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 Darwin’s 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 Darwin’s 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 don’t 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

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 Jupiter’s 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 Jupiter’s 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.