The moment we are born, we begin to die. Time’s arrow makes this process an inescapable reality. Biology offers a more life-affirming perspective on death, albeit death of a different kind. I’m talking about death at the level of individual cells, not the whole physiological system that marks death of the organism.

Our cells “kill themselves” continuously in a process known as apoptosis. This term comes from a Greek word that refers (rather poetically) to leaves falling from trees or petals falling from flowers.

Cell death is programmed by nature, and far from being morbid, it is an altruistic miracle. The body harbors an excess number of primitive, undifferentiated cells known as stem cells. During development, these cells communicate with each other to decide their ultimate fate. Some will end up as the eye, others, as the nose, and still others, the nervous system. The body keeps extra cells to act as “reserves”, just in case sometime goes wrong. For example, neurons that extend toward the muscles are initially produced in excess. After that, a process of “pruning” takes place, whereby excess neurons are killed. The result is a final number of neurons that matches the number of muscles that need these neurons. How wonderful.

For another example, consider our hands and feet. We are so accustomed to the five fingers in each of our hands that we rarely stop to ask how they were formed. Why, for example, do we not end up with the webbed limbs of a duck? The answer again points to cell death. In the embryo, the fore and hind limbs initially look like the blade of a boat paddle. At this stage, our digits are all fused together. A process of programmed cell death then takes place whereby cells occupying the interdigital space die off, opening up space as it were for our fingers to emerge as separate limbs. Wonderful is the fact that nature “knows” that a webbed feet is for ducks and fingers are for us.

Cell killing is also vital for maintaining cell composition and health. If excess cells aren’t killed, they are liable to grow uncontrollably and become tumors. Cell death is also the mechanism for replacement of aged cells. Only a few tissues, such as the lens cells of the eye, are maintained without replacement throughout life. Most other tissues are continuously renewed. For example, the lifetime of a red blood cell is only 120 days, after which it dies and is replaced. Cells at the base of each hair divide, differentiate, then die, leading to the growth of new hairs, something that gives barbers and hairdressers their livelihood. Our skin continuously renews itself by shedding dead cells from the surface after new, differentiated cells arrive. The cells lining our intestine is completely renewed every couple of days. Renewals such as these are for the good of our physiology. In the case of the skin and intestine, renewal protects these vital tissues and organs from the harsh insults their cells have to endure. Once again, cell death is nature’s amazing way of keeping us from breaking down before the times comes for the ultimate breakdown we call death that signals the end of a life.

This essay is adapted from Benny Shilo, Life’s Blueprint: The Science and Art of Embryo Creation, Yale University Press, 2014.