Many of them are tiny, all of them are tough, and they could be your most distant ancestors.
True to their name (which is a Greco-Latin combo for "someone who loves extremes"), extremophiles can batten and fatten in conditions that humans - and most other species - would consider off limits. The first of these sturdy organisms to be discovered, a thermophile, was found in the late 1960s in Yellowstone National Park, hanging out in one of the hot springs. It was a bacterium with a name bigger than itself: Thermus aquaticus (literally, "warm bath water dweller." Species names are often surprisingly prosaic once you translate them.)
Thermus aquaticus not only withstood, but thrived, in temperatures above 160 F. For comparison, try turning on the hot water tap at home, and let it run. It will scald your hand, but the temperature won't exceed 140 F. This is observational proof that you are not a thermophile.
As it turns out, Thermus aquaticus is only middle-of-the-road tough when it comes to taking the heat. One hyperthermophile, Pyrolobus fumarii, can tread water at a scalding 235 F. That's not only above the boiling point, but it will soften the upholstery in your pickup. Other extremophiles operate smoothly in below-freezing cold (psychrophiles), highly acid or base solutions (acidophiles and alkaliphiles), heavy-duty brines (halophiles), and in circumstances of crushing high pressure or dusty dryness (piezophiles and xerophiles). There are varieties that can shrug off nuclear radiation, or dwell well in aviation fuel. Frankly, extremophiles would be recruited for the local SWAT team, if they were big enough to carry weapons.
The Hard Cell
How do they do it? What defenses do these frequently diminutive creatures (many are microbial, although not all - think penguins) mount against environmental conditions that would either pickle or pyrolize you and me? There are two fundamental strategies: erect a barrier against the elements, or change your metabolism.
For example, some halophiles protect themselves from a saline environment by increasing the concentration of salts in their innards. With salinity about the same both within and without the cell, the halophile needn't fear that runaway osmosis will drain it of its precious water.
If you can't defend against a brutal habitat, you can learn to love it. For example, psychrophiles come equipped with special proteins to adapt their lifestyle to the cold. Some of these proteins act as antifreeze to lower the freezing point of water, to prevent its congealing, expanding, and sundering the cell. Other proteins (enzymes) are specially formulated to ensure that chemistry continues even when the temperature dips to the single digits or lower.
Many researchers are looking for ways to exploit the Darwinian inventiveness that has produced these extremophile defense mechanisms. For example, Deinococcus radiodurans, which boasts a highly sophisticated DNA repair shop within its tiny cell walls, is able to recover from exposure to massive doses of molecule-busting, high energy radiation by simply fixing the damage. It's hoped that this talent will prove useful in engineering microbes that can clean up radioactive spills, or possibly even protect us from skin cancer.
Extremophiles in Space
By definition, most of the habitats on Earth are not extreme. Extremophiles are the exception, rather than the rule. And yet when we search the solar system for biology, we expect that if there are alien life-forms nearby, they will most likely be analogs to Earth's extremophiles. This is simply because, whether you're talking about aquifers beneath the cold, ultraviolet-stung sands of Mars, or the deep, salty seas of Europa, you're describing environments as brutal as Green Bay's defensive line. Most of your local flora and fauna would perish straight-away in these nasty niches. But Earth's extremophiles - some of them - could be transplanted to such otherworldly habitats and never look back.
The question is less whether survival in the hypothesized ecological recesses of Mars and a few of Jupiter and Saturn's moons is possible - that seems more than likely - but whether it could arise in the first place. The fact that extremophiles may be our oldest living relatives here on Earth suggests that these highly specialized organisms can appear quickly, and do so in the hellish environments that would exist on many young solar system worlds.
It is, in fact, currently fashionable to argue that life on Earth may have begun, not in Charles Darwin's "warm little pond," but in a seething, scalding and turbulent sub-ocean geyser, where chemical reactions run fast and hard. This, of course, is a habitat that thermophiles are pleased to call home. In the 1970s, DNA studies revealed that thermophiles branched off early from the tree of life, and that they are as old as any creatures we know. This is obviously compatible with the view that they were the first sort of life to arise. After all, although naturally boiling water is confined to only a few places on Earth today, there was a time, more than 3-1/2 billion years ago, when our planet was laced with seething, sizzling caldrons. The earliest life might well have had to be capable of hanging tough in a tough and sweaty world. Thermophiles might have paved the way for today's millions of species.
This scenario, while seductive, is not air tight. It's always possible that life actually arose in more moderate conditions, but nearly all of it was obliterated by a large rock from space. Thermophiles, ensconced near hydrothermal vents miles beneath the sea's surface and protected from the tragedy above, might have been the only survivors of this unrecorded catastrophe. They might appear to be our earliest relatives only because all their ancestors died without trace.
Such uncertainties are part of the allure of extremophiles for the astrobiologist. Perhaps these hardy types are the prototypes for all subsequent life; perhaps not. But there's no doubt that they are Nature's best candidates for a body plan that will work on other worlds that we know. They have engineered their own space suits.
Extremophiles, a biological curiosity here on Earth, could represent the most frequent form of life in the universe. What's exotic and rare on our planet might be both common and commonplace elsewhere. Indeed, perhaps the "extremophile" appellation is too provincial, and we should really call them Vita vulgaris.