T h e R o b u s t n es s o f L A W K I [ L i fe A s W e K n o w It ]:
"Whatever doesn’t kill you will make you stronger."
By: Deidre Tomlinson
Life and Earth have changed together, generation by generation, over a time span of
around four billion years. The histories of the two are inseparable. As stated by
Campbell et al. (1999), life on Earth in notable for both its unity and diversity: unity since
each species shares the same underlying mechanisms for basic life processes, and
diversity since there may be as many as 100 million different species of living things on
Earth. So, what is life [as we know it]? It is "a continuum extending from the earliest
organisms through various lineages to the great variety of forms alive today" that
probably began between 3.5 - 4.0 million years ago, as determined by the scientific
dating of evidence of early life found in ancient rocks. (Campbell et al., 1999)
The living world is highly organized into a hierarchy of structural levels; each
level builds on the level below it with its special qualities or emergent properties. The
biological miracle of life is chemical in nature requiring about 25 of the 92 natural
elements. Only four of these - carbon (C), oxygen (O), hydrogen (H), and nitrogen (N) make up 96% of all living matter. (Clegg, 2003) Note that atoms in living things are no
different than those found in inanimate objects, and since these chemicals of life are
found throughout the universe, there is a large likelihood that life may exist - or will
eventually exist - elsewhere. What differentiates those chemical compounds ignited
with life from the non-living is that lifeforms possess these general properties of life:
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order -- all other characteristics of life emerge from an organism's complex
organization at the cellular level and beyond ...
compartmentalization -- organisms are chemically different from their
environment, sometimes separated from it by a cell membrane
metabolism -- the taking-in of energy [chemical, heat, radiative] and matter from
their external environment and use it for their own purposes (i.e. growth, repair,
reproduction, motility... )
reproduction -- life can only come from life, an axiom known as "biogenesis";
possessing the capacity to bring forth offspring (though may not be physically
able, i.e. infertile)
growth and development -- heritable programs in the form DNA (the genetic
code that carries necessary information required for metabolism and
reproduction) direct the pattern of growth and development, producing an
organism that is characteristic of its species..
homeostasis -- regulatory mechanisms maintain an organism's internal
environment within tolerable limits, even though the external environment may
fluctuate.
evolutionary adaptation -- how characteristics can change over time, leading to
the survival of the species which are well adapted to their environment ...
"Natural law is clear then it comes to adaptation: whatever works.
Evolution is ruthless. Success is defined only according to outcome - survival or
extinction of the species." (Clegg, 2003)
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Until very recently it was thought that life could develop only under 'normal'
conditions. That is: with neutral pH [~7.0], with low ionic strength (similar to that of
blood plasma), at temperatures around 37°C, at normal atmospheric pressure
(corresponding to altitudes up to several kilometres), and in the presence of oxygen.
(Campbell et al., 1999) These restrictions were obviously geocentric in nature, stemming
from the strong conviction that our species is the central in the universe and that any
living things that may exist out there must match up to our definition and parameters of
life. Now, there is recognition that current environmental conditions are not the
standard ones we would have observed during the origin of life, and that hostile
habitats which would have jeopardized life as we know it probably existed in the past.
An understanding of life processes in extreme environments will allow constraints to be
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placed on the bounds of life. An environment would be classified as “extreme” if its
physical and chemical conditions approach or exceed the tolerances for life. Many of
these unusual environments are extreme by more than one criterion. For example, the
deep-sea is a high-pressure, low-temperature water body, while hypersaline lakes in
Antarctica are acknowledged for having high salt concentrations and very low
temperatures. Note that many of these environments are inaccessible and thus have not
been adequately characterized or even explored. Usually, too little or too much of a
single physical factor can adversely affect the function of an organism. These "limiting
factors" are physical or biological necessities whose presence in inappropriate amounts
limits the normal action of the organism, but such is not the case for the some of the
members of the primitive, prokaryotic Archaean domain-of-life. It consists of many
'lovers of extremes' called extremophiles, which "are nature’s pioneers, organisms that
not only survive but thrive in the harshest environments." (Clegg, 2003) Individually
though, their survival strategies are incomplete - each has a response attuned to a
certain, extreme environment. They are subgrouped into: thermophiles [live in
exceedingly hot temperatures], psychrophiles [live in near-freezing or sub-zero aquatic
environments], halophiles [live in very saline water], acidophiles [live in acidic
environments], and alkaliphiles [live in alkaline (basic) environments]. As written by
Clegg (2003): "[t]here is a growing understanding of the 'survival kits' possessed by
extremophiles. Some have protective membranes that inoculate against environmental
assaults, others have 'savior' proteins that are activated to rescue other proteins when
the organism is in life-threatening danger, and still others are metabolic wonders that
embark on a radical form of hibernation when external conditions are punishing." We
will now focus our attention to specific examples of these 'super-survivors' - whether
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they are single-cellular or Homo sapiens - that thrive in hostility in the ocean, on the land,
or deep within the Earth.
