Coloured scanning electron microscope (SEM) of a water bear (Paramacrobiotus craterlaki) in moss. Photo by Eye of Science/Science Photo Library


Life is tough

Human life is fragile but tardigrades and other extremophiles show that life itself is in little danger of disappearing

by David P Barash + BIO

Coloured scanning electron microscope (SEM) of a water bear (Paramacrobiotus craterlaki) in moss. Photo by Eye of Science/Science Photo Library

Life – each life – is special. Living things differ from the nonliving in many ways, such as metabolism, responsiveness to stimuli, and the capacity to reproduce. Living things also maintain internal conditions that are highly nonrandom and low in entropy. Unlike, say, crystals or salt solutions, life as we know it can exist only within generally narrow limits, with specified concentrations of nutrient molecules oxygen and carbon dioxide, as well as waste products. Most organisms, moreover, can tolerate only a narrow range of acid-base balance (pH), ambient pressure and temperature, osmotic concentration of various electrolytes, and so forth. In other words, individual lives are delicate, often painfully so.

They require a demanding balance, one mostly achieved by homeostasis, an array of thermostatic control mechanisms that – like a thermostat in a house – increase something if it gets too low, and decrease it if it gets too high. In his classic text The Wisdom of the Body (1932), the American physiologist Walter Bradford Cannon detailed the ways that life maintains itself within a narrow range of parameters. Cannon showed how the viability of life depends on a pair of conflicting realities. The first is that even small deviations – especially when it comes to an organism’s internal environment – can be lethal. The second is the contradictory fact that, by virtue of having the capacity to maintain such narrow internal horizons, despite variations in the external environment, life perseveres.

Homeostasis makes organisms capable of colonising a range of different environments. This would not be possible if their innards could reflect their immediate surroundings only. Turtles and snails carry their protective houses on their backs; living things have internal houses, maintained by homeostasis, and are obliged to maintain them within narrow limits. Those that can’t are dead. Seneca put it thus: ‘It would be some consolation for the feebleness of ourselves and our works if all things should perish as slowly as they come into being; but as it is, increases are of sluggish growth, while the way to ruin is rapid.’ Aristotle, in his Nicomachean Ethics, made a similar point: it is possible to fail in many different domains, whereas the path to success is narrow. There are numerous ways, for example, for an archer to miss her mark, but only one way to hit it. A last-minute tremor, gust of wind, broken string, slightly twisted bow, inadequately feathered arrow or sudden cough: any of these can make the whole process go awry.

By the same token, there are many ways for a biological system to fail – and thus, to sicken and die – but very little tolerance when it comes to maintaining the demanding conditions necessary for life. Indeed, life can be defined as a concatenation of highly nonrandom, carefully circumscribed events that must all come together to succeed in keeping random, bad things (such as death) at bay. As someone first noted somewhere, there are many more ways to be dead than to be alive, which would suggest that living organisms are the poster children for fragility.

Then there are extremophiles. Extremophiles tell us that everything we think we know about the fragility of life is wrong. Life is indeed extraordinary, not to mention precious and deserving of reverence – but not in any sense miraculous.

The word extremophile didn’t exist until the 1970s. It entered wide circulation only after 1979 when the US Navy’s submersible Alvin revealed ecosystems prospering in deep-ocean hydrothermal vents. The Alvin scientists discovered organisms living in superheated water and largely metabolising hydrogen sulphide, which until then had been thought toxic and incompatible with life. Interest in extremophiles has burgeoned in proportion as scientists have come to appreciate their abundance, as well as their novel physiology. There is a journal devoted to extremophiles, focusing on creatures 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 and archaea, although there is no bright line distinguishing, say, arctic hares, which thrive in very cold habitats, from their rabbit relatives whose habitats are more temperate. But neither compares with those life forms whose existence excites the admiration and wonder of biologists. The concept itself is nonetheless anthropocentric, since denizens of, say, blisteringly hot hydrothermal vents would perish in our ‘moderate’ temperatures and pressures, which for them would doubtless be extreme.

The first solid roads in New York, Baltimore and Washington, DC were paved with natural asphalt taken directly from Pitch Lake in Trinidad. The Europeans who discovered this natural curiosity in 1595 were under the command of Sir Walter Raleigh, who used pitch from the lake to caulk his ship. Then, in 2011, 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 the finding unique.

