Chapter 24. LIFE In Review
What I wish to make clear…is, in short, that from all we have learnt about the structure of living matter, we must be prepared to find it working in a manner that cannot be reduced to the ordinary laws of physics. And that not on the ground that there is any ‘new force’ or what not, directing the behaviour of the single atoms within a living organism, but because the construction is different from anything we have yet tested in the physical laboratory.
Erwin Schrödinger (1943)
BIOCHEMIST JOHN KENDREW, in his book The Thread of Life: “Personally I do not think there is…any difference in essence between the living and the nonliving, and I think most molecular biologists would share this view.” Kendrew’s glib pronouncement has a breezy tone intended to impress upon his readers the committed materialist’s creed: It’s all chemistry, folks! Life consists entirely of the interaction of lifeless atoms—nothing more. But Kendrew is on shaky ground when he claims that a majority of scientifically minded people share his view. He’s welcome to his opinions. But this chapter offers another perspective—one that should leave little doubt about the vast difference between living and the nonliving matter. And that LIFE’s capacities somehow extend beyond the realm of pure chemistry and physics.
One indicator of LIFE’s problematical status is how resistant it is to unambiguous description. In fact, scientists have yet to agree on a simple, clear-cut definition. Science writer Carl Zimmer: “When Portland State University biologist Radu Popa was working on a book about defining life, he decided to count up all the definitions that scientists have published in books and scientific journals. Popa gave up counting after about 300 definitions.” A quick review of that list shows chemists, physicists, and biologists stamping their proposals with a point of view characteristic of their respective fields.
To get a sense of why defining “life” might present challenges, consider some representative offerings. A typical example: “Living organisms are characterized by five properties: they evolve, they recognize themselves, they develop, and they feel. The official NASA version: “Life is a self-sustained chemical system capable of undergoing Darwinian evolution.” Life as a bearer of information: “a material system that can acquire, store, process, and use information to organize its activities.” The biochemist’s take: “a potentially self-perpetuating open system of linked organic reactions, catalyzed stepwise and almost isothermally by complex and specific organic catalysts which are themselves produced by the system.” Two, more technical-sounding examples: “a self-sustaining kinetically stable dynamic reaction network derived from the replication reaction” and “autocatalytic energy-processing systems stabilized far from equilibrium.” Another source lists a number of characteristics and requirements: reproduction, metabolism, nutrition, complexity, organization, growth and development, hardware/software entanglement, information content, and the paradoxical conjunction of permanence and change. (One candidate property for such lists, notably absent: “a susceptibility to death.”)
Several attributes of living organisms prove even harder to pin down. Among them are autonomy, self-determination, powers of self-repair and regeneration, interdependence, resilience and homeostasis. Add to these the capacity to learn, cognition, development and cooperation—features whose dynamic interplay contributes to evolutionary adaptation. Finally, there’s that enigmatic, core quality—or core “condition”—shared by all living beings: call it aliveness.
Some of these features are quantifiable, measureable, or in some fashion open to empirical analysis. Others, like those in that second set of more slippery items, are less easily treated. For instance, what about that continually recurring theme, seen throughout the natural world, of individual autonomy? While there are so many close associations between organisms, individuals—whether microbial, multi-cellular or even individual eukaryotic cells—tend to maintain some degree of independence in their interactions. In labs, human tissue cells are reared in nutritional broths where they assume amoeboid form. Here’s an observable phenomenon that appears to offer a rich trove of opportunities for scientific study: these cells—adrift and abandoned—continue to divide and multiply while searching for and, given the opportunity, reuniting with other marooned cells in a hearty attempt to resume their collective roles. Cells acting as if they were just going about their normal affairs and making due with what’s available. But the implications of these independent cells’ actions and instincts continue to be viewed exclusively in mechanistic terms. In times past, this odd phenomenon would instantly be recognized as a clear-cut manifestation of the universal life-force. It would elicit sage nods of reverent admiration from witnesses gazing down on the Petri dishes.