From space, the liquid portion of our planet reflects a brilliant blue hue ... but it is
beneath the water’s surface that we can see the extensive variety of lifeforms that the
ocean is capable of supporting because of water's unique physical properties: its density,
dissolving power, ability to absorb large quantities of heat without a significant rise in
temperature, etc.
Cells can live effortlessly in this salty marine environment. This would justify
why all living things - terrestrial and aquatic alike - carry a mini-ocean within
themselves. Their blood, their eggs, and the fluids that bathe their cells are all saline.
We humans also contain roughly the same salt-to-water ratio as the sea, which is about
0.7% by weight. (Garrison, 2004) An intriguing survival mechanism of the extremophile
is the ability to internalize components of the threatening environment, essentially
inoculating itself against damage from it. This too is true for those who dwell in the
regions of excessive salinity non-uniformly distributed throughout the ocean. For
example, there are extremophiles that live in highly saline water that would instantly
dehydrate other organisms. They have been found to have molecules that tolerate very
high salt concentrations, which prevent the water in the rest of the cell from absorbing
salt. (Clegg, 2003) Despite the irony of its name, Gross (2001) confirms that the Dead
Sea is actually very much alive with halophiles thriving everywhere as well as
halotolerant algae in particular areas of the sea.
As one of the major ingredients required for photosynthesis, sunlight is crucial for
life of autotrophic flora, and also - by default - for the organisms that feed off of them.
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The amount of light transmitted throughout the ocean is inversely proportionate to the
water depth. Thus, heading towards the bottom of the ocean, just below the brightly-lit
surficial waters, we approach the dim 'disphotic zone' beginning approximately 600
metres below sea level. Even though a small percentage of sunlight reaches this zone, it
is of poor quantity and poor quality (considering only blue wavelengths penetrate to the
greatest depth because of selective absorption by water molecules, suspended particles,
and organic debris). Here in this virtually colourless zone, there is barely enough light
to see, let alone to carry out photosynthesis. If we were to continue our journey southbound, we would eventually meet up with the 'aphotic zone' of perpetual darkness that
extends to the ocean floor. Though a region of no light, the aphotic zone is no stranger
to life. In 1977, researchers came across peculiar organisms such as tube worms
flourishing at a hydrothermal vent off the Galapagos Islands, dismissing the earlier
theory scientists once had insinuating that no living creature could survive the harsh
combination of toxic chemicals, high pressures, high temperatures, and complete
darkness which exist at these vents. Unexpectedly, hydrothermal vents are underwater
oases for unusual lifeforms that are not found elsewhere. This is thanks to the presence
of metal sulphides; the most prevalent being hydrogen sulphide [H2S], synthesized
when seawater reacts with sulphate in the rocks below the ocean floor. Vent bacteria,
which have a symbiotic relationship with the giant tube worms, use hydrogen sulphide
as their energy source in lieu of sunlight in a reaction called chemosynthesis. (Stover, 2004)
Thus, chemicals are the key to vent life, not radiant energy. In addition to the 'lightless'
version of photosynthesis, evolution had made another enormous contribution to this
dark situation by inventing bioluminescence on more than 30 separate occasions!
Garrison (2004) informs us that some species of the deep-sea anglerfish such as the
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Melanocoetus, have bioluminescent organs that both radiate diffuse blue light downward
to mask their own shadows to reduce their chances of being spotted by a potential
predator as well as project patterns of glowing (or flickering) dots or lines to lure in a
potential meal.
