Oxygen
Page 44
Lessons From Evolution on the Future of Ageing • 315
out). This is the distinction between the competing theories of Darwin and the French naturalist Jean Baptiste Pierre Antoine de Monet, better known as the chevalier de Lamarck. Lamarck believed in the inheritance of acquired traits, a theory that was popular in the Soviet Union during the Stalin years. Guided by the Marxist pseudoscience of Lysenko, Stalin hoped that imposing communism for a few generations would imprint the ‘genes’ for communism on the Russian people.
If Lamarck had been right, then a bird could become more like a chicken as it grew, just as a Russian might become a better communist.
The bird would then pass on its newly fledged chicken-genes to its offspring. If this were the case, the chicken would come before the egg. There is nothing illogical about this scenario — it is in fact what bacteria do.
Bacteria do not generate sex cells. When they divide, they pass on any new traits they have acquired to both daughter cells. As it happens, this is not how sexual species pass on their genes. In sex, the body is thrown away as a genetic dead end, while the sex cells contain all the inheritable genes. The genetic changes that led to the evolution of a chicken therefore took place in one or other of the sex cells — or both — and came together in the egg on fertilization. This means the first chicken must have hatched from an egg laid by a bird that was not a chicken. Clearly the egg came first.
The first chicken, of course, did not appear suddenly: there was a gradual transition from non-chickens (actually, the red jungle fowl Gallus gallus) to domestic chickens. The eggs of earlier birds therefore evolved before chickens. Eggs with hard shells were in fact invented by the reptiles, around 250 million years ago. After the Carboniferous, the climate grew cooler and drier, and the great coal swamps dried up. The first reptiles developed scales and shelled eggs to escape the constraints suffered by amphibians, which depended on water. Eggs with shells could be laid on land and did not dry out. This was beginning of the ‘age of reptiles’, which lasted until the demise of the dinosaurs, 65 million years ago. As a related historical accident, the hard shell made copulation necessary. The shell forms before the egg is laid, so fertilization has to take place internally. Thus, all reptiles copulate, and passed on this trait to their descendants — the birds and mammals. A little understanding of the history of life, then, tells us that both copulation and eggs came before chickens —
and the historical narrative makes the idea of infinite regression seem absurd.
316 • LIFE, DEATH AND OXYGEN
The role of oxygen and free radicals in ageing and disease presents a similar problem: which comes first, the radicals or the disease? In the 1950s, Rebeca Gerschman, Daniel Gilbert and Denham Harman argued that the reactive intermediates of oxygen respiration caused ageing and disease.
Such big claims have not been proved experimentally even today. Even so, many people, including some eminent researchers, cling to the belief that antioxidants are a miracle cure. Most researchers in the field, though, would concur with the characteristically trenchant view of John Gutteridge and Barry Halliwell: “By the 1990s it was clear that antioxidants are not a panacea for ageing and disease, and only fringe medicine still peddles this notion.”
Few scientific fields hold a greater promise of fame — a cure for ageing! — than the free-radical field, and none has suffered so many reversals. Much of the excitement that followed the discovery of superoxide dismutase (SOD) in the late 1970s (see Chapter 10) dissipated as pharmaceutical drugs failed to deliver the anticipated miracle. High doses of dietary antioxidants likewise failed to impress. The field too easily became a forum for bad science, in which claims were unsupported by hard evidence. We examined an instance of this in Chapter 9: the claim that plasma levels of vitamin C correlate inversely with mortality. They do, it is true, but the unspoken implication — the reason this study was published in The Lancet — is that if we eat more vitamin C we will be less likely to die. Perhaps this too is true, but the study came nowhere near proof, as the authors themselves were first to admit: if anything, it proved the contrary, as people taking vitamin C supplements gained no extra benefit. The claim is equivalent to saying that the number of hours we spend on our feet correlates inversely with mortality, so if we stand up more often we will live longer.
