(11/1) Death, contrary to conventional wisdom, is not an essential part of life. Some forms of life are immortal. They never die.

Take, for example, organisms that consist of a single cell. Unless they encounter the microbial equivalent of getting hit by a truck, they never encounter death. After bacterial cells or yeasts or protozoans have lived a while, they divide and, presto, they are instantly reincarnated as two youthful individuals. The spark of life is passed to a new generation and yet the old generation leaves no corpse

Through repeated cycles of growth and division, life continually extends its power to convert more of the surrounding world (taking it in as food) into ever more copies of itself, copies of the molecular structures that make up its cells. The small daughter cells, as they are called, feed on bits of their environment and reorganize many of those molecules into parts of their living, growing selves.

Like a crystal whose geometry causes newly arriving atoms to link up in predictable patterns, the molecular structures of life dictate the architecture of newly incorporated matter. Thus, when the cell divides, be it a whole amoeba or a cell of a human embryo, the thing that is transmitted to each daughter cell is an architectural plan. The plan has inherent powers to organize still more nonliving matter as each new cell grows and divides. That plan, of course, is written in the code of DNA and then carried out by molecules dictated by the genes.

It’s worth looking at this from another perspective. The concept that life emerges from architectural arrangements within and between molecules developed quite slowly among biologists. Key players included some very small animals whose survival depends on their astonishing ability to enter into a state of being that conventional biology defined as death. Though quite small, these common but little-known life forms are made up of thousands of cells and many internal organs. They simply dry up, shriveling into kernels. In this state they do not eat or breathe. They do not move. Digestion stops. Nervous systems shut down. All metabolism ceases. For periods of weeks, months, and even years, the little things simply endure inert.

But with the return of moisture, they revive. Within minutes, the desiccated kernels absorb water and swell back to their normal proportions. All of the hundreds of biochemical processes that make for life as it is conventionally defined, resume. The lumps that may have spent years in a bottle on a shelf spring back into action when wetted, wriggling and writhing in a Petri dish, or literally crawling off the microscope slide in search of food. Up to this moment these tiny organisms existed in a limbo that challenged the conventional definition of life.

When most of us went to school, biology text books taught that life cannot exist without water. It is, after all, an essential player in the many chemical processes summed up in the word metabolism. When metabolism ceased, death ensued. Or so it was said.

Once you know about these little animals, you know that belief cannot be true.

These creatures include scores of species that can be found by the billions in almost every habitat on Earth--soil-dwelling animals such as nematodes and tardigrades and aquatic species like the vase-shaped rotifers of freshwater ponds and the brine shrimp that live in salt lakes. Although small--most are less than a millimeter across--they are complex, multicellular animals equipped with digestive tracts, reproductive organs, nervous systems, muscles and other specialized structures and tissues that make them more closely related to higher forms of life than to bacteria or protozoa.

These animals live in a variety of hostile environments, such as the thin soils of the Arctic where they remain frozen most of the time, and in hot desert sands where they receive so little moisture that they are active only a few days each year. And they inhabit virtually all other soils as well, including the yard of every reader of this column.

These animals pose a fundamental challenge to the old definition of life. According to that definition, when these organisms exhibit no sign of metabolism, they are not alive. But are they then dead? When moistened again, is life spontaneously generated? In the 19th century scientists debated these questions, orienting themselves into camps called resurrectionists and anti-resurrectionists. In the fashionable salons of France reviving dry nematodes under microscopes was a popular entertainment. Biologists have named this dormant state cryptobiosis, or hidden life.

Even today it is possible to experience this same phenomenon with brine shrimp "eggs" from a pet store. These aren’t really eggs; they are the dried cysts of embryonic brine shrimp. They come dry in a bottle, which you can keep on a shelf as long as you like. Make up a little slightly salty water in a glass, drop in some "eggs," and wait about 24 hours. You will have a swarm of little swimming larval shrimp, mainly used as food for tropical fish in an aquarium. In the right environment, the shrimp can grow to about half an inch long.

It is now known that as cryptobiotic creatures are drying, their cells make a kind of sugar called trehalose. These molecules wrap themselves around the inner structures of every cell, essentially encasing the connections. In the dry state, all that remains of these organisms is their structural integrity. A dry nematode, tardigrade or rotifer may have been deflated by the loss of water, but it maintains all the basic physical relationships that keep the structures of its cells together and that keep the cells in their proper places within the organism. Just add water, and the proteins and nucleic acids simply regain the ability to flex and wriggle and interact according to the chemical proclivities implicit in their structure.

Life, then, is not the product of some mystical "vital force" that inhabits cells, as once was believed. Life is the structural arrangement of specific molecules which behave chemically in specific ways but only in the presence of water. Water also participates in many chemical reactions and its electromagnetic properties influence the courses of chemical reactions and the shapes of intracellular structures. Take way the water, and all the molecular motions stop, locked in place.

We humans lack the ability to mothball our structures the way a brine shrimp or tardigrade can, and so we must die, but the life within each of our cells is exactly like that of those tiny animals. Actually, if you have a child, you have transmitted an immortal cell from your body—carrying its molecular architecture--to shape the development of that new individual.

Boyce Rensberger is a retired science journalist for The Washington Post and The New York Times and taught at the Massachusetts Institute of Technology.