Book Read Free

Oxygen

Page 11

by Nick Lane


  Snowball Earth, Environmental Change and the First Animals • 71

  Million

  years ago

  500

  0

  Present day

  Cambrian explosion

  Oxygenation of deep waters

  O2 levels reach

  500

  100% of present

  Vendobionts and worms

  Cambrian atmospheric level

  explosion

  Radiation of sulphur bacteria

  600

  First fossil metazoans

  1000

  Alternating isotope

  1500

  Snowball Earth period

  patterns implying

  2-4 successive glaciations

  oxygenation of air

  and surface waters

  700

  2000

  2500

  800

  3000

  First metazoans?

  (deduced from molecular clocks)

  3500

  900

  4000

  4500

  1000

  Figure 4: Geological timeline expanding the late Precambrian period and Cambrian explosion. Exactly when the first metazoans evolved is not known.

  Calculations from molecular clocks suggest a date of somewhere between 700 and 1200 million years ago. The succession of snowball Earths appears to have driven atmospheric oxygen up to atmospheric levels similar to those today.

  72 • FUSE TO THE CAMBRIAN EXPLOSION

  the association (see Chapter 2, page 28). Are there good reasons for linking oxygen with biological opportunity?

  The most obvious basis for a causal link is energy production: oxygen releases much more energy from food than do sulphur, nitrogen or iron compounds acting as oxidants, and is an order of magnitude better than fermentation. The consequences of this simple fact are startling. In particular, the length of any food chain is determined by the amount of energy lost from one level of the chain to the next. This, in turn, depends on the efficiency of energy metabolism. Energy metabolism is generally less than 10 per cent efficient in the absence of oxygen (that is, less than 10 per cent of the total energy available in the food is extracted). If this organism is eaten in turn, the energy available to the predator is less than 1 per cent of that originally synthesized by the primary producer. This is the end of the food chain: below a 1 per cent threshold, there is simply not enough energy available to eke out a living. As a result, food chains must be very short in the absence of oxygen. Bacteria usually become specialized, or compete with each other for scarce resources, rather than ‘eating’ each other. In contrast, oxygen-powered respiration is about 40 per cent efficient in energy extraction. This means that the 1 per cent energy threshold is crossed only at the sixth level of the food chain. Suddenly carnivorous food chains pay and the predator is born. The dominant position of predators in modern ecosystems is not possible without oxygen. It is no fluke that the Cambrian animals were the Earth’s first real predators.

  Predation is a powerful stimulus to weight gain in both predators and prey, either to eat larger prey or to avoid being eaten. Large size requires structural support. The two most important structural components of plants and animals, lignin and collagen, require oxygen for their synthesis. Lignin is best known as the cement that binds cellulose into a strong but flexible matrix in the wood of trees. Because the production of paper necessitates the expensive and time-consuming chore of removing most of the lignin, there has been commercial interest in genetically modifying plantation trees to produce less lignin. If nothing else, the failure of these efforts gives an idea of the importance of lignin: the paltry, stunted, lignin-depleted trees are swept to the ground and crushed in the lightest of winds. Lignin is produced by the reaction of phenols with oxygen. (Phenols, or phenolic antioxidants, are found at high levels in red grapes, and have been linked to the benefits of the Mediterranean diet.) Once formed, it is the most refractory biological polymer known: even bacteria cannot easily break it down for energy.

  Snowball Earth, Environmental Change and the First Animals • 73

  Collagen is the animal world’s answer to lignin. It is a protein and an essential part of the supporting connective tissues in flesh, the skin, around organs and the tendons at joints. Before construction work can begin, additional oxygen atoms must be incorporated into the protein chains of collagen, cross-linking them together to form triple-chain molecules, entwined like rope. As animals get older, more collagen cross-links form, which is why meat from older animals is tougher than that from younger animals. Even tiny errors in collagen synthesis can bring about a pathological bendiness of joints and fragility of skin, such as in Ehlers-Danlos syndrome. The ‘India-Rubber Man’ of circus fame, for example, is said to have had Ehlers-Danlos syndrome. Given the universal importance of lignin and collagen, it is hard to see how large plants and animals could have borne their own weight without oxygen.