Not surprisingly, other fields of medicine have become suspicious of the boy who cried wolf too many times. This feeling was conveyed well by one of the reviewers of my proposal for this book:
I confess to some prejudice against the free-radical field, which is complex and messy and seems at the same time to attract messianic types, who think that free radicals explain all diseases, not to mention ageing. (So if we eat enough free-radical scavengers we shall all live forever.) Of course this is not to say that free radicals are unimportant or uninteresting, only that it is difficult to separate the science from the hype.
Lessons From Evolution on the Future of Ageing • 317
Apart from the hype, there are some genuine scientific difficulties here. I wonder if it is even possible, using direct experimental methods, to prove that free radicals cause disease. The problem is that most free radicals are present at tiny concentrations and for fleeting periods: no sooner are they formed than they transform into something else. The only known way of measuring free radicals directly is a technique called electron spin resonance (ESR), which can detect tiny magnetic signals deriving from the spins of unpaired electrons in free radicals (see Chapter 6). Unfortunately, these transient signals are easily lost against background noise, and the method is not sensitive enough to detect radicals as reactive as the hydroxyl radical, which disappear in billionths of a second. There are ways around this difficulty, but they generate their own problems of interpretation.
The easiest way to skirt these issues is to measure free radicals indirectly, by quantifying the build-up or excretion of their end-products —
in particular, oxidized DNA, proteins and lipids. Now the problem is one of attribution: do the oxidized products really reflect free-radical attack in the body? For example, one oxidized breakdown product of DNA is 8-hydroxydeoxyguanosine (8-OHdG). We have seen that 8-OHdG is formed when hydroxyl radicals attack DNA, but some 8-OHdG is formed as an artefact, and some may be formed by enzymes. Estimates of total DNA oxidation by hydroxyl radicals are in reality a best guess. While it would be perverse to ignore the large body of evidence which suggests that free radicals actually cause disease, definitive claims are no more than hype.
The same applies to other measures of free-radical formation, including standard tests for the products of lipid and protein oxidation. We cannot infer the definite involvement of free radicals from such tests any more than we can infer deliberate arson from the smoking remains of a building. If we accept that oxidized proteins, DNA and lipids are the tell-tale footprints of free radicals, we still do not know whether free radicals cause disease. We have little idea about timing or causality. The signs of oxidation are often concomitant with the signs of disease, but this is not to say that one causes the other. The simplest way to prove that free radicals do cause disease is to block their action using antioxidants. As we have seen, antioxidants rarely cure diseases, let alone ageing. Of the many possible explanations for this — perhaps they are not potent enough, or do not get to the right place in the right amount at the right time — the most inherently believable is that free radicals are only part of the problem. Even when antioxidants do help, it is not easy to prove that they do so by working as an antioxidant. In the case of vitamin C, many of its actions
318 • LIFE, DEATH AND OXYGEN
have nothing to do with antioxidant activity: it may act by stimulating the synthesis of carnitine, or the production of peptide hormones and neurotransmitters. Short of a methodological breakthrough, it is difficult to progress beyond this point with present experimental techniques.1
Set against the potential dead end of experimental research is the intuitive expla
natory power of free radicals as a cause of ageing and disease. The fact is that free radicals are detected in virtually every disease known to man, and that in principle they can explain the progression of ageing and the rising incidence of age-related diseases. The massive accumulation of data is at least suggestive that free radicals have a causal role in many conditions. Many other facts line up with this interpretation. To take just one example: if free radicals are produced by mitochondria, we would expect DNA in the mitochondria to be damaged more than DNA in the nucleus. In Chapter 13, we saw that there are practical problems in measuring this predicted difference — estimates vary by a factor of 60 000.
However, a high rate of damage should lead to a high mutation rate, and the mutation rate of mitochondrial genes is higher than the mutation rate of nuclear genes by an order of magnitude. The free-radical explanation is therefore supported by evidence from other quarters.
