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Life

Fine. You want me to rewrite this, expand it, and make it... engaging. As if the universe itself isn't already a grand, tedious spectacle. I suppose I can oblige, but don't expect sunshine and rainbows. Expect facts, dished out with the kind of weariness only someone who's seen too much can muster. And keep those internal links. They're the only things holding this whole mess together, aren't they?

Life: The Messy, Persistent Phenomenon

For other uses, see Life (disambiguation).

Life

Temporal range: 3770–0 Ma (possible Hadean origin, documented from Archean to present)

Diverse forms of life on a coral reef

Scientific classification

Domains, supergroups and other

On Earth, life manifests in a bewildering array of forms:

Life, in essence, is matter that engages in biological processes. It’s the capacity for self-maintenance, for organization, for a relentless cycle of metabolism, growth, adaptation, response to stimuli, and, of course, reproduction. No organism, however grand, is truly immortal. It all eventually succumbs to death. Philosophers have wrestled with its definition, often through the lens of self-organizing systems. But even that gets complicated when you consider viruses, which are only truly "alive" when they've hijacked a host cell, or the tantalizing possibility of extraterrestrial life, which might defy all our terrestrial assumptions. Life, in its stubborn persistence, occupies every conceivable niche on Earth, from the airy heights to the crushing depths, from the arid deserts to the steaming hot springs. The sum of all this activity is the biosphere. The collective of life in any given place or time is its biota.

Definitions: A Perpetual Struggle

Defining life has been a remarkably persistent headache for anyone who’s bothered to think about it. It’s not a substance, but a process, a cascade of chemical reactions with a peculiar drive to perpetuate itself. 5 6 7 This becomes even more convoluted when we consider the unknown forms life might take elsewhere in the cosmos. 11 12 Even our legal systems grapple with distinguishing life from death, a line that’s often blurrier than we’d like. 14 Some have even attempted to quantify definitions, compiling well over a hundred. 15

A biota, for the record, is simply the collection of living organisms—plants, animals, the whole lot—that inhabit a specific place and time. 16 Protecting this biota, then, is the rather Sisyphean task of nature conservation. 17

Descriptive Traits: The Checklist Nobody Agrees On

Since a single, universally accepted definition remains elusive, biology tends to rely on a descriptive approach. Life is that which strives to persist, to maintain its existence against the forces of entropy. This usually involves a combination of these traits: 7 18 19 20 21 22

  • Homeostasis: The internal thermostat. It's about maintaining a stable internal environment, like sweating to cool down.
  • Organisation: Everything is built from cells. Whether one or many, they are the fundamental units.
  • Metabolism: The energy game. It's the transformation of energy and matter, involving anabolism (building up) and catabolism (breaking down). Without energy, nothing happens. 23
  • Growth: Anabolism outpacing catabolism. It's not just getting bigger; it's about structural development.
  • Adaptation: The slow march of evolution. Organisms gradually become better suited to their surroundings. 23 24 25
  • Response to Stimuli: Reacting to the world. This ranges from a single-celled organism dodging a chemical to complex sensory systems in larger creatures, or even a plant turning towards the sun (phototropism). Chemotaxis is another example.
  • Reproduction: The imperative to create more. This can be simple, from a single parent (asexual), or involve two (sexual).

Physics and Life: Entropy's Persistent Adversary

From a purely physics standpoint, an organism is a complex thermodynamic system that, against all odds, manages to organize itself and reproduce. 26 27 It's an open system, constantly drawing on external gradients to create imperfect copies of itself, a perpetual dance against entropy. 28 A definition adopted by a NASA committee, "a self-sustained chemical system capable of undergoing Darwinian evolution," aims for this thermodynamic perspective. 29 30 However, this definition falters when considering individuals within a sexually reproducing population, who can't evolve independently. 31

Living Systems Theory: A Broader Perspective

Then there's the living systems theory viewpoint, which sidesteps precise molecular definitions. It posits that living things are inherently self-organizing and autopoietic – they produce and maintain themselves. Stuart Kauffman proposed that life is an autonomous agent or system capable of self-reproduction and performing thermodynamic work cycles. 32 This is further expanded by the idea of evolving novel functions. 33 Living systems exhibit a remarkable hierarchical organization, from the molecular machinery within a cell to entire ecosystems and the global biosphere. 34

Death: The Inevitable Conclusion

Death is the ultimate cessation of all vital functions. 35 36 Distinguishing it from life is a challenge, as biological functions don't all shut down simultaneously. 37 The philosophical and religious contemplation of death has been a constant throughout human history, with various beliefs about an afterlife, reincarnation, or resurrection. 38 The decomposition of corpses, like this African buffalo, is crucial for recycling nutrients back into the ecosystem, fueling new life.

