, for it belches from volcanoes continually. Only a small proportion becomes recycled back to the mantle in the long run. Much water locked in the oceanic crust going down subduction zones escapes the grip of the minerals in which it is bound because of heat and pressure, to emerge from the volcanoes above. So water has been destined to accumulate in the outer Earth since the planet stabilised. The Earth’s gravitational field is so strong that even hot, vigorously vibrating water molecules cannot escape its grip under normal circumstances. Ultra-violet light breaks a tiny amount into its component hydrogen and oxygen, when light hydrogen can reach escape velocity and be lost. The existence of oceans seems to bear witness to the overwhelming preponderance of mantle supply to loss by this means from the Earth. Mars shows signs of once having liquid water at the surface, but its lower gravity and sluggish volcanic activity means that such photodissociation has slowly stripped it to its present dryness. Water vapour is a ‘greenhouse’ gas, in the mantle it encourages vigour by reducing the temperature at which rock begins to melt, and it is essential to life. However, its abundance, together with the fact that it happily exists as gas, liquid and solid and quickly transforms from one to another under Earthly conditions mean that its history is one of omnipresence and variability in the short term. In this book its main significance is in the level to which oceans have risen and fallen, and the extent to which it has become temporarily locked as ice above sea level. These are important, but secondary matters.

Although climate is hugely complicated, involving astronomical forces, the disposition of land and ocean, and the circulation of energy by winds and currents, its general state as reflected by surface temperature is governed by the atmosphere’s temporary retention of thermal radiation. That is the ‘greenhouse’ effect (Chapter 1), which keeps surface temperature above the -15°C average that would prevail if there was no atmospheric heat trapping. The central player in this retention is carbon dioxide. Reducing the evolution of the Earth system, life and all, to its bare bones requires us to examine the variation over time of the atmosphere’s content of CO₂ and oxygen. Doing that is no easy matter, because nothing conveniently locks away samples of the atmosphere, except for the bubbles that glacial ice encloses, but that is ephemeral stuff. The clues have to come from sedimentary rocks and what lies within them. Chapter 7 made plain that the adage ‘the present is the key to the past’ works nowhere so well as in the sedimentary record. We can chart a multitude of details about past environments from this uniformitarian principle. As well as making sense of ancient climatic zonation from sensitive indicators plotted on palaeomagnetically reconstructed continents, such methods in the hands of experts show where and when deep-ocean and continental environments are preserved, and so on. Yet that fascinating information says little about atmospheric composition. That demands a focus on rocks that demonstrate chemical processes in which the atmosphere is deeply implicated. Those that involve its CO₂ content are plentiful, being limestones of various kinds. For oxygen, matters are not so straightforward, and we must turn to whatever likely evidence presents itself, some of a truly bizarre nature. The best way to proceed is from the familiar to the increasingly strange, that is, by a backward look in time.

Backtracking the composition of air

The basic architecture of all today’s life on continents and in the seas turns up and is easily recognised in the Cenozoic era of ‘modern life’. Most useful are fossils of mammals, very different from modern ones, but nonetheless divisible from the start of the Cenozoic into the placental, marsupial and monotreme types (us, wombats and platypuses), and all dependent on a high atmospheric oxygen content. On their presence alone, it is safe to say that at the outset 65 Ma ago, the atmosphere was not a great deal different from the modern one. But there are suggestions that there may have been more oxygen early on. Those parts of the continents that were tropical at that time all possess a mantle of ancient blood-red soils, like those of northern Ethiopia and Eritrea (Introduction). These universal tropical laterites contain vast amounts of iron in the form of ferric oxide (Fe₂O₃). They are highly oxidised soils that lock up a great deal of oxygen. Similar ones form today, but by no means everywhere in the tropics. Up to about 35 Ma ago higher oxygen levels than now might have encouraged this intense oxidation. At the very base of the Cenozoic there is barely contestable evidence that the Earth had an atmosphere little different from that inside an intensive-care unit’s survival tents.

