Life

• 1. Introduction

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  Life is matter that runs biological processes, and yet at least 123 separate definitions of it have been compiled, with no agreement among them. That is a strange fact to sit with. The thing every reader of this script possesses, the thing that fills the oceans and the soil and the air, resists a single sentence that pins it down. Part of the trouble is that life is a process, not a substance. It is something matter does, not something matter is. There is a microbe living in 120 degree Celsius sediment more than a kilometre below the floor of an undersea trench. As one researcher put it, "You can find microbes everywhere, they're extremely adaptable to conditions, and survive wherever they are." How did this start? Why does a virus sit on the border of being alive at all? What did ancient philosophers think a living thing actually was, and why were they so often wrong? And if life exists out among the stars, would we even recognise it? These are the questions this documentary follows.

• 2. The Problem Of Definition

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  At least 123 definitions of life have been compiled, and that abundance is itself the clue. Defining life has long been a challenge for scientists and philosophers, partly because of how little is known about what living things outside Earth might be like, if any exist. Legal definitions exist too, though they tend to circle a narrower question, the decision to declare a human being dead and the consequences that follow.

  Most working biologists, faced with no consensus, settle for description rather than definition. Life, in this view, is whatever preserves, furthers or reinforces its own existence in a given environment. That implies a cluster of traits. Homeostasis is the regulation of an internal state, as when a body sweats to cool itself. Organisation means being built from one or more cells. Metabolism transforms energy, building cellular components through anabolism and breaking organic matter down through catabolism. Growth is simply anabolism outpacing catabolism, so the organism gets larger.

  Response to stimuli covers a vast range, from a single cell flinching away from a chemical to the leaves of a plant turning toward the sun, a motion called phototropism. Reproduction completes the list, whether asexually from one parent or sexually from two. A physicist would phrase it differently. From thermodynamics, life is an open system that exploits gradients in its surroundings to make imperfect copies of itself.

  That physical framing produced one famous attempt at a definition, adopted by a NASA committee trying to define life for exobiology and based on a suggestion by Carl Sagan. Life, it said, is "a self-sustained chemical system capable of undergoing Darwinian evolution." The definition was widely criticised. By its own logic a single sexually reproducing individual is not alive, because it cannot evolve on its own. Stuart Kauffman offered another route, casting a living thing as an autonomous agent able to reproduce itself and complete at least one thermodynamic work cycle.

• 3. Organisms At The Edge

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  Viruses are described as "organisms at the edge of life," and the phrase captures why they are so hard to place. They carry genes. They evolve by natural selection. They replicate by making many copies of themselves through self-assembly. By those measures they behave like living things.

  Metabolism is where they fail the test. Viruses do not metabolise, and they cannot make new products without commandeering a host cell. For that reason they are most often treated not as life but as gene-coding replicators, lacking cell membranes and the ability to grow or respond to their surroundings. When microbiology revealed these non-cellular entities, biologists found themselves with something genuinely on the margin.

  That self-assembly inside a host carries a deeper implication. It may support the hypothesis that life itself began as self-assembling organic molecules, which makes the virus not just a puzzle of classification but a hint about origins.

• 4. Souls, Atoms, And Vital Forces

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  Empedocles, writing around 430 BC, argued that everything in the universe was built from four eternal roots: earth, water, air, and fire. Every change was just these four rearranging, and the various forms of life came from an appropriate mixture of them. It was one of the earliest materialist accounts, holding that all that exists is matter and life is only a complex arrangement of it.

  Democritus, around 460 BC, was an atomist who thought the essential feature of life was having a soul, the psyche, itself made of fiery atoms. He dwelt on fire because of the apparent link between life and heat, and because fire moves. Plato disagreed entirely, holding that the world was organised by permanent forms reflected only imperfectly in matter, forms that supplied direction and intelligence.