  A final factor that is often mentioned in relation to rising levels of oxygen is the formation of an effective ozone screen. Ozone (O3) is formed from the action of ultraviolet radiation on molecular oxygen in the upper atmosphere. Ozone is good at absorbing ultraviolet rays, so once a thick ozone layer has built up, the penetration of damaging ultraviolet rays into the lower atmosphere is cut by a factor of 30 or more.

  Much early work focused on the importance of the formation of an ozone layer in permitting the colonization of the land, though this work has been questioned in recent years. James Kasting, for example, has argued that only 10 per cent of the present atmospheric levels of oxygen is required to produce an effective ozone shield. This level may have been attained as long as 2.2 billion years ago, nearly 2 billion years before the invasion of the land took place.

  James Lovelock argues that the living world is much more robust than we credit, and tells an amusing tale of his early work at the Institute of Medical Research in Mill Hill, London, where he and his collaborators tried to sterilize hospital air using high-intensity ultraviolet radiation —

  with a singular lack of success. The bacteria protected themselves under a layer of mucus, and were only destroyed if stripped of their mucus before being irradiated. In such circumstances, high levels of ultraviolet radiation, in the days before the ozone layer had formed, cannot have presented much of an obstacle to bacterial colonization of lakes and shallow oceans. Desiccation on land may have been a more intractable problem; but there is no reason why desiccated bacterial spores should not have survived prolonged dryness then, as today.

  In fact, rather than protection by an ozone layer, size and structural

  74 • FUSE TO THE CAMBRIAN EXPLOSION

  support probably rank as the most important factors behind the colonization of the land by animals and plants. True land-lubbers, which are more or less independent of liquid water, must avoid desiccation. Resisting desiccation while remaining active requires special adaptations that are only possible in larger organisms, such as a waterproof skin combined, in animals, with internal lungs for maximum oxygen uptake and minimum water loss. And large size, as we have seen, is not likely without oxygen.

  We may reasonably conclude that oxygen was a cornerstone of Precambrian evolution. While nobody would propose that oxygen itself stimulates evolution there can be little doubt that rising oxygen levels opened new horizons for Precambrian life. Not one important evolutionary step took place without an associated rise in oxygen, and no rise in oxygen was divorced from a rapid increase in biodiversity and in the complexity of life. Curiously, though, the major injections of oxygen were not brought about by biological innovations, as had been tacitly assumed for many years (with the exception of guts), but by non-biological factors, such as glaciers and plate tectonics.

  Left to its own devices, life on Earth dawdled for billions of years. If the stimuli for change and evolution were little more than accidents of tectonics and glaciation, a quiet world untroubled by geological strife would almost
certainly fail to accumulate much free oxygen. The Earth stagnated for two prolonged periods, which between them account for half its history. From 3.5 to 2.3 billion years ago, the world was dominated by bacteria. Then, after the violent upheavals of 2.3 to 2.0 billion years ago, another equilibrium was established, in which the oxygen levels remained between 5 and 18 per cent of present atmospheric levels.

  This new equilibrium stimulated a blossoming of genetic diversity among the early eukaryotes, but could not provide enough energy for the evolution of large animals. At such low oxygen levels, life is denied size and complexity; and without these, a brain is unthinkable.

  The deadlock was broken by a second series of snowball Earths, which started 750 million years ago and catapulted oxygen to modern levels. Now the evolution of large animals was only a matter of time, and it didn’t take long. The Vendobionts, the Cambrians, the whole plethora of modern life, exploded into being in a period less than that taken up by the preceding glaciations. If nothing else, this relationship between life

  Snowball Earth, Environmental Change and the First Animals • 75

  and environmental conditions should sound a note of caution to those who seek intelligent life elsewhere in the Universe. We must look beyond the mere presence of water to the presence of volcanoes, plate tectonics and oxygen. Perhaps, if life once existed on Mars, it died out as the fires of vulcanism faded within.

