Imagine you are LUCA (Last Universal Common Ancestor), floating in the shallow seas under the boiling Sun. You need energy and you need to reproduce. If you don’t reproduce, you will be torn to pieces one day - no physical matter can survive indefinitely without replication. But if you do replicate, if you succeed in cloning yourself, then you will survive in one sense or another. To clone yourself you need energy and you need a template. The best source of energy is the Sun. Even in the beginning, in the shallow seas, sunlight is splitting water and you are catching oxygen. In just a few hundred million years, your descendants will mimic this trick in photosynthesis, and release masses of oxygen into the air. Other descendants will become living machines that suck up this oxygen to fuel a spectacular leap in the potential of life: size, movement, strength, predation, consciousness, and mind.
The important point is this. From the very beginning, LUCA was using sunlight as an energy source. This had a downside: solar radiation produces a flux of free radicals, and the more energy, the more free radicals. Free radicals can destroy the all-important DNA template. Somehow, LUCA had to venture as close as she could to the sunlight for energy, without getting so close that her DNA template was jeopardized by free radical attack. The health of the first cells must have depended on getting this balance just right. This, in turn, must have meant detecting free radicals and responding somehow if too many (or too few) were present. How could this have been done? Detection and response might have been coordinated by means of proteins that change their function when oxidized by free radicals. Certainly, representatives of all three domains of life have proteins that respond to oxidation. Today, when proteins become oxidized, the cell responds by correcting the free-radical balance, either by swimming away from the source of danger, or by stepping up the antioxidant defences and repair mechanisms.
Free radicals are therefore dynamic indicators of the energy levels and general ‘health’ of the cell. They should have been among the earliest and most important indicators of cellular health, as they are a unique chemical bridge between the most basic traits of life - metabolism and reproduction. The ‘right’ number of free radicals indicates the ‘right’ balance between energy and replication. This is the critical point. Evolution works by building on existing system in the same way that the Spanish conquistadors built baroque cathedrals on the solid Inca walls of Cuzco. In biology, older foundations are rarely obliterated completely.
Nick Lane, Oxygen: The Molecule That Made The World