Nevertheless, it took a long time for apoptosis to emerge into the limelight. Apoptosis is over 20 times faster than mitosis, and sightings of dying cells are rare. In contrast to passive cell death (necrosis) — which features leakage and inflammation — apoptotic cells are engulfed and degraded by neighbouring cells without a trace. Various morphologies that we would regard as apoptotic have been observed since the middle of the nineteenth century. But it wasn't until the 1980s that apoptosis was generally credited, when Robert Horvitz and collaborators mapped the fate of every cell in the nematode Caenorhabditis elegans, including those that were committed to die. It emerged that cell death is determined by a handful of genes. These 'master switches' have been conserved in evolution so that they, or rather their equivalent families, still orchestrate apoptosis in mammals. However, like life, death is not that simple; the wealth of pro- and anti-apoptotic proteins in cells suggests that suicide is as carefully considered by the cell as it was by Camus.

In stating that it is not possible to enter the same river twice, Heraclitus expressed the irreversibility of time. We, too, undergo continuous changes. What controls the changes in the molecules that form our bodies? What provides our sense of permanence? How do our cells constitute a unified whole? Gradually, the idea emerged that the stability of the body is maintained by signals that control the life and death of single cells. This is a powerful concept, implying that there are specific survival and death signals, and corresponding receptors on cells. Such social control of life and death are vital in complex multicellular networks such as the immune system and the nervous system, where communication between cells is crucial.

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