Here lies the source of the problem that has caused so much confusion and resistance. Our difficulty in pinning down the essence of living matter’s unique vitality is rooted in this: LIFE possesses distinctive qualities, tendencies, and predispositions which, solely because of their intangible nature and lack of measurability, have never achieved formal scientific recognition. The lack of formal recognition has created an intellectual barrier—an artificial impediment to crediting the actuality of known LIFE-attributes. The more remarkable manifestations of these problematic features tend to be seen as mere curiosities. (For instance, startling demonstrations of intelligence in lower animals—even plants, which show forms of intelligence that bypass consciousness of self and the need for a centralized brain). Moreover, these distinguishing life-features are precisely what gives the phenomenon in its exclusive status—and are the cause of our seeming inability to fully embrace LIFE’s end-directed agency.
One such feature of LIFE that has come to impress me almost as much as its diversity and complexity is its omnipresence. Another is LIFE‘s unshakable tenacity.
We’ll never know how long it took living matter, after it got started, to spread across the entire globe. But every environmental setting one can imagine had probably been colonized tens of millions of years ago. Note the use of the word “setting.” Location does not an ecosystem make. LIFE alters environments in ways that gradually make them better suited to the needs of their inhabitants. Through time, growing assemblages of organisms collectively modified their surroundings, creating new habitats. Novel ecological niches opened for business. Distinct ecosystems were fashioned, forever to be in a state of flux. Much later, when humans appeared and our activities began to alter existing habitats and create new niches, these too were quickly occupied. Even our dwelling places become ecosystems—home to a surprising array of organisms, the bulk of which go unnoticed.
Living matter is found everywhere, in any environment that’s not too hot or too cold to halt key physiological processes and any environment with at least some available water. These appear to be LIFE’s only limiting factors. Thus we find both plants and animals surviving in the hottest of deserts, where summer ground temperatures can almost boil water. Certain archaeans produce sulfuric acid as waste and subsist in concentrations as acidic as car battery fluid. Algae grow in concentrated salty brine held in channels within polar sea ice at below-freezing temperatures. The photosynthetic algae, which take advantage of whatever sunlight penetrates the ice floes, are food to larval polychaete worms, copepods, and crustaceans. Even more astonishing, a minute species of bacteria-eating nematode worm was found 3.5 kilometers below Earth’s surface living within flooded pore spaces in fractured rock in a South African gold mine at temperatures of almost 50°C (nearly 120°F).
Certain deep-water microbes not only survive but flourish at fantastic pressures and in water well above boiling point. These single-celled organisms are the foundation for entire biological webs that thrive around deep-sea thermal vents called black smokers, where mineral-laden water jets from the seafloor thousands of meters beneath the surface. In a similar vein, there’s an as-yet poorly understood world of mostly anaerobic varieties of bacteria and archaea living within pores between mineral grains inside crustal rock, deep beneath Earth’s surface (and under seafloors as well). Cornell astrophysicist Thomas Gold put it succinctly: “Microbial life exists in all the locations where microbes can survive.” Gold, a daring thinker who made scientific contributions to many fields, speculated in a 1992 paper that the subterranean fauna’s biomass may equal what lives above ground. By one recent estimate based on a decade long world-wide inventory, the subterranean biomass may amount to half of what exists on Earth. On a slightly different note: as further proof of the powers of organic evolution, recent human activity has led to our planet being graced with new bacterial forms capable of metabolizing such unlikely substances as concrete and byproducts of nylon production. Another newcomer subsists in the cooling tanks housing spent radioactive fuel rods. (Hopefully, the proper authorities are keeping a close eye on these developments.)
Pervasive ecological degradation has led to large numbers of people becoming aware of the delicate balance being precariously maintained in so many once-healthy environments. As a result, viewing LIFE as a tenacious and virtually unstoppable “force of nature” is not presently being emphasized so much as the fragility of vulnerable ecosystems. But, for me, an acute awareness of LIFE’s resilience and tenacity took on real meaning thanks to one particularly vivid lesson.