Wherever it is found in sedimentary strata, the Mesozoic-Cenozoic boundary has a unique, thin layer. It contains all manner of oddities, of which more in Part VI. One of the strangest features is that it is full of soot particles preserved in both marine or terrestrial sediments. Soot implies fires, and soot everywhere suggests a global firestorm. Forest fires spring up now when drought makes woodland tinder dry. For the first two decades of the 21^st century huge fires have raged each year somewhere, partly from drought, partly from the outcome of deforestation. Choking sooty smoke sometimes wreathes the whole of south-east Asia, adding to the pollution generated by its rapidly industrializing economies. But these are dry trees that burn. Global conflagration demands that green vegetation burns too, and that is rare today. Calculations from the concentration of this end-Mesozoic soot suggest that much of the terrestrial biomass burned down. While there are good reasons to indicate how such a global barbecue was triggered – probably from an impact by a large extraterrestrial object – that it raged through all biomass can have but one explanation. There must have been more oxygen in the air. The probability that green trees of the modern variety ignite in a fire increases from 1 in 10 for modern oxygen levels to 99 in 100 certainty with a level of 24%. Apart from signifying a catastrophe in which more oxygen-rich air is implicated, these figures suggest an upper limit on the amount of oxygen there can be in the air. Above 35% trees would burn spontaneously all the time, thereby keeping down the oxygen level to which they themselves contribute.

The immediately preceding period to these lugubrious events is called the Cretaceous; it takes its name from a corruption of the German word kret meaning chalk. The white cliffs of the coasts of south-east Britain and northern France are so dramatic because they are composed of Cretaceous Chalk, and that thick unit extends buried as far as the Urals. Chalk is a very fine-grained limestone made of shelly remains of tiny algae. It is, however, but a small part of the products of vast, living carbonate ‘factories’ in both shallow and deep water that encircled the Cretaceous tropics to form the greatest concentration of limestones known from the geological record. The only conceivable source for all the CO₂ buried in this carbonate form is the Cretaceous atmosphere. Such high limestone productivity is a clear sign that considerably more of the main ‘greenhouse’ gas resided in the Cretaceous atmosphere than now. Since large amounts entered long-term storage, for it to be maintained implies a far larger supply from volcanic action then. Yet the complexity of the balance means that it is difficult to judge directly what the atmospheric levels were.

The earlier record of the Mesozoic contains plenty of limestone, but in its very oldest parts and the immediately underlying, and therefore youngest Palaeozoic strata, there is another major atmospheric indicator. Wind-blown sands, and lake beds form distinctive brilliant red outcrops, formed in the dry heart of Pangaea. They are much different from the early Cenozoic laterite soils. For one thing they are much thicker, for another they are not soils formed by slow weathering but accumulations of eroded material. They are red because each sand or silt grain is wrapped in a thin coat of ferric oxide. Even more so than the younger laterite blanket, these Permian and early Triassic redbeds (Chapter 7) lock away massive amounts of oxygen. Here again is evidence for an unusually high oxygen level in the atmosphere that influenced continental chemistry (incidentally, the ‘Red Centre’ of Australia, and red dunes of parts of other modern deserts probably inherit their colour from the early-Cenozoic laterites). That this period and the one immediately preceding it had oxygen-rich air is confirmed from an unusual source. Among the most strange features of the fossil record in the Carboniferous Period is the abundance of insects. Their diversity took a sudden leap about 320 Ma ago. They became very large and they soon adopted flying. One dragonfly had a wingspan of 70 cm and, with a body 3 cm thick, dwarfed any modern descendants. The Carboniferous was a world of monster bugs, but they cannot reach such dimensions now. Insects do not have lungs, but respire passively by diffusion of air through their body-walls. They do not breathe, so diffusion and atmospheric oxygen levels govern their maximum size, especially for those that metabolise rapidly in order to fly. Giant flying insects point to higher oxygen levels in the period when most hard-coal reserves were deposited. To become successful fliers at the same time perhaps implies a denser atmosphere too.