I think that we can best understand the importance of oxygen free radicals by looking at their place in a bigger picture. We cannot prove experimentally that free radicals cause disease any more than we can prove logically that eggs cause chickens, but we can see how far free radicals fit into an evolutionary framework by telling the story of oxygen. The story we have pieced together in this book goes some way towards answering intractable experimental questions. The answers hold significant implications for the future of medicine. Before we peer into future possible worlds, let me recap the story, bringing out the elements most important to our own lives and deaths.
In the beginning there was no oxygen, but there was ultraviolet radiation and water. Without an ozone layer, the intensity of ultraviolet radiation 1 One such breakthrough has already happened: the advent of ‘knock-out’ mice, in which the genes for enzymes like SOD are disabled so that the protein is not produced. Such experiments are valuable, and illustrate the importance of SOD in newborn mice; but the fact that SOD is necessary in baby mice does not mean it is important in ageing people. The extension of lifespan in Drosophila by the overproduction of SOD and catalase is more revealing, but limited, so far, to Drosophila.
Lessons From Evolution on the Future of Ageing • 319
in the air and surface oceans was at least 30 times greater than today. Radiation splits water to produce the same reactive oxygen intermediates that we generate when we breathe — hydroxyl radicals, superoxide radicals and hydrogen peroxide. These unstable intermediates reacted together, and with water, to generate hydrogen and oxygen. Hydrogen was light enough to seep away into space. Oxygen reacted with iron in the rocks and with sulphurous gases emanating from volcanoes, and was trapped in the crust.
In the thin, dry air of Mars, the oxygen intermediates were petrified, literally, as the red iron oxides that lend the planet its colour today.
On Earth, something different happened. Life adapted to the surface oceans. LUCA, the Last Universal Common Ancestor (Chapter 8), had already evolved antioxidant enzymes that could protect her against the reactive oxygen intermediates derived from radiation. Genetic studies suggest that LUCA possessed antioxidant enzymes, including SOD, catalase and peroxiredoxins. More than this, LUCA had a sophisticated metabolism. She could trap oxygen using a form of haemoglobin, and generate energy from it, using the enzyme cytochrome oxidase — the grand ancestor of the enzyme that continues to do the same job for us. LUCA could do all this as long as 3.85 billion years ago, soon after the end of the meteorite bombardment that cratered the Moon and the Earth. Even though free oxygen had not yet accumulated in the air, the earliest ancestor that we know about was already generating energy from oxygen respiration, and was resistant to oxidative stress.
The oxygen intermediates formed by radiation reacted with dissolved iron salts and hydrogen sulphide, gradually depleting these substances from the shallow seas and lakes. Both were early raw materials for photosynthesis, so their depletion raised the selective pressure to find an alternative. In such sheltered environments hydrogen peroxide was relatively abundant, and it was a practical alternative as it could be split by the antioxidant enzyme catalase. Catalase thus doubled as a photosynthetic enzyme. As multiple enzymes clustered around the photosynthetic reaction centres, two catalase units became lashed together to form an ‘oxygen-evolving complex’. This complex could harness the energy of sunlight to split water and release oxygen. Water-splitting, oxygen-producing photosynthesis evolved only once on Earth. In a thought-provoking quirk of fate, all life on Earth that uses water as a raw material for photosynthesis has inherited a water-splitting complex based on catalase units. This could never have happened if life had not learnt how to tolerate radiation first. Perhaps it would never have happened without catalase; and it
320 • LIFE, DEATH AND OXYGEN
almost certainly never did happen on Mars. The ostensible sterility of Mars might be put down to this detail alone.
On Earth, photosynthetic cyanobacteria injected oxygen into the air, and fast: faster than the volcanoes could spew out sulphurous gases, faster than erosion could expose virgin rocks to the air. The crust oxidized, but still there was some oxygen left over. When radiation split water, the hydrogen could no longer escape into outer space: instead it reacted with the excess oxygen to form water again. As oxygen built up, an ozone layer formed, which blocked the penetration of ultraviolet radiation into the lower atmosphere. The loss of oceans slowed to a trickle on Earth but continued apace on Mars and Venus, where no oxygen buffer had formed.
