Cancer is such a ruthless adversary because it behaves as if it has its own fiendishly cunning agenda. Cancer cells come pre-programmed to execute a well-defined cascade of changes, seemingly designed to facilitate both their enhanced survival and their dissemination through the bloodstream. There is even an air of conspiracy in the way that tumours use chemical signals to create cancer-friendly niches in remote organs.

In the frantic search for an elusive "cure", few researchers stand back and ask a very basic question: why does cancer exist? What is its place in the grand story of life? Astonishingly, in spite of decades of research, there is no agreed theory of cancer, no explanation for why, inside almost all healthy cells, there lurks a highly efficient cancer subroutine that can be activated by a variety of agents – radiation, chemicals, inflammation and infection.

Cancer, it seems, is embedded in the basic machinery of life, a type of default state that can be triggered by some kind of insult. That suggests it is not a modern aberration but has deep evolutionary roots, a suspicion confirmed by the fact that it is not confined to humans but is widespread among mammals, fish, reptiles and even plants. Scientists have identified genes implicated in cancer that are thought to be hundreds of millions of years old. Clearly, we will fully understand cancer only in the context of biological history.

Two relevant evolutionary transitions stand out. The first occurred over 2 billion years ago, when large, complex cells emerged containing mitochondria – tiny factories that supply energy to the cell. Biologists think mitochondria are the remnants of ancient bacteria. Tellingly, they undergo systematic changes as cancer develops, profoundly altering their chemical and physical properties.

For most of Earth's history, life was confined to single-celled organisms. Over time, however, a new possibility arose. Earth's atmosphere became polluted by a highly toxic and reactive chemical – oxygen – created as a waste product of photosynthesis. Cells evolved ingenious strategies to either avoid the accumulating oxygen or to combat oxidative damage in their innards. But some organisms turned a vice into a virtue and found a way to exploit oxygen as a potent new source of energy. In modern organisms, it is mitochondria that harness this dangerous substance to power the cell.