While we continue to think about death from philosophical, cultural or religious points of view, we also study it scientifically. Thanatology, the study of death and dying, is a recognized branch of medicine, with its own scientific journals. But thanatology focuses on the psychosocial aspects of death and dying; it does not ask questions about the nature of death itself. The branch of medicine called pathology describes in great detail the changes in the body, and its cells and tissues, that lead to, or accompany, disease and death. A pathologist can tell us whether a tissue is healthy or diseased, alive or dead. But about the precise nature of the razor-thin line separating life from death, the pathologist has little to say.

So what is death? One way to understand the death of a human being is to seek the smallest, ultimately indivisible unit - the "atom" of the ancient Greeks - of human life. That unit, that atom of life, is the cell. The cell is the smallest unit in the human body of which we can say, "This is alive!" And if we can define cells as having life, then it follows that it must be possible to describe them in the absence of life - when they are dead. What does a dead cell look like? What is it missing? Why is it dead? How did it make the transition from alive to dead? How did it die?

These are important questions, because the death of every human being begins with the death of just a few cells. We normally think of death in terms of death of the person - the integrated whole composed of personality, will, memory, passion and the hundreds of other things that make each of us unique. Most of these characteristics are housed in a specific portion of the brain - the cortex - and the loss of "personhood" that results from loss of cortical function is increasingly viewed as one of the most important aspects of human death. But clearly death must also have a biological meaning independent of the human condition. In the death of our cells, we are no different from all of the other organisms on earth condemned to die as a condition of birth. Snails die, as do worms and mushrooms, and their deaths too begin with the death of just a few cells.

The study of death at the level of individual cells has revealed unexpected subtleties and complexities about the nature of death in multicellular creatures like ourselves - for example, the widespread occurrence of suicide among cells in our bodies. Surprisingly, the study of evolutionarily older single-cell organisms suggests that cell aging and death is not an obligatory attribute of life on earth. Obligatory death as a result of senescence - natural aging - may not have come into existence for more than a billion years after life first appeared. This form of programmed death seems to have arisen at about the same time that cells began experimenting with sex in connection with reproduction. It may have been the ultimate loss of innocence.

Trying to grasp the meaning of an infinite and ever-expanding universe has led toward an enormous abyss in human understanding. From the edge of that abyss, we peer anxiously through our telescopes into the fog of the unknowable. If we travel in the other direction - if we turn inward and, with a succession of ever more powerful microscopes trace the process of death down through the level of individual cells and into the molecules and atoms of which they are composed - we come once more to a fog-filled abyss, one that separates the phenomenon we call life from the cold and indifferent physical universe. And we see through our microscope a figure, peering anxiously at us through a telescope...