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Wherever I’ve lived, shelves and windowsills end up as depositories for what I call “nature trinkets.” Lying atop my bookcase at this particular time were a typical assortment of pebbles, crystals, bones, seed pods, feathers, and pieces of weathered wood. One day, on a walk with friends, we were discussing the native peoples who had formerly lived in the region. I pointed out a plant, a type of wild lily, telling my companions that its starchy bulbs had once been an important food source. Using a flat rock I dug one up to demonstrate how difficult and time-consuming it would be to collect the bulbs in quantity. The tuber looked like a marble-sized onion, with the same burnished copper-colored skin…a “shiny object,” which also had the ineffable appeal of a dainty miniature. So I pocketed the little onion look-alike. And onto the top of my bookcase it went.
In time the little brown marble shrank somewhat and grew dusty. In tidying it up, the outer layer of its papery skin flaked off, revealing a fresh-looking but slightly diminished bulb. Months later, I repeated this gesture out of respect. And, again, there was a glossy-skinned micro-onion inside, still firm and hale. Months went by. I decided to move to a new place and, while boxing all my delicate nature trinkets, was stunned to find a green and glistening shoot protruding from the shriveled little bulb’s tip. Something had triggered this do-or-die effort and it had sprouted—after resting on top of my bookcase for two…and a half…years. I was moved close to tears, having been granted a new appreciation for just how resolute and fiercely tenacious living things can be. Though it may sound trivial in the telling, for me this was a genuinely life-changing revelation, a direct link to my decision to write this book.
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Collectively, LIFE possesses an enduring vigor, having survived at least six major mass-extinction events (the latest of which, human-caused, is currently in full swing) along with perhaps nine other garden-variety calamities since the Cambrian era. The result of these past upheavals was, in each instance, a dynamic resurgence marked by considerable evolutionary diversification among the survivors thanks to wide-open, uninhabited territories with niches just waiting to be filled. Both terrestrial and aquatic organisms rebounded after each disaster, with forms both old and new rising to the new day. Take the mass extinction known as the end-Cretaceous event, which occurred around 66 Ma: caused by a sizeable asteroid strike and exacerbated by other factors, the end-Cretaceous event resulted in the extermination of all large animals and perhaps two-thirds of all species. Earth’s entire non-avian dinosaur fauna’s swift disappearance following the catastrophic episode set in motion the rise of mammals (which, in due course, led to the appearance of mankind). Seen from this big-picture perspective, mass extinctions take on new significance—global cataclysm as opportunity for rebirth and renewal. Extinctions impart an evolutionary blank slate, a cleansed and refreshed world presenting endless opportunities.
Perhaps the single most important clue in attempting to understand the nature of LIFE is how long it took to become established on Earth. By all appearances, it sprang up on our planet with surprising ease. If so, this one fact may have more to say about LIFE than all the (so far) fruitless conjecture about how LIFEarose in the first place. The importance of this key point makes it worth looking into in some detail.
Geomorphologists have settled on a figure 4.57 billion years for Earth’s age, defined as the point at which its formation was mostly complete—around one hundred million years after the collapse of the giant molecular cloud that led to the creation of our solar system. (For perspective, Jupiter took shape much earlier—possibly within the first ten million years.) For its first five-hundred million years, our young planet was incessantly bombarded by debris left over from planetary formation while simultaneously being wracked by widespread volcanic eruptions. Until quite recently, this so-called Hadean Era was presumed too inhospitable for life to emerge. Almost all early surface material has subsequently been obliterated so there’s very little remaining evidence from that time to reveal what conditions were like and even fewer hints as to when life might have first appeared.
No surface rock has been preserved from the first 500+ Ma of our planet’s history. Since the rock record at present reaches no further, filling in the story of Earth’s early history is made possible only through analyzing remarkably stable mineral fragments—minute but extremely durable crystals of zircon (ZrSiO₄). These microscopic silicate crystals, almost as hard as diamond but tougher, originally formed in even more ancient rock but are now preserved in somewhat younger sedimentary deposits derived from the older material. The oldest surviving zircons are found in Western Australia, reliably dated at 4.4 billion years.