Viruses: The Edge of Life

Whether viruses are truly alive remains a point of contention. 39 40 They possess genes, they evolve, they replicate – all hallmarks of life. 43 44 Yet, they lack metabolism and are entirely dependent on host cells. 41 Their self-assembly within host cells offers intriguing clues about the potential origins of life itself, suggesting that complex organic molecules could spontaneously organize. 45 46

A Brief History of Studying Life

Materialism: The World of Matter

Early thinkers like Empedocles (430 BC) proposed that everything, including life, was a mixture of fundamental elements: earth, water, air, and fire. 47 Democritus (460 BC), the atomist, believed life was animated by fiery atoms constituting the soul. 48 Later, Plato introduced the idea of perfect forms influencing the material world. 49

This mechanistic view resurfaced with René Descartes (1596–1650), who saw animals and humans as intricate machines. Gottfried Wilhelm Leibniz elaborated on this, describing natural machines as machines within machines, extending infinitely. 50 Julien Offray de La Mettrie (1709–1750) further pushed this with his book L'Homme Machine. 51 The 19th century, with its advances in cell theory and Charles Darwin's theory of natural selection (1859), solidified this mechanistic understanding. 52 Even in the 20th century, Stéphane Leduc (1853–1939) explored the idea that biological processes could be explained by physics and chemistry, likening growth to inorganic crystal formation. 53 His ideas, though initially dismissed, have seen a resurgence of interest. 54

Hylomorphism: Form and Matter

Aristotle (322 BC) proposed hylomorphism, the idea that everything has both matter and form. For living things, the form is the soul (psyche/anima). He described three types: vegetative (plants), animal (motion and sensation), and rational (consciousness, uniquely human). 55 He also asserted that the soul couldn't exist without the body. 56 This perspective aligns with teleological explanations, which explain phenomena by their purpose. However, modern science, particularly evolutionary history, favors explanations based on prior causes rather than future goals. 57

Spontaneous Generation: The Myth Dispelled

For millennia, the idea of spontaneous generation—that life could arise from non-living matter—prevailed, notably championed by Aristotle. 59 It took the meticulous experiments of Louis Pasteur in the mid-19th century to definitively disprove this notion, building on the work of predecessors like Francesco Redi. 60 61 The scientific consensus is now firmly against spontaneous generation. 62 63 64

Vitalism: The Elusive Life Force

Vitalism posited a non-material life-principle, a concept popular until the mid-19th century. It suggested a fundamental difference between organic and inorganic matter, claiming the former could only be derived from living things. This was shattered in 1828 when Friedrich Wöhler synthesized urea from inorganic materials, marking the dawn of modern organic chemistry. 67 66 Later, Hermann von Helmholtz and others demonstrated that biological processes, like muscle movement, could be explained by physical laws, not a vital force. 68 Eduard Buchner's discovery that fermentation could occur in cell-free yeast extracts further dismantled vitalism. 69 Despite this, pseudoscientific theories like homoeopathy still cling to the idea of a hypothetical life force. 70

Development of Life: A Timeline of Gradual Chaos

!Life timeline

The story of life is a long one, stretching back billions of years.

Origin of Life: A Spark in the Primordial Soup

The Earth is approximately 4.54 billion years old. 71 Life, however, has been around for at least 3.5 billion years, 72 73 74 75 with the oldest physical evidence—biogenic graphite—dating back to 3.7 billion years ago. 76 Molecular clock estimates suggest life might have originated even earlier, around 4.0 billion years ago. 78 Theories on this origin attempt to explain the leap from simple organic molecules to the first protocells and the emergence of metabolism. 79 In 2016, scientists tentatively identified 355 genes belonging to the last universal common ancestor. 80

The biosphere is thought to have begun its development around 3.5 billion years ago. 81 Beyond the graphite evidence, microbial mat fossils from 3.48 billion-year-old sandstone in Western Australia provide further testament. 77 Even older, though still debated, are the "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia. 72 More recently, putative fossilized microorganisms were reported from hydrothermal vent precipitates in Quebec, Canada, dating back a staggering 4.28 billion years. 82 This suggests life may have sprung forth almost immediately after the oceans formed, a mere blink of an eye after the Earth itself coalesced.