What encouraged insects’ forbears to colonise the land, as too the first land vertebrates, was the earlier invasion by plant life; a new food supply on the fringes of the oceans. The evolution of early land plants to tree-sized forms fed partially rotted debris to the subsiding swamplands in which coal accumulated. Such burial must have drawn down CO₂ to leave a photosynthetic excess of oxygen, but that is a topic for Parts V and VI. The venture of vertebrates from sea to land was a big step. From the use of gills for intake of oxygen and exhalation of CO₂ waste gas, to the evolution of lungs, as well as modifications in reproduction so that eggs could survive out of water, demanded substantial changes in architecture. Some scientists argue that only a boost in oxygen levels made both possible. Reptile eggs laid in the air are interesting. Coated in a hard shell to prevent drying, they must pass oxygen through pores to supply the developing embryo. The amphibian to reptile transition took place late in the Carboniferous to Permian highly oxygenated world.

A preoccupation with soil

Before the colonisation of land there are no such handy clues to the oxygen content of the atmosphere. For the earlier Earth it is a matter of trying to glean whatever oxygen-related signs happen to present themselves. The redbeds laid down on land show that air of their time had enough potential to mop up electrons from the outer shells of iron atoms to stabilise them as ferric (Fe³⁺) ions, and contained enough oxygen to lock them in red ferric oxide and hydroxide. Because such ferric iron minerals are among the most insoluble common compounds in rocks, once formed they tend to remain as bright signatures that the atmosphere was oxidizing and contained oxygen. In rock sequences older than about 2.2 Ga, continental redbeds are seen nowhere. That time marks the point at which the atmosphere attained sufficient oxygen for oxidation to perform this useful trick. However, that particular oxygen level is difficult to estimate. The chemistry of atmosphere-water-rock being a great deal more complicated than just a relationship between oxygen and iron in the proverbial test tube, we need a more penetrating view. Iron first enters the regime of surface processes locked in crystals within igneous rocks, mainly as silicates plus some sulfides. To become mobile and so available for redistribution by water into newly deposited sediments, the iron-rich igneous minerals must be rotted, and the main agency for this comprises the H⁺ ions released by the weak carbonic acid in rainwater. To cut a long story short, that igneous iron is mainly in its ferrous (Fe²⁺) form, with one more electron in its outer shell than ferric iron. Water can dissolve and move ferrous iron, provided that extra electron is not snatched up by oxygen. So the level of CO₂ in the atmosphere has an important control over how much iron might be released by weathering. The relative proportions of CO₂ and oxygen therefore control the ability of redbeds to form on the land surface. Experiments suggest that for redbeds not to form oxygen must have been at around the same concentration as CO₂ or lower. Today oxygen is 600 times more abundant than carbon dioxide. We cannot infer from redbed evidence alone that there was very little oxygen 2.2 Ga ago, nor that there was a great deal more CO₂; just that the two gases were in very different proportions compared with today’s air.

There are two other indicators of an important change in the state of the atmosphere at that time. In the presence of oxygen, igneous iron sulfide quickly reacts to form sulfuric acid and a ferric hydroxide slime or ochre. In rocks older than 2.2 Ga, some sediments contain rounded grains of these sulfides that have been transported unchanged by water. At much the same threshold of oxidation potential uranium switches from being insoluble to highly soluble. The older sediments with sulfide grains sometimes contain rounded grains of uranium minerals, which would have broken down rapidly under later continental conditions. The only way out of the bind regarding an estimate for the actual concentrations of both interrelated gases is an independent assessment of one or the other. The only handy guide is to the level of CO₂ in aerated soils.

Under conditions sufficiently reducing for iron dissolved in water to be moved in soil, it may precipitate as some ferrous iron compound. There are three choices; as a carbonate, as a silicate or not at all. In the last case it stays dissolved, eventually to reach rivers and ultimately the ocean. Examining the few fossil soils older than 2.2 Ga reveals that none of them contain iron carbonate, but iron silicates had been precipitated in some of them while they formed. Geochemical experiments show that iron carbonate would form in soil only if CO₂ was at least 0.4% by volume of the air contained in soils, which is 100 times more abundant than it is at present. The two lines of evidence suggest that before 2.2 Ga oxygen amounted to less than 0.4% of the air and CO₂ must have been more abundant than that. An abundance of CO₂ before 2.2 Ga goes some way to account for an enhanced ‘greenhouse’ effect in the early atmosphere needed to stop the Earth from freezing over when the Sun was less radiant (Part III). Carbon dioxide is a likely candidate for the early Earth’s ‘rescue’ from becoming locked into a perpetually icebound condition, while nowadays that emitted by human actions may drive it in the opposite direction.