Most marine organisms are cold-blooded, meaning that their internal
temperature is directly related to the temperature of their surroundings. Generallyspeaking, the warmer their environment, the more quickly their metabolic processes
will proceed. If one were to conduct such an experiment in a laboratory (or in this case
an aquarium), he/she would find that the tropical fish in a heated aquarium would
require more food and more oxygen than goldfish of the same size living in an unheated
- but otherwise identical - aquarium. As a result, the former would grow faster, have a
faster heartbeat, reproduce more rapidly, and swim faster ... until they've reached their
temperature threshold.
They can only endure so much heat before they expire. All
living things are made up of proteins or enzymes that break down when the organism is
exposed to extreme life-threatening conditions. Through the study of extremophiles,
though, scientists have discovered that there are special proteins (or "molecular
chaperones") that are either manufactured or mobilized at times when conditions stress
an organism to the verge of death. Thermophiles thus are thought to have adapted
because of “heat stress proteins”, which are molecules activated upon organisms'
encounters with various stressors. They repair the other proteins to prevent the cell
from dying. More complex organisms also have heat shock proteins - even humans.
Fevers that accompany an infection are somewhat analogous in function to these
proteins; they are activated to help the body recuperate from the damages of illness.
(Clegg, 2003) A more assertive mechanism used by extremophiles is simply to
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neutralize the threat from the hostile environment. Some thermophiles (or other more
complex organisms) have adjusted to exceptionally cold environments by creating
internal conditions that prevent the external threats from having any real effect. An
example provided by Gross (2001) is of the polar fish that swim around in the belowfreezing Arctic and Antarctic waters because their bodies contain nature’s equivalent of
anti-freeze: a highly concentrated blend of 'anti-freeze' proteins, certain sugars, and
amino acids.
Another physical factor to consider is that of hydrostatic pressure. Sperm whales
can be found 2,440 metres below the surface. They definitely are not affected by the 244
atmospheres of pressure - the amount that corresponds with the aforementioned depth exerted on them. (Gross, 2001)
The geomorphology of the Earth's lithosphere ranges from the trenches of deeplycarved valleys to mountain-tops that reach the heavens. On the peaks that characterize
the Andean and Tibetan landscapes, the inhabitants have biologically evolved in a
manner that allows them to survive in this locale. This has truly been beneficial since
"early settlers to the high plateaus likely suffered acute hypoxia, a condition created by a
diminished supply of oxygen to body tissues. At higher elevations, the air is much
thinner than at sea level. As a result, a person inhales fewer oxygen molecules with
each breath." (Mayell, 2004) Symptoms include headaches, vomiting, sleeplessness,
impaired thinking, and possibly death at elevations greater than 7,600 metres. The
Andeans adapted by developing the capability to carry more oxygen in each red blood
cell due to their greater amount of hemoglobin [protein] than those who live at sea level.
That is, they breathe at the same rate as those at sea level, but the Andeans can deliver
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oxygen more effectively throughout their bodies, which helps them to counterbalance
the effects of hypoxia. The Tibetans compensate for the air's low-oxygen content by
taking more breaths per minute than their sea-level neighbours. "Andeans go the
hematological route, Tibetans the respiratory route." (Mayell, 2004) Other physiological
adaptations that these high-altitude residents possess are: expanded blood vessels to
facilitate effective blood flow, changes in lung volume and chest dimensions, and
synthesis of larger concentrations of nitric oxide gas from the air they breathe. Meer et
al. (1995) point out that these findings are exclusive to mountain natives and not to
those who acclimatize to high altitudes post-puberty, clearly indicating that these
modifications are limited to the critical period of physical growth. Also, keep in mind
that while hypoxia is the most difficult to adjust to, other severe conditions such as the
brutal cold, lack of moisture, and concentrated cosmic radiation also make this
geographical setting a challenging one for humans to live in.
Optimal conditions for plant growth are sufficient sunlight, carbon dioxide gas,
and water. The green plant Welwitschia mirabilis can live in places plagued by drought,
unaffected by the humidity scarceness of only 10 mm of rainfall per year for thousands of
years. (Gross, 2001)
The toxicity (and smell) that we associate with rotting meat and nuclear waste do
not conjure happy thoughts in our heads, but these breeding grounds for the microbe
Deinococcus radiodurans represent warm, nutrient-rich havens. "Did you know ... that the
bacterium Deinococcus radiodurans can survive radioactive treatment at a thousandfold
of the radiation density that would kill a human and can reassemble its genome from
hundreds of fragments without error?" (Gross, 2001)
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"In the beginning of life on Earth, there was an enormous meteorite bombardment,
which could have dried out the seas. That would have been a problem [for surface life],
but if you go down 500 meters below ground, it would have been quite nice."
