91av tackles eight of the deepest challenges faced by science – from reality and consciousness, to free will and death, in The Big Questions special features.
No human discovery could have more profound ramifications than finding what’s known in the business as a “second genesis” – an origin of life independent of that on Earth. With our present sample of one known living world, the possibility remains that Earth is unique and that we are utterly alone in the universe. But if we find a second genesis in our own cosmic backyard, then we will know that life is a universal imperative. The unproven conviction that the cosmos teems with life drives many of us in the nascent discipline of astrobiology – a field that one wit described as “the only science without a subject matter”.
Earthbound biologists are exceptionally good at finding life. A single cell, a snippet of DNA, even an idiosyncratic collection of carbon-based molecules can point unambiguously to the presence of living beings, but those are signs of Earth life. What if life elsewhere is different, based on an exotic alien anatomy and biochemistry? Unlike Justice Potter Stewart, who in his 1964 Supreme Court ruling on obscenity boasted some proficiency at recognising pornography, “I know it when I see it”, I think the chances are good that we ɴDz’t know alien life when we see it. So what exactly is life, and how can we detect it?
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Scientists care about definitions, so they convene conferences to discuss the matter. A recent meeting called “What is life?” attracted a hundred scientists, who mingled with assorted philosophers and theologians to debate the issue. Opinions differed dramatically, but the most contentious debates occurred within the scientific ranks. One very senior expert on lipid molecules argued that life began with the first semi-permeable lipid membrane. An equally august authority on metabolism countered that life began with the first self-sustaining metabolic cycle. On the contrary, claimed several molecular biologists, the first living entity must have been an RNA-like genetic system that carried and duplicated biological information. One mineralogist even proposed the decidedly minority view that life began not as an organic entity, but as a self-replicating mineral.
The unresolved debate was reminiscent of the classic story of the blind men and the elephant. Asked to describe the beast, each one’s perspective varied, based on which feature was close at hand – the slender rope-like tail, the mighty tree-like legs, the twisting snake-like trunk, and so forth. Each man’s version was wrong, but each possessed an element of the more complex elephantine truth. Perhaps the disparate claims of what constitutes life are likewise mere parts of the more complex truth of life’s identity and origin.
Israeli origins expert Noam Lahav at the Hebrew University of Jerusalem underscored the problem of defining life in his 1998 book Biogenesis. Lahav tabulated a century of scientific definitions penned by 48 different authorities. The entry by the distinguished evolutionary biologist John Maynard Smith describes life as “any population of entities which has the properties of multiplication, heredity and variation”. Alternatively, information theorist Stuart Kauffman claims that “life is an expected, collectively self-organised property of catalytic polymers”. Other experts propose that “life is the ability to communicate”, “life is a flow of energy, matter and information”, “life is a self-sustained chemical system capable of undergoing Darwinian evolution”. The definitions go on and on, and no two are quite the same.
Perhaps this should come as no surprise. Science is not the only profession to struggle with the question “what is life?”. Bioethicists and theologians debate it in relation to the beginning of human life: does life start at moment of conception, when the fetal brain first responds, or when the unborn heart first beats? At the other end of the human journey, doctors and lawyers require a definition of life in order to deal ethically with patients who are brain dead or otherwise terminally unresponsive.
Scientific efforts to define life are less ethically complex and emotionally charged, but the lack of scientific agreement still represents an obvious problem. It’s difficult to be sure you have discovered life on other worlds – or deduced the process of its origin on Earth – when you can’t define what it is. In spite of generations of labour by hundreds of thousands of biologists, in spite of countless studies of living organisms at every scale, a general definition that distinguishes all imaginable living objects from the myriad non-living ones remains elusive.
Scientists who study life desire an unambiguous definition, and they adopt two complementary approaches in their efforts to distinguish that which is alive from that which is not. Many apply the “top-down” approach. They scrutinise all manner of living and fossil organisms to identify characteristics of the most primitive entities that are, or were, alive. This strategy is limited, however, because all known life forms, whether living or fossil, are based on sophisticated cells containing DNA and proteins. Any definition of life based on top-down research is correspondingly myopic.