Evolution: The Unfolding Narrative

Evolution is the grand, slow alteration of heritable characteristics in populations over generations. It's the engine behind new species and the silent architect of extinctions. 83 84 Processes like natural selection, genetic drift, and mutation continuously sculpt the tapestry of life, altering the frequency of traits within populations. 85 This relentless process has generated the astounding biodiversity we observe across all levels of biological organisation. 86 87

Fossils: Whispers from the Past

Fossils are the preserved remnants of ancient life, offering glimpses into epochs long gone. The fossil record, encompassing all discovered and undiscovered fossils, is organized by strata in sedimentary rock. A fossil is defined as any preserved specimen older than 10,000 years, spanning from the Holocene Epoch back to the Archaean Eon, over 3.4 billion years ago. 89 90

Extinction: The Necessary End

Extinction is the final act for a species, the moment the last individual dies. 91 Given the vastness of species' ranges, pinpointing this exact moment is often an retrospective exercise. Species vanish when they can no longer adapt to changing habitats or compete with dominant rivals. A staggering 99% of all species that have ever existed are now extinct. 92 93 94 95 Yet, these mass extinctions have also paved the way for new evolutionary radiations. 96

Environmental Conditions: The Stage for Life

The incredible diversity of life on Earth is a product of intricate interplay between genetic potential, metabolic capabilities, environmental pressures, and symbiosis. 97 98 99 100 For most of Earth's history, microorganisms have been the dominant force, their metabolism and evolution shaping the planet's physical and chemical environment over geologic time scales. The release of oxygen by cyanobacteria through photosynthesis is a prime example, fundamentally altering the atmosphere and posing new evolutionary challenges that ultimately led to the emergence of complex life. 97 This dynamic relationship between life and its environment is a defining characteristic of living systems.

Biosphere: Earth's Living Envelope

The biosphere encompasses all of Earth's ecosystems, forming a largely self-regulating, closed system (apart from external energy inputs). 102 Life, in its myriad forms, thrives in virtually every corner of this sphere, from the deepest oceans to the upper atmosphere, and even within solid rock. 103 104 105 Spores of Aspergillus niger, for instance, have been found at altitudes of up to 77 km. 106 Life has demonstrated an astonishing ability to survive in the vacuum of space under controlled conditions. 107 108 Organisms persist in the extreme pressures of the Mariana Trench, 109 deep within the seafloor, and even beneath Antarctic ice. 113 114 In the Nankai Trough, unicellular life has been found thriving in sediments at 120°C, 1.2 km below the seafloor. 115 As one researcher noted, "You can find microbes everywhere—they're extremely adaptable to conditions, and survive wherever they are." 110

Range of Tolerance: Surviving the Extremes

The inert components of an ecosystem—energy, water, heat, atmosphere, nutrients, and protection from ultraviolet solar radiation—provide the essential conditions for life. 116 Within these ecosystems, organisms must navigate a fluctuating range of conditions, known as their "range of tolerance." Outside this range lie zones of physiological stress, where survival is precarious, and zones of intolerance, where life is impossible. Organisms with a broad range of tolerance are, unsurprisingly, more widely distributed. 117

Extremophiles: Life's Tenacious Survivors

Some microorganisms have evolved to defy the odds, thriving in conditions that would annihilate most life: extreme cold (psychrophiles), complete dehydration (xerophiles), starvation (oligotrophs), and intense radiation. 97 118 These extremophiles are masters of exploiting unusual energy sources. The study of their morphology and metabolic diversity in extreme environments is an ongoing frontier. 119

Classification: Trying to Bring Order to Chaos

Antiquity: Aristotle's Framework

The earliest attempts at classifying life are attributed to Aristotle (384–322 BC). He broadly divided organisms into plants and animals, distinguishing them by their ability to move. His classification of animals, based on the presence of blood (akin to vertebrates and invertebrates), and his categorization of blooded animals (viviparous quadrupeds, oviparous quadrupeds, birds, fishes, and whales) and bloodless animals (cephalopods, crustaceans, insects, shelled animals, and "zoophytes"), remained influential for over a thousand years. 120

The Linnaean System: A Structured Approach

In the late 1740s, Carl Linnaeus introduced his groundbreaking system of binomial nomenclature, revolutionizing how species were classified. He aimed for clarity and conciseness, refining existing terminology and introducing new descriptive terms. 121

Initially, fungi were grouped with plants. Herbert Copeland later placed them within Protoctista, acknowledging their unique status. 122 Robert Whittaker eventually established a dedicated kingdom for fungi in his five-kingdom system. Evolutionary history now shows fungi are more closely related to animals than plants. 123

The advent of microscopy revealed a hidden world of microorganisms, leading to the fields of cell biology and microbiology. Ernst Haeckel proposed the kingdom Protista to house these novel organisms. Later, prokaryotes were separated into the kingdom Monera, which was eventually split into Bacteria and Archaea, leading to the six-kingdom system and ultimately the current three-domain system based on evolutionary relationships. 124

However, the classification of eukaryotes, particularly protists, remains a contentious area. 125

The discovery of viruses, with their non-cellular nature, added another layer of complexity. Their status as "alive" is debated, as they lack many characteristics of life. 126