Loss of water in this way may have cost Mars and Venus their oceans.
Water retention was the first gift of photosynthesis. The second was oxygen itself. At several times in the long Precambrian era, catastrophic geological upheavals — snowball glaciations and bouts of mountain building — overturned the lengthy periods of evolutionary stasis, and buried so much organic matter that excess oxygen was injected into the air. Each time, life leapt forward. In the first injection, 2.7 billion years ago, our own ancestors, the eukaryotes, made their first wispy appearance, leaving behind tell-tale molecular fingerprints — sterols similar to cholesterol. In the next, larger, injection, which followed the snowball Earth and mountain-building episodes around 2.3 to 2.2 billion years ago, the eukaryotes made their first robust appearance in the fossil record. Soon after, the fossil record began to show signs of multicellular algae; but little else happened in the next billion years. Then came the greatest of all geological roller-coaster rides: a succession of at least two snowball Earths, in which ice shrouded the Earth episodically over a 160 million-year period and finally pushed atmospheric oxygen up to modern levels.
Afterwards, as the ice retreated and the dust settled, the first large animals floated on-stage — jellyfish-like bags of protoplasm, the vegan Vendobionts. With their bulk they had guts and made faeces. Their heavy faecal pellets sank and were buried in the ocean depths, depleting the organic matter in the ocean. Not only this, but faecal burial prevented the breakdown of organic matter by respiration (and therefore the consumption of oxygen), and so contributed to the oxygenation of the overlying oceans. The bacteria were forced to retreat, taking their stinking, sulphurous underworld with them. A new, oxygenated, ecosystem yawned open, a vast blank canvas awaiting the hand of nature. With oxygen came a fourfold leap in organisms’ ability to extract energy from their food, and
Lessons From Evolution on the Future of Ageing • 321
with that came predation. Now, for the first time, it really paid to eat, and extended food chains became possible. Life burst into the vacant eco-space in the Cambrian explosion, 543 million years ago. The oceans filled up with scampering, armour-plated monsters, the hunters and the hunted.
With predation came evolutionary arms races, in which hunters and hunted competed for size (which depended on oxygen for energy and to build structural support). With size came the complex adaptations that enabled the colonization of the land.
Behind the scenes, oxygen was puppeteer. The first single-celled eukaryotes were degenerates, scavengers in a
world of assiduous bacteria.
They had lost the metabolic prowess of LUCA and scraped a living by fermenting organic remains or engulfing bacteria. They needed oxygen to make their membranes, but couldn’t bear too much of it. Then one day a eukaryotic cell happened to swallow an oxygen-guzzling purple bacterium. Suddenly it could swim with apparent impunity through the shallow seas, protected from oxygen by its internal vacuum cleaner. The insider deal blossomed into a Mephistophelean pact: as the purple bacteria turned into modern mitochondria, they exchanged their surplus energy for a life spent dicing with death. Oxygen mutated DNA, forcing genes to change and evolve. It must have been one of the factors that drove the evolution of the most efficient of all genetic cleansing mechanisms, sexual recombination. But mitochondria presented a unique and profound problem of their own: they retained some genes that were necessary for the function of the eukaryotic cell as a whole. Stranded in the belly of the furnace, and prevented by their host cell from dividing as fast as bacteria, mitochondrial genes could not be rejuvenated by sex nor by bacterial-style selection. They could only degenerate. The solution was not sex but sexes. Given two sex cells that would combine to form the next generation, one cell could be powered by mitochondria destined for disposal, like the fuel tanks of a rocket, while the other could maintain its mitochondrial population in a dormant state, like hibernating astronauts, ready for duty on arrival in the next body. Early in embryonic development, the pool of sleeping mitochondria would be siphoned off and kept
‘on ice’ for the next generation.
With sex and gender came redundancy. When only some genes are passed down to the next generation, all other attributes are subsidiary to the transmission of these genes. Redundancy allows the specialization of support cells and ultimately of whole bodies. Bodies fundamentally became