By contrast, a small army of investigators pursues the so-called “bottom-up” approach. They devise laboratory experiments to mimic the chemistry of Earth’s ancient environments. Eventually, the bottom-up goal is to create a living chemical system in the laboratory from scratch – an effort that might clarify the transition from non-life to life. Such research leads to an amusing range of opinions regarding what is alive, because each scientist tends to define life in terms of his or her own chosen speciality – cell membranes, metabolic cycles, RNA, viruses and even silicon-based artificial intelligence all have their passionate proponents.
“Eventually, the bottom-up goal is to create a living chemical system in the laboratory from scratch”
Into this mix, philosophers and theologians inject a more abstract view and speculate on the full range of phenomena that might be said to be alive – robotic life, computer life, even a self-aware internet. Such debates can at times sound like a science fiction convention, but defining life is no idle exercise. After all, if NASA is to look for life on other worlds, a clear definition is essential for planning future missions.
Scientists excel at many things, but compromise is not always one of them. Nevertheless, Gerald Joyce of The Scripps Research Institute, serving on a NASA exobiology panel, has tried to achieve this. He proposed one of the descriptions in Lahav’s list as a “working definition” for life in the context of space exploration: “Life is a self-sustained chemical system capable of undergoing Darwinian evolution.”
This succinct and widely cited metric combines three distinct characteristics. First, any form of life must be a chemical system. Accordingly, computer programs, robots or other electronic entities are not alive. Life also grows and sustains itself by gathering energy and atoms from its surroundings – the essence of metabolism. Finally, living entities must display variation. Natural selection of more fit individuals inevitably leads to evolution and the emergence of more complex entities. This NASA-inspired definition is probably as general, useful and concise as any we are likely to come up with – at least until we discover more about what is actually out there.
Armed with such a definition, we can imagine that our planet’s earliest life may have been vastly different from anything we know today. Many experts suspect that the first living entity was not a single cell as we know it, for even the simplest cell incorporates astonishing complexity. That first life form probably did not use DNA, given the intricacies of the genetic code, nor did it necessarily rely on proteins, the chemical workhorses of cellular life.
As a geologist trained in the ways of rocks, my favourite hypothesis is that the earliest entity to fit NASA’s trial definition might have been a molecular coating on rock surfaces. Such “flat life” would grow as a layer only a few nanometres thick, exploiting energy-rich mineral surfaces while slowly spreading like lichen from one rock to the next. If such life still exists on Earth today, yet lacks diagnostic DNA or proteins, how would we know?
In spite of such well-intentioned efforts to define life, any attempt may be doomed to failure for the simple reason that the transition from the non-living to the living world was inherently gradual. French anthropologist Claude Lévi-Strauss, who investigated the mythologies of many cultures, identified a deep-seated human tendency to reduce complex situations to oversimplified dichotomies: friend and enemy, heaven and hell, good and evil. The history of science reveals that scientists are not immune to this mindset. In the 18th century, the Neptunists, who favoured a watery origin for rocks, fought with the Plutonists, who favoured heat as the causative agent. Both, it turns out, were more or less right. A similar contentious and ultimately misleading dichotomy raged between 18th-century catastrophists and uniformitarians, the former espousing a brief and cataclysmic geological history for Earth and the latter holding that geological processes are gradual and ongoing. More recently, once-doctrinal distinctions between plants and animals or single-celled and multicellular organisms have become similarly blurred.
Any attempt to formulate an absolute definition that distinguishes between life and non-life represents a similar false dichotomy. The first cell did not just appear, fully formed. Rather, life must have arisen through a sequence of emergent events – diverse processes of organic synthesis followed by molecular selection, concentration, encapsulation and organisation into various molecular structures. The emergence of self-replicating molecules of increasing complexity and mutability led to molecular evolution through the process of natural selection, driven by competition for limited raw materials.
“Any attempt to formulate a definition that distinguishes life from non-life represents a false dichotomy”
What today appears as a yawning divide between non-life and life obscures the fact that the chemical evolution of life occurred in this stepwise sequence of successively more complex stages. When cells emerged, they quickly consumed virtually all traces of the earlier stages of chemical evolution. “Protolife”, a rich source of food, was wiped clean by voracious cellular life.