This limited evidence for a ‘Great Oxidation Event’ at around 2.2 Ga is supported by other more intricate geochemical arguments. But a few geochemists have refuted that hypothesis by citing nuances within the available data; a good illustration of the flux of ideas when information is in short supply. Later in geological history, more data on the relative abundances of the crucial gases O_2­ and CO₂ shows up, even to the extent of suggesting actual concentrations in the atmosphere. The data are often abstruse, but one source of information that applies to CO₂ after the land became vegetated is easier to understand. Plants need to take in carbon dioxide through their leaves in order to photosynthesise, and that is via pores or stomata in the outermost cells. The number of leaf stomata per square centimetre is a measure of atmospheric concentration of CO₂ – the more of that gas is available, the less densely packed are these pores. Figure 8.1 shows how estimates of atmospheric composition have changed through time.

Figure 8.1 (a) Variation of atmospheric oxygen through geological time; (b) Atmospheric oxygen (blue) and carbon dioxide (red) values for the Phanerozoic Eon. Precambrian CO₂ data are patchy and disputed, so are not shown .

The enigma of Precambrian iron ores

By far the world’s largest repositories of iron ore are the banded iron formations or BIFs of the older Precambrian. They are made mainly of ferric oxide, and that presents an enigma, as you saw in Chapter 7. Before 2.2 Ga, the air and the continental surface experienced conditions that permitted ferrous iron to remain dissolved. Continental redbeds did not form, yet ferric oxides precipitated in marine environments over great lengths of time. They do represent oxidizing conditions and both iron and oxygen being available in large amounts in sea water. However, the minute layers in BIFs show continuous transport of dissolved iron into the BIF basins, subject to intricate controls that may be partly tidal, partly seasonal and perhaps even varying with the Milankovic astronomical cycles of changing solar energy supply. As regards the chemistry at work, we can visualise a simple scenario. If oxygen in seawater then was available only in the vicinity of the BIF basins, everywhere else iron would exist in its reduced, soluble, ferrous form. Where the one met the other, oxidation would ensure rapid precipitation of ferric oxide. For such vast repositories of iron-rich sediment to build up day by day demands that both ferrous iron and oxygen were in continual supply, for if either one ran out the reaction would stop and other kinds of sediment would accumulate. Even if oxygen was abundant in the early Precambrian atmosphere, it is inconceivable that it would enter sea water only in specific locations. In any case oxygen only constituted a tiny proportion of the air, if there was any at all.

As far as we know for such a world, the only source of abundant oxygen in the oceans would have to be some kind of photosynthesis; a living process is implicated in formation of the ores for industry’s most used metal. The ultimate source of iron is the Earth’s mantle, from which partial melting draws it off in the form of various kinds of basaltic magma. The bulk of this magma emerges today as part of plate tectonics to form oceanic crust, and more occasionally as flood basalts associated with rising plumes from the deep mantle. Whatever their origins, oceanic magmas crystallise to form silicate minerals that contain iron in their structure. The associated heat warms seawater that has seeped into the lithosphere, causing igneous minerals to rot, dissolving parts of them, including iron, and circulating these hydrothermal fluids in the lithosphere. Such fluids must eventually well out. Today they source the ‘black smokers’ discovered in association with mid-ocean rift systems. The ‘smoke’ consists of fine insoluble compounds precipitated by reaction between the emerging fluids and modern seawater. That is oxygenated, and all the iron falls out either as ferric oxides and hydroxides, or as sulfides that combine iron with sulfur.

