That much is true, but the story is more complicated than this. Until recently the prevailing wisdom was that when oxygen appeared, thanks to photosynthesis by certain bacteria (cyanobacteria), it was toxic to most forms of life and this caused a mass extinction, an 'oxygen holocaust'. But some bacteria were able to adapt to oxygen and eventually to use it to produce energy, and the evolution of complex life was the result.
Nick Lane explains why this biologically plausible idea is wrong. Oxygen did not appear suddenly but rather in fits and starts punctuated by numerous episodes of 'snowball earth', and there was no oxygen-induced mass extinction. Moreover, the older theory postulated that oxygen, once it appeared, always remained at a fairly constant level in our atmosphere, but Lane thinks that it has fluctuated considerably; during the Carboniferous period, for example, it may have been as high as 35 per cent, which probably accounts for the existence of large insects such as giant dragonflies at this time.
The first seven chapters describe the evolution of complex life in relation to oxygen in considerable detail. Lane does not just narrate the story but provides the evidence on which his account is based. This adds an extra dimension to the book that is not always found in science writing for a non-specialist audience. It is a difficult thing to do well but I think Lane brings it off. Time and again he shows how an idea that initially seems plausible and convincing has had to be refined or replaced completely as knowledge advances.
Although there was no mass extinction due to oxygen, it remains true that oxygen is potentially lethal, because respiration necessarily involves the production of very reactive molecules called free radicals. At one time it was thought that these were the cause of ageing, and the idea that taking antioxidants will prevent or at least delay ageing are still widespread in alternative medicine. But attempts to affect ageing by giving antioxidants such as vitamin C have not succeeded and the theory is no longer accepted in mainstream science, at least in its original form. Lane has an interesting chapter on the vitamin C story to show how complex the question really is. Eating fruit and vegetables is good for you but the benefits are probably not due to their antioxidant properties. (Rather disarmingly, Lane admits to not liking fruit himself and also to enjoying smoking.)
And yet the older theory was not completely wrong. Oxidation does lie at the heart of the ageing process after all, although not in a simple way. The existence of complex cells like those that make up our bodies depends on the fact that they include structures known as mitochondria. These were once free-living organisms that entered into a symbiotic relationship with another species. Complex life could not exist without mitochondria. They produce energy by means of respiration, which entails the release of free radicals. This causes 'oxidative stress', which is a key concept in the discussion. Our cells have evolved the means of coping with this, which is fine when we are young but as we age the mechanism begins to wear out and break down. Ageing mitochondria leak free radicals and this may eventually lead to cell death (apoptosis).
So are free radicals bad for you? Yes and no—another twist in the story. Some of them are not simply unwanted by-products of respiration but have a vital role as signals within the cell. Resistance to infection depends on this system being turned on. So we are caught in a double bind.
I suggest that there is a trade-off between oxidative stress as a signalling pathway that musters our defences against infection and oxidative stress as a cause of ageing. In effect, the diseases of old age are the price we pay for the way we are set up to handle infection and other forms of stress in our youth. In both cases the shadowy agent pulling the strings is oxidative stress.This idea has important implications for medicine, Lane believes. Much research effort today is directed to trying to find individualised treatments for the diseases of old age, on the assumption that there is a remediable (probably genetic) cause if only it can be identified. Lane thinks this is a mistake.
This is the rising field of pharmacogenomics and woe betide anyone who says it is misguided. It is, though. Particular genes, or even whole genotypes, may predispose us to the common diseases of old age, but in a wider sense this is irrelevant.We cannot expect to cure diseases such as osteoarthritis or Alzheimer's disease by means of drugs. What we can hope to do is to improve the way our cells deal with oxidative stress, either by reducing it at source or by diminishing its effects. In this way we may be able to delay the onset of disease until later in life—ideally beyond the point at which we die of 'old age' due to final mitochondrial failure. This could mean a healthy life span of about 115 to 120 years.
The ideas outlined above appear again in Lane's later books, especially Power, Sex, Suicide, sometimes with minor updates and modifications. This does not mean that Oxygen is not worth reading in its own right. If you enjoy any of them you will probably want to read the others as well.