  Whether oxygen was linked with opportunities or extinctions in the modern age of plants and animals, the Phanerozoic, is a question for the next chapter. I can find no evidence to support the idea that free oxygen caused a global holocaust in the Precambrian, but there is a big difference between our modern oxygen level of about 21 per cent, and the postulated Carboniferous high point of 35 per cent, around 300 million years ago. Our experience with diving gas mixtures alone suggests that prolonged exposure to high levels of oxygen can provoke lung damage, convulsions and sudden death, to say nothing of the raging infernos and stunted plant growth predicted by most biologists. Did oxygen really reach fever pitch? If so, how did life cope? And if life flourished, what does this say about our health today as we pop another multi-antioxidant pill to stem the ravages of ageing?

  C H A P T E R F I V E

  The Bolsover Dragonfly

  Oxygen and the Rise of the Giants

  The small english mining town of bolsoverin Derbyshire

  enjoyed an unexpected 15 minutes of fame in 1979. While working a coal seam 500 metres [1640 ft] beneath the surface, local miners dislodged a gigantic fossilized dragonfly with a wing-span of half a metre

  [20 in], rivalling that of a seagull. Experts from the Natural History Museum in London confirmed that the fossil dated to the Carboniferous period, about 300 million years ago. The giant was dubbed the Bolsover dragonfly, but although one of the oldest and most beautifully preserved of fossil insects, it was far from unique. Similar fossils from the coal measures of Commentry in south-east France had been described by the French palaeontologist Charles Brongniart as long ago as 1885, and giant dragonflies had since been unearthed in North America, Russia and Australia. Gigantism was unusually common in the Carboniferous.

  The Bolsover dragonfly belongs to an extinct group of giant predatory flying insects, thought to have sprung from the same stock as the modern dragonflies ( Odonata) and known as the Protodonata. Like their modern counterparts, the Protodonata had long narrow bodies, huge eyes, strong jaws and spiny legs for grasping prey. Pride of place went to the largest insect that ever lived, the colossal Meganeura, which had a wingspan of up to 75 centimetres [30 in] and a diameter across the upper body

  — the thorax — of nearly 3 centimetres [just over an inch]. For compari-

  Oxygen and the Rise of the Giants • 77

  son, the largest modern dragonfly has a wingspan of about 10 centimetres

  [4 in] and a thoracic diameter of about 1 centimetre [1/3 in]). The prototype giant dragonfly differed mostly from its living relatives in the structure of its wings, which were primitive in the number and pattern of veins. The giant size and primitive wing structure led the French scientists Harlé and Harlé to propose in 1911 that Meganeura could never have managed to fly in our thin modern atmosphere. They argued that such a giant could only have found the power to fly in a hyperdense atmosphere containing higher levels of oxygen than the present 21 per cent. (If the extra oxygen was added to a constant amount of nitrogen, the air as a whole would be more dense.) This startling claim echoed down the corridors of twentieth-century science, to be repeatedly and vigorously rejected by the palaeobiological establishment. In 1966, the Dutch geologist M. G. Rutten could write, in a charmingly antiquated style that has passed forever from the scientific journals:

  Insects reached sizes of well over a metre during the Upper Carboniferous. In view of their primitive means of breathing, by way of trachea through the external skeleton, it is felt that these could only survive in an atmosphere with a higher O2 level. As a geologist, the author is quite satisfied with this line of evidence, but other geologists are not. And there is no way of convincing one’s opponent.

  Insect flight mechanics are notoriously complex. A famous, albeit spuri-ous, tale from the 1930s tells of an unnamed Swiss aerodynamicist who was said to have proved, on the basis of calculated flight mechanics, that the bumblebee cannot fly (in fact, he proved that the bumblebee cannot glide, which is quite true). We should not smile too superciliously, though; we have not progressed that far since then. In a detailed 1998 review of dragonfly flight, J. M. Wakeling and C. P. Ellington concluded that our grasp of dragonfly aerodynamics is limited by a poor understanding of the interactions between the two sets of wings, and admitted that we are unable to model their aerial performance with any confidence. In the face of such sweeping ignorance, we can hardly reach any firm conclusions about the composition of the ancient atmosphere on the basis of theoretical flight mechanics alone.

