Imagine a world without photosynthesis. It wouldn’t be green, for a start. Our emerald planet reflects the glory of plants and algae, and ultimately their green pigments, which absorb light for photosynthesis. First among pigments is the marvellous transducer that is chlorophyll, which steals a beam of light and conjures it into a quantum of chemical energy, driving the lives of both plants and animals.

The world probably wouldn’t be blue either, for the azures of the heavens and the marines of the oceans depend on clear skies and waters, cleansed of their haze and dust by the scouring power of oxygen. And without photosynthesis there would be no free oxygen.

In fact there might not be any oceans either. Without oxygen there is no ozone; and without that, there is little to cut down the searing intensity of ultraviolet rays. These split water into oxygen and hydrogen. The oxygen is formed slowly and never builds up in the air; instead it reacts with iron in the rocks, turning them a rusty-red colour. And hydrogen, the lightest of gases, evades the tug of gravity and slips away into space. The process may be slow but it is also inexorable: the oceans bleed into space. Ultraviolet radiation cost Venus its oceans, and maybe Mars too.

So we don’t need much imagination to picture a world without photosynthesis: it would look a lot like Mars, a red dusty place, without oceans, and without any overt signs of life. Of course, there is life withoutphotosynthesis, and many astrobiologists seek it on Mars. But even if a few bacteria are found hiding beneath the surface, or buried in an icecap, the planet itself is dead. It is in near-perfect equilibrium, a sure sign of inertia. It could never be mistaken for Gaia.

Oxygen is the key to planetary life. No more than a waste product of photosynthesis, oxygen really is the molecule that makes a world. It is let loose by photosynthesis so fast that it finally overwhelms the capacity of a planet to swallow it up. In the end, all the dust and all the iron in the rocks, all the sulphur in the seas and methane in the air, anything that can be oxidised is oxidised, and free oxygen pours into the air and the oceans. Once there, oxygen puts a stop to the loss of water from the planet. Hydrogen, when released from water, inevitably bumps into more oxygen before it finds its way out into space. Swiftly it reacts to form water again, which now rains back down from the heavens, drawing to a halt the loss of the oceans. And when oxygen accumulates in the air, an ozone shield forms, ablating the searing intensity of the ultraviolet rays, and making the world a more habitable place.

Oxygen doesn’t just rescue a planet’s life: it energises all life, and makes it big. Bacteria can do perfectly well without oxygen: they have an unparalleled skill at electrochemistry, they are able to react together virtually all molecules to glean a little energy. But the sum total of energy that can be derived from fermentation, or by reacting two molecules like methane and sulphate together, is negligible in comparison with the power of oxygen respiration – literally the burning up of food with oxygen, oxidising it fully to carbon dioxide and water vapour. Nothing else can provide the energy needed to fuel the demands of multicellular life. All animals, all plants, all of them depend on oxygen for at least part of their life cycle. The only exception that I’m aware of is a microscopic (but multicellular) nematode worm that somehow gets along in the stagnant oxygen-free depths of the Black Sea. So a world without free oxygen is microscopic, at least at the level of individual organisms.

Oxygen contributes to large size in other ways too. Think of a food chain. The top predators eat smaller animals, which might in turn eat insects, which eat smaller insects, which live on fungus or leaves. Five or six levels in a foodweb are not uncommon. At each step energy is wasted, for no form of respiration is ever 100 per cent efficient. In fact, oxygen respiration is about 40 per cent efficient, while most other forms of respiration (using iron or sulphur instead of oxygen, for example) are less than 10 per cent efficient. This means that, without using oxygen, the energy available dwindles to 1 per cent of the initial input in only two levels, whereas with oxygen it takes six levels to arrive at the same point. That in turn means that long food chains are only feasible with oxygen respiration. The economy of the food chain means that predators can operate in an oxygenated world, but predation as a lifestyle just doesn’t pay without oxygen.

Predation escalates size, of course, driving arms races between predator and prey. Shells combat teeth, camouflage tricks the eye; and size intimidates both hunter and hunted. With oxygen, then, predation pays; and with predators size pays. So oxygen makes large organisms not just feasible but also probable.

It also helps build them. The protein that gives animals their tensile strength is collagen. This is the main protein of all connective tissues, whether calcified in bones, teeth and shells, or ‘naked’ in ligaments, tendons, cartilage and skin. Collagen is by far the most abundant protein in mammals, making up a remarkable 25 per cent of total body protein. Outside the vertebrates, it is also the critical component of shells, cuticles, carapaces and fibrous tissues of all sorts – the ‘tape and glue ’ of the whole animal world. Collagen is composed of some unusual building blocks, which require free oxygen to form crosslinks between adjacent protein fibres, giving the overall structure a high tensile strength. The requirement for free oxygen means that large animals, protected with shells or strong skeletons, could only evolve when atmospheric oxygen levels were high enough to support collagen production – a factor that might have contributed to the abrupt appearance of large animals in the fossil record at the beginning of the Cambrian period, some 550 million years ago, soon after a big global rise in atmospheric oxygen.

The need for oxygen to make collagen may seem no more than an accident; if not collagen why not something else with no requirement for free oxygen? Is oxygen necessary to give strength or just a random ingredient that happened to be incorporated and then forever remained part of the recipe? Wedon’t really know, but it’s striking that higher plants, too, need free oxygen to form their structural support, in the shape of the immensely strong polymer lignin, which gives wood its flexible strength. Lignin is formed in a chemically haphazard way, using free oxygen to form strong cross-links between chains. These are very difficult to break down, which is why wood is so strong and why it takes so long to rot. Eliminate lignin from trees – a trick that manufacturers of paper have tried, as they need to remove it laboriously from wood pulp to make paper – and the trees slump to the ground, unable to sustain their own weight even in the lightest breeze.

So without oxygen there would be no large animals or plants, no predation, no blue sky, perhaps no oceans, probably nothing but dust and bacteria. Oxygen is without a doubt the most precious waste imaginable. Yet not only is it a waste product, it is also an unlikely one. It is quite feasible that photosynthesis could have evolved here on earth, or Mars, or anywhere else in the universe, without ever producing any free oxygen at all. That would almost certainly consign any life to a bacterial level of complexity, leaving us alone as sentient beings in a universe of bacteria….