Summary: Over the past two decades, advances in a number of scientific disciplines have helped us better understand the nature and evolution of life on Earth. These scientific developments also have helped lay the foundation for astrobiology, opening up new possibilities for the existence of life in the Solar System and beyond.
[[Excerpts from the testimony of Jack D. Farmer, Director and Principal Investigator of the NASA funded Astrobiology Program at Arizona State University, for the "Life in the Universe" hearings before the House Subcommiteee on Space and Aeronautics]]
Over the past two decades, advances in a number of scientific disciplines have helped us better understand the nature and evolution of life on Earth. These scientific developments also have helped lay the foundation for astrobiology, opening up new possibilities for the existence of life in the Solar System and beyond.
A New Look at Life
Carl Woese of the University of Illinois published the first universal tree of life in 1987. The universal tree is based on genetic sequence comparisons, which showed that there are three major domains - Archaea, Bacteria and Eukarya. These three domains consist of dozens of kingdoms, nearly all of which are microbial. This is in contrast to the traditional five kingdom view of the biosphere (Animals, Plants, Fungi, Protists and Monera), where multicellular plants and animals are given prominence.
Perhaps one of the most fundamental things we have recognized from the universal tree is that we live on a microbial planet. Microscopic life dominated the first 85% of biospheric history.
Evidence from Paleontology
During Charles Darwins time, there was limited awareness of the importance of microbial life in the evolutionary history of the biosphere. The oldest fossils known were shelled invertebrates that appeared at the base of the Cambrian Period, now dated at 540 million years. Stromatolites (sediments produced by ancient microbial communities) were first described around 1850, at about the same time Darwins Origin of Species was published.
The interval of Earth history preceding the Cambrian (called the Precambrian) was regarded as being largely devoid of fossils and life. However, in 1993, J. William Schopf of UCLA reported bacterial microfossils from stromatolite-bearing sequences in western Australia dated at nearly 3.5 billion years. Then in 1996, Steven Mojzsis of the University of Colorado described possible chemical signatures for life from 3.9 billion-year-old rocks from Greenland. These are the current record holders for the oldest life on Earth.
These advances in Precambrian paleontology have pushed back the record of life on our planet to within half a billion years of the time when the first viable habitats existed on Earth. This suggests that once the conditions necessary for life's origin were in place, it arose very quickly. Exactly how quickly, we dont yet know, but certainly on a geologic time scale, it was much shorter than previously thought.
Impact Frustration of Early Biosphere Development
Prior to 4.4 billion years ago, surface conditions on the Earth were unfavorable for the origin of life. Frequent asteroid impacts produced widespread oceans of molten rock at the Earth's surface. Easily vaporized compounds, like water, and elements important for biology, like carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorous, were lost to space through a combination of volatile escape and impact erosion.
About 4 billion years ago, the rate and size of impacts dropped off, allowing the Earth to retain the water and organics delivered by comets and other icy objects. A stable atmosphere and ocean developed, providing the first suitable environments for life. However, models also suggest that as late as 3.8 billion years ago, the emerging biosphere may have experienced one or more giant impacts. These impactors would have been capable of vaporizing the oceans and sterilizing surface environments.
The deepest branches of the universal tree -- those presumably lying closest to the common ancestor of life -- all share an interesting property: a preference for very high temperatures. For some scientists, this implies that life probably got started at high temperatures, perhaps around the deep-sea hydrothermal vents. For others (myself included), it seems more likely that we are not seeing the environment of life's origin, but rather environments that prevailed after the last giant impact. These forms may simply be the descendants of organisms that were able to survive by hiding out in hydrothermal environments.
The Subsurface Biosphere
In 1979, oceanographer Robert Ballard and biologist J. Frederick Grassle piloted the deep submersible Gilliss to sites more than a mile and a half deep on the sea floor. Their mission was to describe in detail the volcanic vents and their associated faunas. At these locations, scientists got a first glimpse of living ecosystems based entirely on chemical energy.
As this type of exploration continued, complex vent communities were discovered in virtually every ocean basin, proving the remarkable ability of these organisms to colonize even the most widely dispersed habitats. There are now hints of photosynthetic organisms that are able to utilize the weak thermoluminescent radiation given off by the hot vents. This has opened up the intriguing possibility that photosynthesis may have evolutionary roots in deep sea vent settings.
More recently it was discovered that life also thrives in deep subsurface environments where interactions between water and rock yield available energy. While many subsurface organisms utilize the "filtered-down" organic compounds produced by photosynthetic surface life, some species are able to make their own organic molecules from the purely inorganic substrates that come from simple weathering reactions between groundwater and rock.
The Extremes of Life
Microbial species are now known to occupy almost the entire range of pH from 1.4 (extremely acid) to 13.5 (extremely alkaline). Life also thrives in extreme temperatures, with some species showing growth up to 114 degrees C (thermal springs at Vulcano, Italy and deep sea vents) and other species surviving down to -15 degrees C (brine films in Siberian permafrost). Life also occupies an equally broad range of salinity, ranging from fresh water up to sodium chloride saturation (about 300 percent), where salt precipitates.
In addition to environmental adaptation, some microbial species show evidence of remarkably prolonged viability. In even the driest deserts on Earth, some species survive by living inside porous rocks where they find a safe haven from UV radiation. They spring to life only when the water needed for growth becomes available. Microbes have been isolated from Siberian permafrost, where they had remained in deep freeze for about 3 million years. Bacteria have been germinated from 30 million-year-old spores that were preserved in amber. Salt-loving microbes have been cultured from rock salt that is hundreds of millions of years old.
The Search for Extraterrestrial Life
These findings hold special importance with regard to potential habitats for life elsewhere in the Solar System. For example, we must now consider the possibility of a subsurface biosphere on Mars or on Jupiters moon Europa.
Mars may have an extensive ground water system located several kilometers below the surface. This possibility was bolstered by the recent discovery of small channels caused by surface fluid seeps. If liquid water is proven to be the agent that formed these features, then the biological potential for Mars will be dramatically enhanced.
Liquid surface water also may have been present at the martian surface for a few hundred million years early in the planet's history. If surface life developed on Mars during this Earth-like period, it quite likely left behind a fossil record.
Refrigeration is known to be an effective means for the preservation of organisms. Carl Sagan first suggested that microorganisms from an earlier period in martian history might still exist there today in a perpetually frozen state, preserved in ground ice.
Could the same hold true for Jupiters moon Europa? Measurements of the magnetic field of Europa, obtained during the Galileo mission, have strengthened arguments for the existence of a salty ocean lying beneath an exterior shell of water ice. It seems quite plausible that water welling up from below may carry organisms or their by-products. These materials would eventually freeze and become cryopreserved in ices at or near the surface.
One thing seems clear: on Earth, life occupies virtually every imaginable habitat where liquid water, an energy source, and basic nutrients coexist. Whether or not this is true on other worlds is one of the premier questions facing astrobiology today. While we can effectively build on what we have learned about life on Earth, the question of extraterrestrial life requires exploration. This is perhaps the most compelling aspect of astrobiological science, and a standard by which we can measure our progress.