We humans think we're pretty tough, pretty smart, and pretty much the Overlords of the Earth. Well, we are sorta smart, and we've used those smarts to make us much more tough than we are naturally. In some sense, we have, indeed, become the Overlords of the Earth. Technological wonders notwithstanding, the range of conditions that we can endure is still very limited. Of course, we view the rest of the cosmos from our own perspective. As such, we define places that we find personally uncomfortable as "extreme environments", but does this term have any real meaning for biology on Earth and for exobiology on other planets?

I believe that the answer is a resounding "yes!"

What are the limits for Earth life?

Our kind of life, that is life made with carbon compounds floating in water, does have some absolute constraints based on the nature of our chemistry. The boiling and freezing points of water, acid and alkaline extremes, the presence or absence of oxygen, and other factors provide the physical window frame within which life can expand. This may sound deceptively simple, but the truth is, evolution has the time and the mechanisms at its disposal to allow life to push these seemingly rigid physical boundaries.

The most obvious example of a significant physical limit on our kind of life is the need for liquid water. Life, as we currently understand it on Earth, is limited by the boiling and freezing points of water. This is the solvent in which all our life chemistry takes place. Of course, some organisms have found ingenious ways to stretch or avoid these temperature constraints. Adding anti-freeze compounds such as highly concentrated sugars, salts, and amino acids to their cells has allowed some organisms like the tiny crustaceans known as "water bears" and many species of microorganisms to live in sub-freezing temperatures. Indeed, the chemistry and adaptations of creatures like the icefishes of Antarctic waters means that they MUST live at cold temperatures. Above 4°C, these animals suffer from "heat exhaustion" and may die.

On the other end of the temperature spectrum, some organisms live in a high pressure environment which depresses the physical boiling of the liquid. The mechanical destruction that boiling causes is thus avoided. However, there are also actual heat limits for proteins and the DNA in every cell. Above these temperatures, the proteins and nucleic acids undergo an irreversible process called "denaturing". This means that the critical three-dimensional structures that allow the molecules to perform their functions are destroyed.

While we don't know how hot the ultimate high temperature may prove to be, there are reliable data pointing at 110°C to 115°C as relatively common limits for microbes. Some reports of up to 131°C have appeared recently but remain to be validated. In the super hot volcanic vents at the bottom of the world's oceans, some scientists have found what they think are microorganisms growing at temperatures as high as 230°C but these experiments have not yet been successfully repeated by other investigators.

In organisms more complex than microbes, the possibility exists that behavior can modify the physical constraints. Staying with our consideration of temperature, this last common alternative strategy is simple relocation or other physical avoidance of undesirable temperature ranges. All of us are aware of trees losing their leaves, bears going into hibernation, and birds flying south in the winter. But fewer people are aware of how common this technique is in the living world. Of course, people do it too, every time we step into an air-conditioned car in Tucson or into a heated ski hut in Aspen.

Many desert organisms avoid the burning hot surfaces of their environment during the day, and burrow into the cooler, wetter subsurface levels. Among frogs, those delicate and moist amphibians, some have developed the technique of spending most of their lives wrapped up in damp mudballs at the bottom of dry lake beds. Once in a while, when rain occurs in their arid habitats, they quickly revive, feed, mate, and then return to their comfy mudballs to wait out the next long dry spell. Antarctic nematode worms are also masters of the "dry and crispy" school of survival. They spend much of their lives totally dried out and blowing around in the strong winds of the dry valley deserts. The microbes take this "opting-out" strategy to the extreme as they shut down entirely, dry out their cellular contents and wait for times to improve. In fact, microorganisms are routinely freeze-dried for storage in laboratories. Many parents would love to repeat this experiment with their teenagers but it doesn't seem to work in higher lifeforms.

Earth life's extremes: are we there yet?

How much farther can life push its boundaries beyond our current understanding? It is amazing to realize that our knowledge of the extreme environments and the limits to life on Earth have expanded greatly in only the past 20 years. The intensified search for life in harsh and difficult environments on Earth has been one important benefit of our interest in the possibilities of life elsewhere in the universe. In turn, the accelerated pace at which we uncover ever greater limits to life on Earth has inspired us to speculate more realistically on the forms and constraints that such extraterrestrial life may take on other planets.

We know that understanding extreme environments is important to understanding biology and evolution in general. It is also critical in helping us search for life on other planets. But, what exactly IS an extreme environment?

On Earth, we have environments ranging from the superheated waters of submarine volcanic vents to the ultra-dry bitter cold of the Antarctic Dry Valleys. We find organisms living in caves dripping with sulfuric acid and others thriving in intensely alkaline solutions. We find creatures happily existing in saturated salt solutions, enduring megadoses of ionizing radiation, or deriving their food and energy sources from unpromising inorganic materials like manganese, iron, and sulfur compounds. Intuitively, we conclude that all their environments are extreme. After all, we'd hate to be there ourselves, but can we develop a more objective definition?

Scientists don't agree on the definition of "extreme" environment. Every scientist probably has her or his own definition. Mine is really a relative and operational scale.

For me, extreme environments are those which require significant modification of organisms living in them from their immediate ancestors. For example, the Antarctic icefishes have had to do a lot of "unfishy" evolving to suit their environment, like losing most of the hemoglobin in their blood. I think that is rather extreme. On the other hand, it may be that someday we will find a whole planet where the inhabitants arose and evolved under the conditions that our icefish enjoy. In that instance, they would be living in conditions that are the norm for their biology and not extreme at all!

Is "extreme" here "normal" out there?

Are there environments on Earth, which may resemble those we expect to find on other worlds? Herein lies the strongest connection between Earth's extreme environments and life on other planets. When we look at our own planet's most challenging environments, we are really looking for clues to what may be the normal conditions on other planets. We want a hint of what we may be searching for when we investigate those other worlds for signs of life.

We know that there are a number of environments that mimic some features of the dry, cold surface of Mars. The Antarctic Dry Valleys and their permanently ice-covered lakes may represent one stage of Mars' past development. While we have only recently become aware of the deep subsurface microbial inhabitants of Earth living at many kilometers depth, their possible counterparts in the deep surface of Mars may be all that is left of a once more extensive Martian biology.

When we look in amazement at the thriving communities of weird clams and exotic worms growing around deep sea volcanic vents we are struck by the fact that they are supported without deriving energy from our sun. But perhaps, in the briny, dark ocean of Europa (and perhaps Callisto?) these sorts of communities will be the norm. We will be better prepared to recognize and study them because our minds have been expanded by our knowledge of our own terrestrial biology.

The time factor

Do Earth's extreme environments last long enough to suggest that we could expect similar environments to be maintained on Mars or other planets over very long periods of time? This is a tricky question. At one time, not so long ago, the consensus of the scientific community was negative. But, every time we have another fossil find that pushes back the earliest signs of life on this planet, we realize that the origin of life may have taken an astonishingly short period of time. We also see amazingly rapid evolutionary jumps, a notion now known as "punctuated equilibrium" within the history of life.

This may mean that we have greatly over-estimated the necessary time periods for both the origin and subsequent evolution of life. We know that some of the extreme environments on Earth have persisted for millions of years. Indeed, there are many suggestions that the origins of life on this planet occurred in a hot, sulfurous, and non-oxygen environment that we now call very extreme!

Extremophiles of the Universe unite!

And we, we extremophiles who study the weird and exotic extremophile inhabitants of Earth's strangest places, Well, we're really just practicing for the day when we can ply our skills in corners of the universe where we are the extreme lifeforms and the inhabitants are just doing business-as-usual.