On the pre-2.2 Ga world, such precipitation reactions would consume any oxygen dissolved in deep ocean water, leaving an excess of dissolved, originally magmatic ferrous iron in solution. Heated by hydrothermal activity iron-rich deep water would rise and mix with the rest of the water column, making seawater steadily more iron rich. However, should this circulation meet patches of water oxygenated by photosynthetic organisms, oxidation would instantly precipitate the dissolved iron there and nowhere else. That seems to be a plausible explanation for the massive BIFs of early Precambrian times. Mapping out their distribution in each of the world’s great iron-ore provinces shows that BIFs do give way laterally to other submarine sediments. In almost all cases, among these other sediments are limestones with a distinctive structure. They contain layer after layer arranged in strange, bulbous structures. These are stromatolites, in which microscopic examination occasionally reveals filamentous mats separating the layers. They are biologically precipitated limestones, albeit devoid of tangible fossils. Part IV takes up the question of the organisms responsible. The association of BIFs and stromatolitic limestones suggests that the latter were the required photosynthetic sources of the oxygen in BIFs. Oxygen is potentially a threat to all life, particularly to single-celled prokaryotes. Produced by life itself, only the vigorous intervention of the mantle’s iron release mopped it up before its toxic effects polluted the world, or life itself evolved a means of both tolerating and using it.

However persuasive this account of BIF formation might seem, it is not the only one. Some types of modern bacteria, including photosynthesisers that do not produce oxygen, are able to fix iron as Fe-3 hydroxides where there is very little oxygen or none at all. Given sufficient nutrients, such bacteria are able to deposit ferric iron at rates that match those needed to build up great thicknesses of BIFs; about 100 m per million years. That would need 10²² individual cells infesting each BIF basin. What seems to be an unimaginable number of bacteria amounts to only about 40 thousand cells per cubic centimetre of the seawater, which is actually far fewer bacterial cells than the number that build plaque on our teeth! Also, in the geosciences it is never wise to demand that there was ‘nothing’ of some material at such and such a time. In the case of atmospheric oxygen, recent studies of the isotopes of chromium in an Archaean palaeosol and a BIF the lies on top of it suggest that about 3 Ga ago the atmosphere did contain oxygen at between 6 x 10­^-5 to 3 x 10^-3 the atmospheric level at present; not much but enough to be detected by a technical break-through. One explanation is that oxygen-producing photosynthesisers even that far back had colonised the land.

The inescapability of CO₂ having been an increasingly dominant atmospheric gas the further back in time we venture, that its presence was the only escape mechanism from a permanently frozen early world, and the fact that it is the prime building block for life emhasise the supreme interconnection of life, atmosphere, oceans, sedimentation and climate. The record of oxygen, a waste product of now dominant photosynthetic life takes this web of checks and balances deeper still. For the early part of Earth history, it is tied to the mantle through its literal connection with iron. There have been hints in this chapter too that iron is not a mere bit player that serves merely as a buffer for oxygen. Through the grand unification it is also central to living processes at the core of the eye-wateringly named enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO for short. RuBisCO is involved in the first major step of carbon fixation by photosynthesis into energy-rich bio-molecules such as sugars. Today it is probably the most abundant enzyme on the planet of the living, and you will meet it again.

We leave with a question implied by this chapter’s opening paragraph, ‘How much life has there been?’  There is a way to look at its relative abundance through time, if not to answer the question fully. Sedimentation always has a chance to bury some organic tissue, if it is rapid enough to interrupt the carbohydrate to water plus carbon dioxide direction of life’s central chemical equilibrium. Although the molecules change through heating, some trace of them remains, provided erosion does not remove those sediments. Simplifying all the starting materials and the various stages of their degradation to the organic carbon content in sedimentary rock throws up a surprise. Over the 542 Ma long Phanerozoic Eon, the average carbon content of sedimentary rocks has averaged 0.5%. At Isua in West Greenland (Chapter 5) the earliest of all recogniseable sediments, close to four billion years in age, contain about 0.6%. Did life arise at about that time, had it been around for some time beforehand or can sedimentary carbon on its own give a false signal? Whatever, once life did emerge on our home world it radically transformed it, again and again.

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