In 2013, microbiologists found abundant bacteria in a cold, dark lake (of more traditional water), half a mile under the Antarctic ice. A month later, researchers encountered microbes occupying the Mariana Trench, the deepest place on Earth. There are also ‘infra-terrestrials’ that live, incredibly, inside rocks in the deep ocean, nearly 2,000 feet below the sea floor, which itself is 8,500 feet down, and thus not just utterly dark and devastatingly cold, but subject to immense pressure. (This, too, was reported early in 2013, which qualifies it as an annus mirabilis for the discovery of extremophiles.)

The continuing revelation of extremophile lifeforms, and the subsequent recognition that life is resilient and widespread, has helped to undercut the myth that aliveness is necessarily special, much less evidence for divine intervention. As recently as the 19th century, even many scientists believed that the mere existence of life was a miracle, not explicable in material terms. The supposed supernatural specialness of life was encapsulated in the doctrine of vitalism, which held that living things contained some sort of metaphysical ‘life spark’ that was not subject to the basic laws of physics and chemistry. Early in the 20th century, the French philosopher Henri Bergson claimed that life was characterised by a unique ‘élan vital’, which led the English biologist Julian Huxley to respond that this was as satisfying as attributing the movement of a train to its ‘élan locomotif’.

Extremophiles are in a sense antitheological and a cure for life-worshipping mysticism, another nail in the coffin that proclaims living things to be divinely created because they couldn’t possibly derive from natural processes. They also expand the possible playing field within which life initially evolved. Given that organisms can succeed in extreme environments, they might have first developed in them as well. It was long presumed, 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 might indeed have been the case. But the existence of thermophiles thriving in superheated, hydrothermal deep ocean vents, along with the discovery of numerous other extremophiles, raises the prospect that perhaps aliveness first emerged in what we – sunny children of what is, for us, a relatively easy, superficially life-friendly environment – until recently considered impossible conditions.

Boil them, freeze them, dry them, drown them, expose them to radiation – tardigrades just shrink

Astrobiologists pay special attention to organisms abounding in extreme conditions – superheated (thermophiles), supercooled (cryophiles), without oxygen (anaerobes), and intensely salty (halophiles) – but also getting nutrition from methane (methanotrophs) and surviving, even thriving, among heavy concentrations of arsenic, cadmium, copper, lead and zinc, metals toxic for most ‘normal’ creatures. There are, moreover, radiation-resistant organisms that can cheerfully gargle with the effluent from nuclear reactors. And don’t overlook the cryptoendoliths (‘hidden’ ‘inside’ ‘stones’) that seem perfectly happy occupying cavities within rocks.

Most extremophiles are microbes, but not all. There are, for example, a group of wingless, mostly eyeless insects known as grylloblattids, more commonly ice bugs or ice crawlers. They live, as one might expect, in very cold environments, typically under frozen rocks. My personal favourites, however, are tardigrades. These multicellular creatures are rarely more than one millimetre in length and often invisible to the unaided eye. They have four legs along each side, each outfitted with tiny claws. They also have a clearly discernible mouth, and are impossibly adorable. Purists don’t include tardigrades among extremophiles, since they don’t appear to be adapted to extreme environments per se – that is, like us, they do best in comparatively benign conditions, which, in the case of tardigrades includes the moist, temperate miniworld of forest moss and lichens.

Their probability of dying increases in proportion as they are exposed to highly challenging circumstances, so, unlike classic extremophiles, tardigrades are evidently adapted to what human beings, at least, consider moderate circumstances. However, they are extraordinary in their ability to survive when their environments become extreme. Not only that, but whereas typical extremophiles specialise in going about their lives along one axis of environmental extremity – extreme heat or cold, one or another heavy metal, and so forth – tardigrades can survive when things get dicey along many different and seemingly independent dimensions, simultaneously and come what may. You can boil them, freeze them, dry them, drown them, float them unprotected in space, expose them to radiation, even deprive them of nourishment – to which they respond by shrinking in size. These creatures, also known as water bears, are featured on appealing T-shirts with the slogan ‘Live Tiny, Die Never’ and in the delightful rap song that describes their indifference to extreme situations, entitled Water Bear Don’t Care.