Our challenge, then, rather than to define life in absolute terms, is to establish a progressive hierarchy of steps leading from a prebiotic Earth enriched in organic molecules to cellular life. The nature and sequence of these steps may vary in different environments, and we may never know the exact sequence – or sequences – that occurred on Earth. Yet many of us suspect that the chemical path has a similar, inexorable direction on any habitable planet or moon.
Such a stepwise scenario informs attempts to define life. To pin down the exact point at which such a system of gradually increasing complexity becomes “alive” is intrinsically arbitrary. “What is life?”, then, is fundamentally a semantic question. Nature holds a rich variety of complex chemical systems, and scientists increasingly are learning to craft such systems in the laboratory. Yet no matter how curious or novel their behaviour, none comes with an unambiguous label: “life” or “non-life”.
Philosopher Carol Cleland of the University of Colorado in Boulder and planetary scientist Christopher Chyba of Princeton University have compared recent attempts to define life with unfruitful 18th-century efforts to characterise water. Before the discovery of molecules and atomic theory, water could be described only using a series of non-unique traits. Water is clear and wet, but so are many oils – and muddy water isn’t all that clear. Water sustains life, but so do many foods – and water with a few invisible pathogens can kill you. Water freezes when it gets cold, soaks into wood, flows downhill, on and on the list grows; but none of these traits, nor any combination of them, is both necessary and sufficient. No definition devised in the 18th century could have captured the true essence of water – the molecule composed of two atoms of hydrogen and one of oxygen. By the same token, they argue, scientists in the early 21st century are in no position to define life. Better, therefore, to keep an open mind and simply describe the characteristics of whatever we find. And if life arose through a sequence of steps, then perhaps each one represents a taxonomically distinct, fundamentally important stage in life’s emergence. In that case, each step deserves its own label.
How are we to come closer to a definition of life? Ultimately, the key to defining the progressive stages from non-life to life lies in experimental studies of relevant chemical systems under plausible geochemical environments, coupled with targeted exploration of our nearest planetary neighbours. The concept of emergence simplifies the experimental endeavour by reducing an immensely complex historical process into a more comprehensible succession of measurable steps: the emergence of metabolism, of genetic polymers, of self-replicating molecular systems. Each one provides a tempting focus for laboratory experimentation and theoretical modelling.
This fuzzy definition also informs the search for life elsewhere in the universe. It’s plausible, for example, that Mars, Jupiter’s moon Europa and other bodies in our solar system progressed only part way along the path to cellular life. If so, that’s crucial information for NASA’s astrobiologists. If each step in life’s origin produced distinctive and measurable molecular, isotopic and structural signatures, and if such markers can be identified, then these chemical features become observational targets for space missions. It’s possible, for example, that primitive prebiotic molecular, isotopic and structural forms are inevitably eaten by more advanced cells and survive as “fossils” only if cellular life never developed in their environs. Thus prebiotic features may serve as extraterrestrial “abiomarkers” – clear evidence that molecular evolution never progressed beyond a certain pre-cellular stage. As scientists search for life elsewhere in the universe, they may be able to characterise extraterrestrial environments according to their degree of progress along this path.
Saturn’s cloud-enshrouded moon Titan provides a tempting case in point. Titan possesses a methane-rich atmosphere one-and-a-half times as thick as Earth’s. Organic molecules, which colour the atmosphere a hazy orange, rain onto the surface to form thick accumulations of organic gunk. Lakes of methane and ethane occur side by side with expanses of rock-hard water ice, though conditions are generally much too cold for liquid water or significant chemical progress toward life.
Even so, from time to time the impact of a large comet or asteroid may have melted regions of ice. For periods of hundreds or even thousands of years, gradually cooling ice-covered lakes might have supported the first steps in the path toward life, only to become frozen again. Such primitive biochemistry, though lost forever on Earth’s scavenged surface, might conceivably survive in Titan’s deep freeze.
The question “What is life?” provides a scientific benchmark. We can say that any rigid demarcation of the natural world into living versus non-living represents a false dichotomy. We now understand that life arose as a gradual, sequential process of emergent steps from geochemical simplicity to biological complexity. We are poised to reproduce those enigmatic transitional states in the laboratory, and with luck might discover them frozen on other worlds. And so, perhaps within a few decades, we may at last know what it is we’re looking for.
Read more: The biggest questions ever asked
