Extremophiles: They Love, Well, Extremes!

I recently wrote about the important idea that life deserves reverence, but not worship. This brings us to a fascinating array of critters that show the resilience of life.

Enter: extremophiles. This word didn’t exist a few decades ago, and has only found widespread use in the 21st century. It refers to organisms that survive – even thrive – in environments that are extremely hot, cold, highly acidic or alkaline, and so forth, circumstances that would be lethal for most living things. Not surprisingly, extremophiles tend to be relatively simple creatures, notably invertebrates and especially bacteria, although there is no bright line distinguishing, say, mammals such as arctic hares – which thrive in very cold habitats – from their close relatives such as more “traditional” rabbits, which inhabit temperate environments. In a sense, the former are extremophiles relative to the latter, but neither compares with those life forms whose mere existence has excited the admiration and wonder of biologists.

The first asphalt roads in such eastern U.S. cities as New York, Baltimore and Washington, D.C. were paved with natural asphalt taken directly from Pitch Lake, in Trinidad. The first Europeans to discover this natural curiosity, in 1595, were under the command of Sir Walter Raleigh, who used pitch from this lake to caulk his ship, noting in his diary that the material was "most excellent... It melteth not with the sun as does the pitch of Norway." More than four centuries later, scientists discovered abundant microbial life luxuriating in this same lake of liquid asphalt, an environment that would seem more appropriate to one of Dante’s infernal regions than a life-affirming Petri dish. Nor is this finding unique.

In 2013 bacteria were reported to be abundant in a cold, dark lake (of more traditional water), one-half mile under the Antarctic ice. A month later, microbes were found occupying the deepest place on Earth, the Marianas Trench.[iii] There are also “infra-terrestrials” which live, incredibly, inside rocks in the deep ocean, nearly 2,000 feet below the sea floor, which itself is under 8,500 feet of ocean, and thus not just utterly dark and devastatingly cold, but subject to immense pressure. (This, too, was reported early in 2013, which qualifies as an “annus mirabilis” for the discovery of extremophiles.) Living things – thermophiles – have also been found in super-heated oceanic vents, at temperatures up to 122 degrees Celsius, which is considerably hotter than boiling water.

It remains unknown exactly how life first appeared on Earth, not least because it occurred more than four billion years ago, and left no currently discernible fossil evidence. There have, however, been numerous hypotheses and no shortage of feasible speculation, including famous research by Stanley Miller in the 1950s, which showed that complex organic molecules can arise when simpler chemicals are recirculated in a laboratory and provided with electrical energy (simulating lightning storms). It had long been assumed, especially by non-scientists, that the details of how life first evolved constitute one of the greatest evolutionary mysteries, perhaps the greatest. In fact, that’s not the case. Although it would certainly be interesting to unravel precisely how nucleic acids (whether DNA, RNA, or some precursor) first achieved the ability to replicate itself, and also how cells first appeared in the pre-Cambrian, the truth is that there are lots of feasible routes, for both and there are many other, far more challenging evolutionary mysteries.

Moreover, the abundance of extremophile life-forms and the subsequent recognition that life is resilient and widespread has if anything helped undercut the myth that aliveness is so special that its appearance is not only a huge missing piece of the evolutionary puzzle, but also prima facie evidence for divine intervention, a belief that was widespread – even among many scientists – as late as the early 19th century. The supposed supernatural specialness of life was encapsulated in the doctrine of vitalism, which proclaimed that living things contained some sort of metaphysical “life spark,” which was not subject to the basic laws of physics and chemistry. In addition to its metaphysical and theological appeal, evidence for vitalism came from the assumption that unlike inorganic compounds such as salts, oxides and simple acids and bases, organic compounds (including proteins and carbohydrates) were imbued with a unique vitality such that they not only characterized living organisms but they could only be produced in plant or animal bodies, and could not be synthesized in the barren confines of a merely inorganic laboratory.

In 1773, a French chemist isolated crystals of urea from the urine of different animals, as well as from human beings. Even when it turned out that this substance was a comparatively simple compound, vitalism was not undermined, since the greatest chemists maintained that urea could only be made by animals with kidneys. (“Organic” compounds were well known at the time, their name deriving from the dogma that they existed only by virtue of the action of appropriate living organs.) But since previously identified organic compounds were complex and urea was relatively simple, the prospect arose that perhaps it could be artificially synthesized after all.

Then, in 1828, Friedrich Wöhler – who was already renowned as the discoverer of aluminum - undertook the synthesis of the inorganic compound, ammonium cyanate. What he got, to his surprise and consternation, was urea: surprise because he wasn’t expecting this result, and consternation because at the time, Wöhler himself was a firm believer in vitalism. He wrote to his mentor, the famous chemist Jacob Berzelius, "I cannot, so to say, hold my chemical water and must tell you that I can make urea without thereby needing to have kidneys, or anyhow, an animal, be it human or dog." It was no longer true that bona fide organic compounds could only be made by living organisms and then extracted from biological sources such as blood, urine, and so forth. This led Wöhler to write additionally to Berzelius, lamenting “the great tragedy of science, the slaying of a beautiful hypothesis by an ugly fact.”

From today’s perspective, it isn’t at all clear that the chemical synthesis of organic compounds is ugly – although it is certainly a fact, one that is replicated innumerable times throughout the world and has given birth to the field of organic chemistry, which currently synthesizes about a million different compounds each year, including many hormones, vitamins and life-saving antibiotics. But the hypothesis of vitalism - whether or not beautiful - is dead as can be. The success of organic chemistry thus contributed to the demise of the yet another paradigm: that life itself, and not merely its human component, was metaphysically unique. (The notion nonetheless persisted in some circles of thought – humanist rather than biological. Thus, the philosopher Henri Bergson argued as late as 1907 that living things were alive by virtue of their “élan vital,” which led biologist Julian Huxley to note that this was like attributing the movement of a railroad train to its “élan locomotif.”)

Which brings us back to those extremophiles, the discovery of which hasn’t been quite as earth-shaking and paradigm-breaking as the demolition of vitalism, but which nonetheless added another nail in the coffin that proclaims life to be a creation of God because it couldn’t possibly derive from natural processes. Given that life can succeed in extreme environments, maybe it first evolved in them as well. It was long assumed, for example, that life must have originated in some sort of warm, shallow, benevolent puddle that offered the kind of comfortable incubator that such a delicate flower would require. This may have been the case. However, the existence of thermophiles thriving in super-heated, hydrothermal deep ocean vents, along with the discovery of numerous other extremophiles has raised the prospect that perhaps life first emerged in what we – sunny children of a relatively easy, superficially life-friendly environment – have until recently considered impossible conditions.

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It has also evoked considerable interest from the community of astrobiologists, those scientists interested in ascertaining whether life exists elsewhere in the solar system and/or beyond. Hence, the special significance of life abounding not only in conditions that are super-heated (thermophiles) as well as super-cooled (cryophiles), but also without oxygen (anaerobes), in intensely salty environments (halophiles), getting nutrition from methane (methanotrophs), not only surviving but even thriving among heavy concentrations of such traditionally toxic heavy metals as arsenic, cadmium, copper, lead and zinc, as well as some radiation-resistant organisms that can cheerfully gargle with the effluent from nuclear reactors.

Resilient indeed, but wholly natural and in no way supernatural.

David P. Barash is professor of psychology emeritus at the University of Washington. His most recent book is Through a Glass Brightly: using science to see our species as we really are (2018, Oxford University Press), from which this material is adapted.