(Monastersky, 1997) The Earth’s interior is shielded from natural disasters by its hard,
lithospheric skin. This inner portion of our planet does not have access to atmospheric
oxygen nor the sun's energy while being situated in a location of blazing temperatures
and crushing, confining pressures.
In Gross' (2001) article, he tells us that there was bacteria thriving on stone and
water found hundreds of metres below ground. Bacteria cultured from the deep rock
samples were of a different sort entirely: they were anaerobic organisms, which die
when exposed to oxygen. Colonies of such anaerobic bacteria have recently been
recovered from depths of 7 km more in the Earth's crust. They are exactly the type of
bugs one would expect to find in the oxygenless depths of the crust. (Monastersky,
1997) Deep in the Earth's crust, these 'intraterrestrials' are scratching a living from little
more than barren rock. So, if life can survive there, why not inside another planet?
The Earth looked radically different from when bacteria first colonized the land and the
oceans. (Atmospheric) oxygen was scarce, which was a blessing considering it was a
poison to many early plants. Today, things - including atmospheric chemistry - have
changed drastically. Since their species' conception, bacteria have been subjected to
major environmental fluctuations that parallel the differences between the Earth we
know today and certain places in the universe deemed the most hospitable, according to
evidence complied over the generations. (Noever, 1998)
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The National Science Foundation (1998) forewarns us that “while even a
comprehensive study of life in extreme environments will not necessarily provide the
answer, scientific investigations in the full possible range of habitats in our universe will
provide a better understanding of how our planet functions from geological through
biological processes." The fossil & geologic records do however present an opportunity
to gain insights into the history of extreme environments on Earth and may offer a
perspective on the potential for life on other planets. (National Science Foundation,
1998) … and as assumed as the oldest organisms on Earth, extremophiles have been
voted by biologists as 'the lifeforms most likely to be found on other planets'.
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R eferen ce L i s t
Campbell, N.A., Reece, J.B., and Mitchell, L.G. 1999. Biology: Concepts and
Connections, 3rd ed. Menlo Park, CA: Pearson-Benjamin Cummings.
Clegg, E. 2003. "How Extremeophiles Thrive in Impossible Conditions: Lessons for
Business from Nature’s Super-Survivors", Agility Factor [online]. Available from:
<http://www.human-landscaping.com/clegg/thrive.html> [Accessed on: 28 March,
2004] .
Garrison, T.S. 2004. Essentials of Oceanography, 3rd ed. U.S.: Thomson-Brooks/Cole.
Gross, M. 2001. Life on the Edge: Amazing Creatures Thriving in Extreme
Environments. New York: Perseus Books.
Mayell, H. 2004. "Three High-Altitude Peoples, Three Adaptation to Thin Air", National
Geographic News: 25 February.
Meer, K., Heymans, H.S.A., and Zijlstra, W.G. 1995. "Physical Adaptation of Children to
Life at High Altitude", Eur J Pediatr 154(1): 263-272.
Monastersky, R. 1997. "Deep Dwellers”, Science News Online [online] 151(13). Available
from: <http://www.spaceref.com/redirect.html?id=0&url=www.sciencenews.org/
sn_arc97/3_29_97/bob1.htm> [Accessed on: 29 March, 2004].
National Science Foundation. 1998. "What Conditions Determine the Limits of Life?"
[online], Life in Extreme Environments (LExEn) Workshop Report. Available from:
<http://www2.ocean.washington.edu/lexen/lex60.html> [Accessed on: 29 March,
2004].
Noever, D. 1998. "Extremophiles: Life on the Edge", Space Science News: 1 September.
Stover, D. 2004. “Creatures of the Thermal Vents”, Popular Science [online]. Available
from: <http://www.spaceref.com/redirect.html?id=0&url=seawifs.gsfc.nasa.gov/
OCEAN_PLANET/HTML/ps_vents.html> [Accessed on: 29 March, 2004].
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