In years past, scientific speculation about how life began on Earth envisioned primordial soups and slimy goo as the incubators in which the first tiny microorganisms developed, billions of years ago. More recently, microbiologists have examined places formerly seen as too harsh and inhospitable to foster biology, seeking answers about how life developed and adapts.

Hydrothermal vents, for example — areas on the ocean floor that expel seawater superheated deep in the Earth’s crust — and the exotic creatures that live there have received increased attention.

Now scientists are scrutinizing another location for clues in unraveling some of life’s riddles — deep in the ground, underneath the planet’s surface.

"We’ve pretty much left Darwin’s warm little pond in the dust," said David Stahl, a University of Washington professor of civil and environmental engineering whose work focuses on ecology and evolution. "The subsurface is being increasingly viewed as an important and largely unexplored part of the biosphere. Depending on how you calculate it, its biomass may exceed the biomass on the surface of the planet."

Stahl’s work examining sulfide-eating bacteria from deep in the Earth has revealed genetic similarities in the microorganisms from different parts of the world, hinting that the existence of such life may be widespread throughout the planet’s subsurface. Those microorganisms, he added, are among the more primitive, in terms of their metabolism, of Earth’s life forms.

He and UW colleague James T. Staley, a microbiology professor in the School of Medicine, will join Abigail Salyers of the University of Illinois Urbana-Champaign and Edward F. Delong of the Monterey Bay Aquarium Research Institute for a 2:30 p.m. session today on "The New Biology of Rocks" at the annual meeting of the American Association for the Advancement of Science in Boston. The session is in the Hynes Convention Center, Plaza Level, Room 109.

Stahl has spent the past several years gathering samples from wells drilled in eastern Washington for use as deep-injection sites to dispose of hazardous waste. The drillers tapped into an artesian system and couldn’t use the wells for disposal, so they were capped. Industry’s loss was science’s gain — the wells provide ready access to tiny microorganisms that exist deep within the earth.

These deep subsurface habitats are are radically different from "surface" biology. They never see the sun, so energy from photosynthesis, which provides the platform upon which life on the Earth’s surface is built, isn’t an option. Oxygen and fixed carbon are scarce, so they must "eat" inorganic compounds — such as hydrogen, CO2 and sulfate — that originate from geothermal processes in the Earth’s hot interior.

Stahl and his collaborators compared those bacteria with bacteria they extracted from deep in African gold mines and from geothermal springs in Yellowstone, and found some strong similarities based on comparative sequencing of genes. That provides a yardstick for estimating evolutionary relationships among them. The studies suggest that the organisms are specialized for life in the subsurface and are very widely distributed on Earth.

So did life start in the rocks?

Stahl will be the first to bluntly say, "We don’t know." But it’s a new place to look for answers about biological diversity and how that diversity relates to environments that used to be seen as barren. And who knows what further research will uncover?

"There are some things that make it an attractive option — at the time we think life arose, the Earth was undergoing almost constant bombardment by asteroids and the like, which should have been enough to kill anything that developed on the surface, including microorganisms," he said. "This scenario would provide shelter from what was happening on the surface. But the bottom line, at this point, is that we really don’t know enough to know. We’re just getting to the point that we can start to ask these questions."