There is a fine line between animate and inanimate that makes it difficult to define life clearly. And similarly, in the animate world there is a fine line between life and death. Many organisms are capable of a state of suspended animation, but for fairly short periods, as in hibernation. In some cases, however, the level of unresponsiveness is easily interpreted as death. Certain micro-organisms, seeds and endospores can become inanimate for very long periods, even for centuries. It is not yet known why this ability is not more common, or whether the potential for suspended animation exists in more organisms than exhibit it.
Hibernation. The most familiar form of suspended animation is the periodic type, hibernation. Some animals that inhabit regions with severe winters withdraw into a deep sleep during the months when their food sources are unavailable. The hibernators include a range of creatures: bears, bats, snakes, turtles, and even some species of fish. In hibernating animals the metabolic and circulatory systems slow to a bare minimum, greatly reducing respiration, pulse and body temperature. The energy to sustain hibernating animals is supplied by fat accumulated in anticipation of the deep sleep.
Many plants and trees in cold climates also become dormant during the winter. In all such cases, the dormancy is seasonal and therefore short-term. There are seeds, however, that may survive in a dormant state for years or even centuries. In one recent example, well preserved lotus seeds ranging in age from 100 to more than 1000 years were successfully germinated by scientists at the University of California, Los Angeles.
DNA. Ancient organisms that were trapped in substances such as amber, salt or ice provide a wealth of scientific information because they are so perfectly preserved. Although most such organisms are recovered dead, the situations vary. Although organisms in amber are dead, the DNA from some specimens is so well preserved that an extinct genome can be reconstructed. The half-life of DNA is about one million years, so retrieving viable material from fossil remains is problematic. DNA preserved in amber, however, may be easier to recover.
Water drops encased in ancient salt have been found to maintain colonies of microbes for centuries. Scientists at Binghamton University in England were able to examine countless generations of the microbes trapped in a single droplet, the living as well as the very well preserved dead. More important, the DNA of the dead bacteria was in such good condition that it was possible to compare the microbial genomes over an exceptional span of time.
Microbes. Deep ice may harbor many types of dormant microbes. The tiniest bacteria ever discovered, H. glaciei, were found in a frozen state deep under a Greenland glacier. The microbes, more than 100,000 years old, were successfully revived. In Antarctica, bacteria frozen for some 8 million years in the oldest ice known were revived. Despite DNA damage and extremely slow cellular processes, the bacteria did revive. Those bacteria groups that are able to reduce to such an attenuated state for unlimited periods (e.g., Bacillus subtilis) are called endospores. In general, they are able to maintain the dormancy in many ordinary environments. They revive quickly when conditions improve. One of the most famous cases, and one in which the evidence of isolation from contaminants seemed especially strong, involved 2000-year-old endosperms of Thermoactinomyces vulgaris, which were completely sealed in an ancient Roman fort until their discovery.
Viruses. Viruses are able to lie dormant indefinitely by slipping into the genome of a host organism. The host has silencer genes that can also suppress a virus in this way --by leaving it in the genome in a suspended state. In both cases, the result is an accumulation of dormant viruses in genomes that may or may not wake up and resume their pirating of the host cells. The biochemical mechanisms behind virus dormancy are not understood, but they are of major interest to evolutionary biologists and medical researchers.
Plants. "Resurrection plants" represent a diverse group known as poikilohydric plants, characterized by the ability to sustain profound dehydration, even for decades. Although appearing dead, if exposed to water, these plants spring to life rapidly. Almost immediately they flower and germinate. Resurrection plants are found in desert areas where rains are irregular. The most familiar example is probably the Rose of Jericho (Anastatica hierochuntica), native to North Africa, but there are varieties elsewhere in many parts of the world, including Africa and Central and South America. The ability of resurrection plants to minimize metabolic functions for great lengths of time and withstand extreme desiccation is based on processes similar that of seeds.
Seeds Many seeds can survive for prolonged periods, and some can maintain dormancy for centuries. Lotus seeds, for example, ranging in age from 100 to more than 1000 years, have been successfully germinated by scientists at the University of California, Los Angeles.
Eggs. A Cornell University scientist retrieved from deep lake sediments the century-old eggs of zooplankton. Surprisingly, the eggs were successfully hatched in the lab. The scientist reported genetic differences between the ancient zooplankton and the contemporary forms from the same lake.
Embryos. Embryos generally develop according to species schedule. However, in a few species, birth is deferred until environmental conditions are optimal. One rare example is the pig-nosed turtle of Australia. This species, like most fresh-water turtles, lays its eggs on dry ground. However, the pig-nosed turtle eggs do not hatch until the nest area is flooded by seasonal monsoon-like rains. When the floods are slow to arrive, the embryos are able to maintain a state of suspended animation. This ability belongs only to the pig-nosed turtle.
For years, scientists have worked on technologies for freezing human embryos. In 2005, a human embryo that had been frozen for 13 years was revived and implanted in the mother. The fetus developed normally and a baby girl in excellent health was born nine months later.
The tardigrade One of the most fascinating microbes is the multicellular but microscopic tardigrade, pictured above. Varieties of this organism are found in all environments --even the most extreme-- on Earth. The tardigrade has the most unusual ability to transition to a freeze-dried state of suspended animation, indistinguishable from death, if environmental circumstances make it necessary. It can maintain that state until conditions make survival possible. Scientists are subjecting tardigrades to all sorts of environments, including space, in an attempt to discover the secret of this remarkable dormancy.
Human revivals. Revival from presumed death. Numerous cases have been confirmed of people who were thought to be clinically dead but were later revived. In each case the victims --all from accidents-- had no detectable pulse or respiration when examined, but then surprisingly, they revived in the hospital. The most reliable cases seem to be victims of freezing on land or in water. The longest "death" seems to be the case of a young Japanese hiker. He was found frozen and without a pulse on a mountain some three weeks after a fall that left him unconscious. Other cases involve skiers, hikers, and lost children, all with the same profile: an accident followed by a lengthy frozen state. The victims, all discovered after a considerable time, have neither pulse nor respiration. When taken to the hospital they were successfully revived and found normal.
Cryogenics. Cryogenics is an extreme preservation process in which the body is reduced to an absolutely minimal state of life through a combination of careful freezing and replacement of blood with special fluids. This halts the cellular processes, including those associated with aging and disease. The objective is to preserve the body in exactly the condition it was in at the time of the operation. There is serious medical and scientific interest in suspended animation. For cases of trauma or disease, it would prevent further deterioration until proper medical treatment or a cure is available. And for scientists anticipating the preparation of humans for long distance space travel, suspended animation might provide a way to transport humans and animals to destinations that might require years of travel. Animal experiments have preserved dogs and other animals for several hours; the animals were fully revived with blood transfusions.