One such organism is a microscopic animal called a "tardigrade." J.A.E. Goeze, who published the first paper on tardigrades in 1773, said, "Strange is this animal ... because it resembles a bear in miniature." Because of this description, and because tardigrades normally live in water, they are also known as "water bears."
Tardigrades look more like a candy Gummy Bear than a grizzly bear -- they have the bright orange, red or green colors of Gummy Bears, and a gummy surface texture. Their inflated round bodies have four pairs of stubby little legs. They use these clawed limbs to walk, grasping onto lichen or moss as they amble along.
If their environment dries up, the tardigrades undergo a process called "anhydrobiosis" ("life without water"). A sugar called trehalose moves into their cells to replace the lost water, and the tardigrade curls into a little ball called a "tun." Their metabolism lowers to a death-like 0.01% of normal, or is entirely undetectable. Depending on how long they have been in an anhydrobiotic state, tardigrades can become active again within a few minutes to a few hours after exposure to water.
Anhydrobiosis is just one type of a range of adaptable techniques called cryptobiosis. The other types of cryptobiosis are cryobiosis (cold temperatures), osmobiosis (salt water), and anoxybiosis (reduction of oxygen). Cryptobiotic animals were first documented in 1702 by Anton van Leeuwenhoek, when he observed tiny life forms in sediment collected from rooftops. He dried the "animalcules" to preserve them, and when he later added water he saw the creatures begin to move around. (The animals van Leeuwenhoek studied were probably rotifers-a microscopic organism that uses a wheel-like organ to swim and feed).
Because of their ability to withstand hostile conditions, tardigrades and other cryptobiotic organisms are of interest to astrobiologists. Some tardigrades can survive in temperatures as low as minus 200 degrees Celsius (minus 328 F). Others can survive temperatures as high as 151 degrees C (304 F). Tardigrades can survive the process of freezing or thawing, as well as changes in salinity, extreme vacuum pressure conditions, and a lack of oxygen. Tardigrades also are resistant to levels of X-ray radiation that are hundreds of times more lethal to humans and other organisms.
It is thought that tardigrades are widely distributed because they are carried on the wind, still clinging to their little bits of dried moss. This theory seems to be supported by the discovery of tardigrades on remote volcanic islands, where they could only have been deposited by wind or birds.
Garey believes that the tardigrade's preference for mosses and lichen is due to the wet/dry cycles these plants undergo. In areas that don't experience wet/dry cycles, tardigrades tend to be out-competed by other animals like nematodes. But nematodes are not as good at surviving without water as are tardigrades.
"In mosses and lichen, with their wet and dry cycles, tardigrades have found their ecological niche," says Garey. "While other organisms like nematodes and rotifers can also undergo anhydrobiosis, tardigrades are the most efficient at the process -- they do it best."
Like other animals, the ancestors of tardigrades probably first appeared during the Cambrian explosion, 540 million years ago. Tardigrades share a common ancestor with arthropods, nematodes, and onychophorans ("velvet worms"), because these animals all grow by molting (shedding their cuticle outer layer). These molting animals are classed together under the name Ecdysozoa.
Arthropods are a hugely diverse group of organisms -- they include such different animals as centipedes, lobsters, and fruit flies -- but they all have jointed appendages and a hard exoskeleton. Like arthropods, tardigrades have leg-like appendages that they use to move around, but unlike arthropods, tardigrade appendages are unjointed.
Tardigrades and nematodes both have a spear-like mouth part called a "stylet" that they use to pierce their prey and suck their juices as though through a straw. But tardigrades have two stylets, while nematodes only have one (arthropods, meanwhile, have jaws).
While tardigrades have been classified as nematodes, arthropods or onychophorans in the past, today tardigrades have their own separate phylum, Tardigrada.
Scientists aren't sure exactly when the tardigrade phylum first emerged. For one thing, there aren't many tardigrade fossils.
"A few examples of tardigrades that look just as they do today have been discovered encased in cretaceous amber," says Miller. "That would place them at about 100 million years old in their present form. Few other records exist because of their small size and soft bodies; they do not fossilize well and are even more difficult to see."
Garey is studying the DNA of tardigrades to pin down where they belong in the evolutionary diagram called the Tree of Life. An organism's position in the Tree can indicate when they appeared in the course of evolutionary history. But placing a precise date on their emergence has proved to be difficult.
"It's hard to really date the emergence of tardigrades, nematodes, and other such animals," says Garey. "Nematodes, for instance, have faster evolution rates than other animals. Also, dating based on nucleotide substitutions -- so-called molecular clock dating -- results in dates ranging as far as 700 million to 1.5 billion years ago."
By studying tardigrade DNA, Garey and his team also hope to figure out how all the different tardigrade species are related to each other.
The ability of terrestrial tardigrades to undergo cryptobiosis has led some to suggest that they could be transferred by Panspermia -- that is, between different planets via meteorites. Although he finds the concept highly unlikely, Garey says, "If you had to pick an animal candidate, I'd pick a tardigrade."
Perhaps future research will lend some credibility to this idea. Miller is mentoring a group of students in the NASA Student Involvement Program, and they have proposed a project to fly tardigrades on the space shuttle. This project could determine how tardigrades are affected by low gravity and test whether tardigrades can survive in space.
The dispersion of tardigrades is not well understood, and demands closer study. For instance, tardigrades seem to be more common in Temperate and Polar Regions than in the Tropics, but no one knows why. And some habitats that would seem to suit tardigrades perfectly are found not to support any tardigrade populations.
"In the wild, populations of tardigrades are patchy," says Garey. "You might find one area that is rich in tardigrades, while another nearby is completely barren. We don't know why, so more research needs to be done in this area."
Because most of the research on tardigrades has been done in Europe, tardigrade populations in South America, Australia, Asia, Africa, and North America are not as well documented. The same is true for the oceans: marine tardigrades have a higher diversity, and therefore may have more species, than tardigrades on land, but so far the marine environment is mostly unexplored.
"We know of 140 marine tardigrade species, but there are probably thousands more," says Garey.
To study the ecology of tardigrades in the wild, you first have to find them. Their small size makes identifying and collecting tardigrades a challenge, but Garey and his team are developing methods to extract DNA from the sediment in which the microscopic animals live.
Miller, meanwhile, is working on the description of several new species of tardigrades, and has a number of canopy, ecological, diversity, and taxonomic projects under way. He also is working on National Science Foundation grant proposals to study the tardigrades of China, Australia, and North America.