Miles below the ocean surface exist some of the most fascinating habitats for life on Earth -- and clues to habitats for possible life beyond Earth.

From time to time, hydrothermal vents, known as "black smokers," occur along these ridges and erupt as underwater geysers. At these sites, cold ocean water seeps through cracks in the seafloor to hot spots underground. The water gets superheated to several hundred degrees Celsius and is spit back up in a mineral-rich broth of scalding fluid. And in this bizarre environment, life flourishes.

Active vents are inhabited by a complex ecosystem of organisms containing both microbial and more complex animal life. (There is no plant life in the deep ocean, because sunlight cannot reach down that far to drive the process of photosynthesis on which plants depend). The animal life includes tube worms, shrimp, clams, mussels and crabs.

Until a little over 20 years ago, no one knew that deep-sea hydrothermal vents existed, much less that they were teeming with life. The first such vent was discovered in 1977 east of the Galapagos Islands. Since then, dozens of vents have been discovered and explored along ridges in the Atlantic and Pacific.

The study of how animal populations evolve and disperse geographically is known as "biogeography." Hydrothermal vents offer a unique opportunity for biogeographers because the underwater environment is affected by fewer factors than are land environments.

"People study biogeography on land and its always got superimposed on it the effects of latitude and climate," said Van Dover. Hydrothermal vents, in contrast, are "largely decoupled from climate. They are isolated from what goes on above."

Most vent organisms, scientists believe, can exist in their adult form only near an active vent site (although many of these organisms have swimming larval states that can travel for great distances). Individual vents remain active for anywhere from a few decades to a few thousand years. When a vent shuts off, the adult animals living there die. Yet as soon as a new vent emerges, it is rapidly colonized.

Within a few years, a new vent undergoes a complete transformation from uninhabited to fully populated.

By studying the similarities and differences among the animals that live at different vents, scientists have begun to piece together a picture of how organisms move from one vent to another, what the natural barriers are to such movement and how the geography of the deep ocean affects the evolution of the species that inhabit it.

Most of the research into the fauna (animal life) that inhabit hydrothermal vent systems has been done in the northern region of the Mid-Atlantic Ridge and along the East Pacific Rise, which runs roughly parallel to the west coast of South America.

Although similar types of animals can be found at both Atlantic and Pacific vent sites, there is more similarity among the vent ecosystems along the same ridge than there is between the two ridges. For example, shrimp are found at both Atlantic and Pacific sites, but one particular type of shrimp, known as "swarming shrimp," is found only in the Atlantic.

The Pacific is a very old ocean, while the Atlantic is relatively young, having fully formed only about 120 million years ago. One question scientists are interested in is how the animals that inhabited the Pacific ridge system made their way to the younger Atlantic ridge.

Some vent organisms, for example, vent shrimp, havent been around all that long. They are thought to have evolved only 20 million years ago. So they couldnt have arrived by way of the Tethys Sea, because by 20 million years ago it had closed up. Another possible route for organisms to have traveled is through the Indian Ocean around the Cape of Good Hope to the South Atlantic.

In either case, the Indian Ocean vents may provide a "missing link" between Atlantic and the Pacific vent ecosystems. Early photographs from the Indian Ocean site taken by Japanese scientists show shrimp and mussels that appear very similar to those found at Atlantic vents.

"If you had shown me one of those pictures and asked me where that picture came from," said Vrijenhoek, "Id have told you it came right from the Mid-Atlantic Ridge." But, he cautions, "we could get fooled just by superficial appearance." He is looking forward to the results of the DNA analysis that his colleagues will perform on these animals.

Recently developed, highly efficient DNA-based tools have dramatically changed the way scientists study evolution. Scientists like Vrijenhoek use these tools to determine the similarities and the slight mutational changes between the genes in organisms found at different vent sites. Using this information leads to a better understanding how the life cycle of an organism interacts with the changing typography of the seafloor to affect both the geographic dispersal and evolution of that organism.

"We do the same thing that a forensic scientist would do," Vrijenhoek said. "We basically extract DNA from the organism and then we use that DNA to look at the degree of relationships within populations, and then between populations."

For example, Vrijenhoek and his colleagues have found what he calls "genetic discontinuities" among populations of vent amphipods (small crustaceans) that dont appear among populations of other vent organisms. This is due, he explained, to the fact that there is no swimming larval stage in the amphipods' life cycle. As a result, one population of organisms can easily be cut off from another, causing the two populations to drift apart genetically.

"The amphipods probably just ride up and down these ridge axes like a corridor," said Vrijenhoek. "So if theres a disruption in that corridor, through a transform fault or lack of habitat or something like that, they simply cant get from point A to point B."

The isolated populations then evolve along separate pathways.

Genetic isolation is less likely to occur among populations of animals that have a swimming larval stage because they can more easily cross such physical barriers. This is just what is seen in mussels, clams and tube worms.

Although there is plenty of work yet to be done in the Indian Ocean, Van Dover and Vrijenhoek are also excited about taking a look at other vent sites that remain completely unexplored. The southern Atlantic is one such region.

But if given a choice (and funding), Van Dover would head for the Arctic Ocean because of its isolation.

"The Arctic basin has been separated from the Atlantic and the Pacific since the Arctic Ocean was formed by shallow fill," she said. "So the deep fauna of the Atlantic and the Pacific, the ones that occur at the vents, may not have gotten up into the Arctic. If you wanted to pick the place to go find the most unusual vent organisms, Id have to choose the Arctic."