  Even so, the idea that giant insects may have required hyperdense, oxygen-rich air to fly was never quite discredited, and stubbornly refused to go away. We shall see that empirical measurements may yet succeed

  78 • THE BOLSOVER DRAGONFLY

  where theory has failed. Other factors imply that oxygen levels fluctuated during the modern era of plants and animals — the Phanerozoic (see Figure 1, page 27). Unequivocal geological evidence shows that the deep oceans contained little dissolved oxygen for at least a short spell, corresponding to the mass extinction at the end of the Permian period (250

  million years ago); and for this to have happened we can only presume that atmospheric oxygen levels fell, at least slightly. Conversely, if we are to believe the principle of mass balance (see Chapter 3, page 34), the vast amount of coal — which is essentially organic matter — buried in the Carboniferous and early Permian period must surely have forced the oxygen levels to rise. The question is, by how much?1

  The chief difficulty in calculating changes in the air is to identify which factors control the composition of the atmosphere over geological time, and which are relatively trivial. Early attempts to model atmospheric evolution would have had us believe that oxygen levels swung from less than zero to several times the present value. If nothing else, these studies drew attention to our surprising ignorance of the factors that actually do control oxygen levels in the atmosphere. The difficulties in modelling atmospheric change may of course reflect no more than an erroneous starting assumption: that changes took place when they didn’t. However, before dismissing the problem as one of our own making, we should note that the same difficulty applies to steady-state models, in which oxygen levels remain constant. We do not know how an unchanging oxygen level is maintained in the face of other environmental changes, which are known to have happened.

  Take fire as an example. Because fires consume oxygen they are assumed to limit the accumulation of oxygen in the atmosphere. In the absence of human meddling, fires are typically ignited by lightning strikes.

  Under present
conditions, most lightning strikes do not start fires because forest vegetation is damp, especially when electrical storms are accom-1 Rutten mentions the old argument that, because the bulk of photosynthesis is thought to come from oceanic plankton, it is not certain whether the flora of the Carboniferous really produced a measurable excess of oxygen. This argument is wrong: the important parameter is not absolute productivity, but burial. The complete decomposition of plankton means that far less carbon is buried at sea compared with the situation on land, where plants are more refractory to decomposition.

  Oxygen and the Rise of the Giants • 79

  panied by torrential rain. But if wet organic matter burns freely in air containing more than 25 per cent oxygen, as we are told, then, given an atmosphere with such levels, lightning could trigger conflagrations even in rain forests. The higher the oxygen level, the greater chance of fire; and as the fires rage they use up excess oxygen. If oxygen levels rise too high, fire would restore the balance.

  This simple scenario tends to be accepted uncritically, but is in fact quite misguided. Only if the forests are vaporized will the balance be maintained (just as we vaporize food when we burn it for energy during respiration, giving off carbon dioxide gas and water vapour in our breath).

  Anyone who has seen the gutted remains of a forest after a fire knows that a large amount of charcoal is formed. Charcoal is virtually indestructible by living organisms, including bacteria. No form of organic carbon is more likely to be buried intact.

  We have already seen that oxygen can accumulate in the air only if there is an imbalance between the amount of oxygen produced by photosynthesis and the amount consumed by respiration, rocks and volcanic gases. Permanent burial of organic matter is the most important way of disrupting this balance, because it prevents the consumption of oxygen by respiration. Organic remains that are buried are not oxidized to carbon dioxide, so the oxygen is left over in the air. As charcoal is more likely to be buried intact than normal decaying plant matter, the net result of a forest fire is to increase carbon burial, and thus to raise atmospheric oxygen. This in turn makes fire more likely and pushes up oxygen levels until finally life on land is destroyed. Only then, when all organic production and photosynthesis on land has ceased, can oxygen levels dwindle slowly, as the gas is removed by reaction with eroded minerals and volcanic gases. If perhaps a spore survived, life can strike up again; but if it does so, the cycle of flames and destruction will be repeated endlessly. Fire is a very poor control of atmospheric oxygen.

 

‹ Prev