Tardigrades might be the toughest creatures on Earth. You can put them in a laboratory freezer at -80 degrees Celsius, leave them for several years, then thaw them out, and just 20 minutes later they’ll be dancing about as though nothing had happened. They can even be cooled to just a few degrees above absolute zero, at which atoms virtually stop moving. Once thawed out, they move around just fine. (Admittedly, they aren’t speed demons; the word ‘tardigrade’ means ‘slow walker’.) Exposed to superheated steam – 140 degrees Celsius – they shrug it off and keep on living. Not only are tardigrades remarkably resistant to a wide range of what ecologists term environmental ‘insults’ (heat, cold, pressure, radiation, etc), they also have a special trick up their sleeves: when things get really challenging – especially if dry or cold – they convert into a spore-like form known as a ‘tun’. A tun can live, if you call their unique form of suspended animation ‘living’, for decades, possibly even centuries, and thereby survive pretty much anything that nature might throw at them. In this state, their metabolism slows to less than 0.01 per cent of normal. Compared with them, a deeply hibernating mammal is living at lightning speed.

Given that tardigrades possess the kind of powers we otherwise associate with comic-book superheroes, it might seem that they are creatures out of science fiction, but maybe it’s the other way around. Liu Cixin’s novel The Three-Body Problem (2010), a Chinese blockbuster that broke all records for sci-fi literature in its home country, became the first book not originally published in English to win the coveted Hugo Award for best science-fiction novel in 2015. It describes extraterrestrials known as Trisolarans, whose planet is associated with three suns, the real-life interactions of which – as physicists and mathematicians understand – would generate chaotically unstable conditions.

Trisolarans, therefore, are unpredictably subjected to extreme environments depending on the temporary orientation of their planet relative to its chaotically interacting stars: sometimes lethally hot, other times cold, sometimes unbearably dry and bright, other times dark, and so forth. As a result, these imagined extremophiles have evolved the ability to desiccate themselves, rolling up like dried parchment, only to be reconstituted when conditions become more favourable.

I don’t know if Liu was aware of real-life, Earth-inhabiting tardigrades when he invented his fictional Trisolarans, but the convergence is striking. (In the interest of scientific open-mindedness, it should perhaps also be considered that maybe tardigrades are real Trisolarans, refugees from a planet that was chronically exposed to intense environmental perturbations. This would explain the puzzling fact that tardigrades appear hyper-adapted, able to survive extremes that greatly exceed what they experience here on Earth.)

When they reconstitute, tardigrades can incorporate pieces of non-tardigrade DNA floating around

But tardigrades have two more arrows in their extremophile quiver, neither of them shared with Liu’s Trisolarans. In 2017, a team of cell biologists discovered that tardigrades possess genes that produce a peculiar array of constituent chemicals, known as ‘intrinsically disordered proteins’, which help induce a solid internal state in these animals when they experience desiccation. In addition, some scientists claim that fully one-sixth of tardigrade DNA consists of genetic material from other species, although this finding has been disputed. It has long been known that some cross-species (horizontal) transfer of DNA takes place among many species, but not on the scale claimed for water bears.

Most animals sport less than 1 per cent foreign DNA. Tardigrades might have this additional trick: when they dehydrate in response to environmental exigency, their DNA breaks into fragments, after which it is reconstituted upon rehydration. As tardigrade genes reconstitute themselves from the tun state, their cell and nuclear membranes become leaky, whereupon they could have acquired the habit of incorporating various pieces of non-tardigrade DNA that are inevitably floating around.

All species possess some capacity to repair errors in their genome. Individuals and hence species lacking this ability will have been selected against, simply given the unavoidable tendency of DNA to mutate, even without the extra challenge of adjusting to extreme conditions. Although presumably the stitching in of extra-specific DNA would be initially random, it is possible that some of these genomic additives contributed to their possessor’s fitness. If the early reports of unusually large amounts of extra-specific DNA in tardigrades turns out to be accurate, it will be yet another example of their charm, whereby in so many wonderful ways the extreme adaptations of these aptly named extremophiles italicise the potency of natural selection.

In his book What Is Life? (1947), the mathematical biologist J B S Haldane wrote: ‘The Universe is not only queerer than we imagine but queerer than we can imagine.’ Extremophiles also demonstrate that life is more resilient. In a world in which nature has been subjected to ever more pain and stress, they represent a kind of cosmic optimism, evidence of the strength and durability, if not of human beings, then at least of life itself.

This essay is adapted from the author’s Through a Glass Brightly: Using Science to See Our Species as We Really Are (2018, Oxford University Press)