  Aristotle, around 322 BC, built the theory called hylomorphism, in which everything material has both matter and form, and the form of a living thing is its soul. He described three kinds: the vegetative soul of plants that grows and nourishes, the animal soul that moves and feels, and the rational soul, the source of reasoning, which he believed belonged to man alone. Each higher soul contained the powers of the lower ones. Because form could not exist without matter, he concluded the soul could not exist without the body.

  Rene Descartes, who lived from 1596 to 1650, revived the mechanistic view, treating animals and humans as assemblages of parts working together like a machine. Gottfried Wilhelm Leibniz pushed the idea down to the smallest scale, writing in his Monadology of 1714 that "the machines of nature, that is living bodies, are still machines in their smallest parts, to infinity." Julien Offray de La Mettrie, who lived from 1709 to 1750, carried it further in his book L'Homme Machine.

  Vitalism ran in the opposite direction, insisting on a non-material life-principle. It began with Georg Ernst Stahl in the 17th century and held that organic material could only come from living things. That belief collapsed in 1828, when Friedrich Wohler made urea from inorganic materials, the synthesis now treated as the starting point of modern organic chemistry. Later, Hermann von Helmholtz showed that no energy is lost in muscle movement, undercutting the idea of any "vital force" needed to move a muscle. The belief survives only in pseudoscience now, such as homoeopathy, which blames disease on disturbances in a hypothetical life force.

• 5. An Almost Instantaneous Emergence

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  Earth is about 4.54 billion years old, and life appears almost as soon as the planet could hold an ocean. The oldest physical traces date back 3.7 billion years, found as biogenic graphite in metasedimentary rocks from Western Greenland. Microbial mat fossils survive in 3.48 billion-year-old sandstone from Western Australia.

  Molecular clock estimates, gathered in the TimeTree public database, push the origin of life to around 4 billion years ago. In 2017, researchers announced putative microfossils in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada, possibly as old as 4.28 billion years. That would be the oldest record of life on Earth, suggesting what one account called "an almost instantaneous emergence of life" after ocean formation 4.4 billion years ago. In 2016, a set of 355 genes was tentatively identified as belonging to the last universal common ancestor.

  From that single ancestor, evolution did the rest. It is the change in heritable characteristics of populations over generations, driven by natural selection, sexual selection, and genetic drift acting on genetic variation. The process produced biodiversity at every level of organisation, and it also produced loss. Over 99 percent of all species that have ever lived are now extinct, and mass extinctions may have accelerated evolution by clearing room for new groups to diversify.

  Life also remade the planet it lived on. When cyanobacteria began releasing molecular oxygen as a by-product of photosynthesis, they triggered a global change in the environment. Oxygen was toxic to most life at the time, a novel evolutionary challenge that ultimately led to Earth's major animal and plant species. The organisms changed the world, and the changed world reshaped the organisms.

• 6. Everywhere There Is A Gradient

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  Spores of Aspergillus niger have been detected in the mesosphere, at an altitude of 48 to 77 kilometres, which gives a sense of just how far life reaches. The biosphere, the global sum of all ecosystems, holds organisms in soil, in hot springs, inside rocks at least 12 miles underground, in the deepest ocean, and at least 40 miles up in the atmosphere. Under test conditions, life forms have even survived in the vacuum of space.

  Life thrives in the deep Mariana Trench and inside rocks up to 580 metres below the sea floor, beneath 2,590 metres of ocean off the northwestern United States, and 2,400 metres beneath the seabed off Japan. In 2014, life forms were found living 800 metres below the ice of Antarctica. The International Ocean Discovery Program found unicellular life in 120 degree Celsius sediment 1.2 kilometres below the seafloor in the Nankai Trough subduction zone.

  Survival in these places depends on what ecologists call the range of tolerance. Within that range an organism manages temperature, water, light, nutrients, and the rest. Just outside it lie zones of physiological stress, where survival and reproduction are possible but not optimal. Beyond those lie zones of intolerance, where the organism almost certainly cannot persist. A wide range of tolerance lets a creature spread; a narrow one confines it.

