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  1. Some Like It Hot

    Most people who visit Yellowstone National Park go there to take in the scenery and to catch a glimpse of the park’s iconic Old Faithful geyser. But not Jack Farmer. When he ventures out to Yellowstone, he goes in search of clues to what life might have been like on Earth – and Mars – some four billion years ago.

    Farmer is an astrobiologist who heads the Arizona State University Lead Team of the NASA Astrobiology Institute (NAI). What fascinates him about Yellowstone are the microscopic organisms that inhabit the boiling waters of its hot springs. He refers to these overheated environments, hot springs and deep-sea hydrothermal vents, as “doorways into the early Earth.” The organisms that live there, he says, are among the most primitive on our planet.

    To understand why, one has to go back about four billion years, to a time just after Earth was formed. Comets and asteroids rained down on the newly created world. The largest of these impacts, although they occurred only rarely, could have smashed into Earth with such force that they boiled away the planet’s oceans.

    Even if life had already gained a foothold at that time, it would have been nearly wiped out. “Anything living on the surface probably would have been killed off,” says Farmer. Only those organisms that lived beneath the surface and were able to withstand tremendous heat could have survived.

    Many scientists believe that all life present on Earth today is descended from these heat-adapted microbes, the primeval survivors of Earth’s earliest mass extinctions.

    Scientists classify living creatures into three broad domains: Archaea, Bacteria and Eucaryota. Almost all multicelled organisms – including all plants and animals – are eucaryotes. Most of the hyperthermophilic (extreme-heat-loving) organisms that inhabit hot springs and deep-sea hydrothermal vents are microscopic single-celled archaeans and bacteria.

    Complex plants and animals, the eucaryotic life forms that we’re most familiar with, can’t withstand temperatures above 50 degrees C (122 degrees F). Many of the microorganisms that Farmer studies, in contrast, would “freeze” to death at these temperatures. They’re comfortable with the thermostat turned up to between 80 degrees C (176 degrees F) and a scorching 114 degrees C (237 degrees F), well above the boiling point of water.

    Until the 1960s, scientists didn’t even bother to hunt for life in extreme environments like Yellowstone’s hot springs. Because plants and animals couldn’t live under such conditions, scientists assumed that nothing could. “We had a very multicellular-centric view of life,” Farmer explains.

    But new molecular tools that enable scientists to study the genetic makeup of living beings have revealed that our planet is dominated not by the relatively large life forms we encounter in our daily life, but by tiny microbes. With few exceptions, wherever liquid water is found on Earth, microscopic life is also found.

    Basically,” says Farmer, “we live on a microbial planet. If you want to understand life, you don’t look at large multicellular things, like we used to 20 years ago. You look at microbial things.” This viewpoint is particularly important for getting a handle on what life on Earth might have been like shortly after it got started. Back then, there were no large multicelled organisms.

    While microbiologists believe that some contemporary archaeans and bacteria resemble the microbes that inhabited early Earth, they would like additional proof. But only fossils – few in number and difficult to interpret – remain as hard evidence of what life was like three to four billion years ago. As Farmer explains, “The only real historical record is in the rocks.”

    That’s why he is interested not only in how the microbes that inhabit Yellowstone’s hot springs live, but how they die. Or, to be more precise, how they become fossils. “My approach,” he explains, “has been to start with the modern and use modern analogs to understand how biological information gets transferred into the rock record and then to use that as the stepping-off point to go into the past.”

    The lasting evidence that gets left behind by these modern-day hot-spring organisms, Farmer has found, is not the organic debris of their decaying bodies. Rather, it is subtle but detectable changes in the textures of the rocks caused by the biological action of the microbes.

    The organic structures produced by some microbes act as nucleation points (seeds) around which crystals form that are composed of the minerals that precipitate from the springs. The organisms become heavily encrusted with these minerals. When they die, their bodies decay, but the minerals that formed around them become part of the rock.

    Moreover, some primitive organisms live by eating the minerals in rocks. When they get up from the table, they leave behind distinct corrosion patterns. Farmer refers to these microbe-sized structures as “biofabric information.”

    “There are very clear suites of microtextures that are very strongly biomediated,” says Farmer. “The organics and all of the unstable biochemical information get stripped away. So you’re often left only with mineralogical and biofabric information.”

    Comparing the microtextures created by modern-day organisms to the textural evidence preserved in billion-year-old rocks helps scientists understand how the ancient predecessors of modern microbes lived and evolved.

    This technique may also prove useful in the search for evidence of past life on Mars. “Most of these kind of textural features,” Farmer points out, “exist at a scale that you could potentially see with a rover, particularly if you had a microscopic imaging system.

    “In 2003,” he adds, “we’re going to send rovers to Mars that will have just that capability. For the first time on Mars we’ll be able to image the fabrics in rocks.” If ancient Martian microbes left a familiar signature in the rocks, detecting that signature could provide the most compelling evidence yet of life on another world.

    What Next?

    Farmer, who chairs the NAI’s Mars Focus Group, is actively involved with the process of selecting the sites where the 2003 rovers will land. Not surprisingly, the candidate sites he favors are those that show evidence of former hydrothermal activity!

    Looking still further into the future, Farmer believes that returning samples of Martian rocks to Earth for more detailed analysis will be important. And, he points out, “mineralogical and fabric information is crucial” for selecting the right samples to return.