A search for the earliest evidence of life has been ongoing for several decades, thanks to advances in radiometric dating technologies. Peer-reviewed journal reports continually push back the date. And every year or so one of these reports is called into question, refuted, or gains further support in a model demonstration of the stepwise advancement of scientific knowledge.
The earliest indirect evidence of life—what is known as a “biosignature”—came in 2015 when University of California researchers identified 4.1 billion year old graphite inclusions within a 3.8 billion year old zircon from the Jack Hills of Western Australia. This minute zircon, smaller than the period at the end of this sentence, contained graphite inclusions with a high ratio of ¹²C relative to ¹³C (a higher fraction of carbon’s lighter isotope being a characteristic feature of biological carbon assimilation through photosynthetic activity). The earliest undisputed evidence of life, reported in 2017, comes from microfossils found in 3.5 billion year old Australian Apex chert derived from hydrothermal vent deposits. Even more tantalizing: published results from another study released around the same time present evidence of fossilized microorganisms in rock at least 3.8 (but possibly as much as 4.3) billion year old—also putative hydrothermal vent deposits from a remote area in northern Quebec. Like the Apex chert, these microscopic fossils consist of “structures [that] occur as micrometre-scale haematite tubes and filaments with morphologies and mineral assemblages similar to those of filamentous microbes from modern hydrothermal vent precipitates.” Time will tell if any of these contenders for most-ancient life prove to be legitimate.
Further complicating matters: recent studies call into question the severity of the Late Heavy Bombardment, which peaked at roughly around 700 Ma after the solar system’s formation. A new theory holds that, while the episode may have taken place as conjectured, significant portions of Earth’s crust had not melted and large bodies of water likely remained on Earth’s surface (that is, never boiled away completely). This supposition is based on the fact that zircons form in several ways, all of them involving water-based chemistry. Zircons are ubiquitous in granites, which make up the bulk of continental crustal rock. The presence of these ancient crystals signifies there was water on the planet’s surface soon after Earth took shape and global conditions that were far milder than has long been assumed, a scenario known as the Cool Early Earth theory.
Where does all this leave us?
If life existed on Earth 4.1 billion years ago, its commencement predated the Late Heavy Bombardment by at least 200 million years. A recent study involving extensive computer modeling indicates that, under even the heaviest conceivable bombardment, Earth’s surface would never have melted entirely. The study concludes that heat tolerant microbes living in the vicinity of underwater hydrothermal vent systems could have survived the heaviest periods of bombardment. The 3.5 billion year old microfossils found in the Australian cherts lived in such environments. Analysis of their remnants, based on different carbon isotope ratios, shows the presence of five different taxa that employed several different metabolic strategies—indicative of a diverse symbiotic microbial community that had originated much earlier.
If the microfossils from northeastern Canada and reported in 2017 (reputedly at least 3.8 billion years old) prove to be closer to the upper age limit of 4.3 billion: they, too, would have arisen much earlier and had time to evolve from more primitive forbears. If this figure is substantiated, it means that life in fact arose almost as soon as physically possible—only two or three hundred million years after the Earth formed.
So, what might all this have to say about LIFE’s fundamental nature?
Asked to speculate on how long it took life to originate, Stanley Miller (of the Miller–Urey Experiments, Chapter Eighteen) once suggested that “a decade is probably too short, and so is a century. But ten- or a hundred thousand years seems okay, and if you can’t do it in a million years, you probably can’t do it at all.” How Miller arrived at these conclusions is interesting, given the complete lack of any solid evidence for the origin of life despite his many years of looking for an answer to the question
But now, shifting to new themes…it’s time to review where mankind and our views about LIFE fit into the bigger picture.
©2020 Tim Forsell 25 Oct 2020
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