  The specialists at the far edges are the extremophiles. To survive, some microorganisms have evolved to withstand freezing, complete desiccation, starvation, and high levels of radiation. They excel at exploiting uncommon sources of energy, and characterising the metabolic diversity of these communities is still ongoing work. The same study of how life endures extremes feeds directly into one question, what life elsewhere might be able to take.

• 7. Naming The Living

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  Aristotle, who lived from 384 to 322 BC, made the first classification of organisms, splitting living things into plants and animals mainly by their ability to move. He divided blooded animals, roughly our vertebrates, into five groups: viviparous quadrupeds, oviparous quadrupeds, birds, fishes, and whales. The bloodless animals, the invertebrates, he split into cephalopods, crustaceans, insects, shelled animals, and zoophytes, creatures that resembled plants. His scheme held for more than a thousand years.

  Carl Linnaeus, in the late 1740s, introduced binomial nomenclature, trimming the long many-worded names of the past into precise descriptive terms. The fungi gave his system trouble. Linnaeus first treated them as plants, briefly filed them among the Vermes in Animalia, then returned them to Plantae. Herbert Copeland later placed them with single-celled organisms in his Protoctista, and the matter was settled only when Whittaker gave fungi their own kingdom in a five-kingdom system. Evolutionary history shows the fungi are in fact closer to animals than to plants.

  Ernst Haeckel united newly discovered single-celled organisms into the kingdom Protista, after they had first been split awkwardly into animal-like protozoa and plant-like forms. The prokaryotes were later separated into the kingdom Monera, which itself divided into Bacteria and Archaea. That progression led to a six-kingdom scheme and finally to the current three-domain system, built on evolutionary relationships. The ability to sequence whole genomes has since allowed a metagenomic view of the whole tree of life, revealing that the majority of living things are bacteria and that all share a common origin.

• 8. The Machinery Inside

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  Carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur are the six elemental macronutrients that build all known life. Carbon is the most abundant of them in organisms, prized for forming multiple stable covalent bonds, which lets organic molecules assemble into the enormous variety of arrangements that fill organic chemistry. Five of those six elements make up DNA, the exception being sulfur, which instead appears in the amino acids cysteine and methionine.

  Deoxyribonucleic acid carries most of the genetic instructions used in growth, development, functioning, and reproduction across all known organisms and many viruses. Most DNA molecules are two strands coiled into a double helix, each strand a chain of nucleotides. Every nucleotide holds one of four nitrogen-containing bases, cytosine, guanine, adenine, or thymine, joined to a deoxyribose sugar and a phosphate group. The base pairing rules, adenine with thymine and cytosine with guanine, mean each strand carries all the information needed to rebuild the other, which is how information survives cell division. Within cells this DNA is organised into chromosomes, and eukaryotes keep most of theirs inside the nucleus.

  The cell is the basic unit of structure in every living thing, and all cells arise from pre-existing cells by division. Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf Virchow, and others in the early nineteenth century. There are two primary kinds. Prokaryote cells, the bacteria and archaea, lack a nucleus and membrane-bound organelles but carry circular DNA and ribosomes. Eukaryote cells hold a membrane-bound nucleus and organelles such as mitochondria, chloroplasts, and the Golgi apparatus, and the conventional model is that they evolved from prokaryotes through endosymbiosis.

  Multicellular life may have begun as colonies of identical cells sticking together through cell adhesion. The members of a true multicellular organism, unlike colony members, developed specialisations that made them dependent on the whole. About 800 million years ago, a minor genetic change in a single enzyme, GK-PID, may have been what let organisms make the leap from one cell to many.

  No such machinery has yet been confirmed anywhere but Earth, though many regard extraterrestrial life as probable or even inevitable. The subsurface of Mars, the upper atmosphere of Venus, and the subsurface oceans of the giant planets' moons are all candidates for microbial life. Lichen has survived for a month in a simulated Martian environment, and a proposed "Confidence of Life Detection" scale now stands ready for the day someone reports finding life beyond this one planet.