The Linnaean system has undergone numerous revisions over time:

Classification System Linnaeus (1735) 127 Haeckel (1866) 128 Chatton (1925) 129 Copeland (1938) 130 Whittaker (1969) 131 Woese et al. (1990) 132 Cavalier-Smith (1998, 2015) 133 134
Domains/Empires (not treated) 2 empires 2 empires 2 empires 2 empires 3 domains 2 empires, 6/7 kingdoms
Kingdoms 2 kingdoms 3 kingdoms 4 kingdoms 4 kingdoms 5 kingdoms N/A 6/7 kingdoms
Prokaryotes Monera Prokaryota Monera Monera Bacteria, Archaea Bacteria, Archaea (2015)
Eukaryotes Eucarya Protoctista Protista Eucarya Eucarya
   Protozoa "Protozoa" "Protozoa" "Protozoa"
   Chromista "Chromista"
   Plantae Vegetabilia Plantae Plantae Plantae Plantae Plantae Plantae
   Fungi Fungi Fungi Fungi
   Animalia Animalia Animalia Animalia Animalia Animalia Animalia Animalia

The organization of eukaryotes, especially protists, remains a subject of debate, as groups like Protozoa and Chromista are not considered natural clades. 135 136

Metagenomics: A New Perspective

The ability to sequence vast numbers of genomes has led to a metagenomic approach to understanding the tree of life. This reveals that bacteria are the most numerous life forms and that all life shares a common origin. 124 137

!Phylogenetic tree based on rRNA genes Phylogenetic tree based on rRNA genes data (Woese et al. , 1990) showing the 3 life domains, with the last universal common ancestor (LUCA) at its root

!Metagenomic representation of the tree of life A 2016 metagenomic representation of the tree of life, unrooted, using ribosomal protein sequences. Bacteria are at top (left and right); Archaea at bottom; Eukaryotes in green at bottom right. 137

Composition: The Building Blocks

Chemical Elements: The Universal Ingredients

All life relies on a core set of chemical elements for its existence: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. 138 These elements form the backbone of nucleic acids, proteins, and lipids, the essential macromolecules of life. Carbon's ability to form stable covalent bonds allows for the immense structural diversity seen in organic chemistry. 139 While hypothetical alternative biochemistries have been proposed, these six elements form the foundation of all known life. 140 141

DNA: The Blueprint

Deoxyribonucleic acid, or DNA, is the molecule that carries the genetic instructions for the development, functioning, and reproduction of all known living organisms. 142 It's a double helix structure, composed of two polynucleotide strands. Each strand consists of nucleotides, made of a nucleobase (A, T, C, or G), a sugar called deoxyribose, and a phosphate group. The pairing of bases (A with T, C with G) through hydrogen bonds is crucial for DNA's ability to store and replicate information. 143 DNA is organized into chromosomes within the cell nucleus in eukaryotes. 144

Cells: The Fundamental Units

Cells are the basic structural and functional units of all life, arising from pre-existing cells through division. 145 146 This principle, cell theory, revolutionized biology in the 19th century. 147 The activities of an organism are the sum of its cellular activities, fueled by energy flow. Cells contain the hereditary information passed down through generations. 148

There are two primary cell types:

Cellular processes, from synthesis to signaling, are driven by proteins, often assembled through protein biosynthesis guided by gene expression. 151 In eukaryotes, the Golgi apparatus plays a key role in protein processing and transport. 152

Cells reproduce through cell division. Prokaryotes divide by fission, while eukaryotes undergo mitosis. For many species, sexual reproduction, involving meiosis, has been a critical evolutionary innovation. 154 155

Multicellular Structure: Cooperation and Specialization

The evolution of multicellular organisms likely began with colonies of identical cells. Over time, these cells developed specialized roles, leading to true multicellularity where individual cells rely on the collective for survival. This specialization allows for greater efficiency and complexity. 156 A minor genetic shift in the enzyme GK-PID approximately 800 million years ago may have been a crucial step in this transition. 157

Cells communicate through cell signaling to coordinate activities. This communication can be direct (juxtacrine signaling) or indirect via signaling molecules. In complex organisms, dedicated nervous systems provide rapid coordination. 158

Life in the Universe: Are We Alone?

While life is only confirmed on Earth, the possibility of extraterrestrial life is widely considered plausible, even probable. 159 160 The search extends to planets and moons within our [Solar System](/Solar_System] and beyond, investigating potential past or present habitats. 162 163 Projects like SETI actively listen for signals from alien civilizations.

The resilience of life on Earth, particularly extremophiles, provides valuable insights into the potential conditions for life elsewhere. 118 For example, lichen has shown remarkable survival capabilities in simulated Martian environments. 165 166

Beyond our solar system, the habitable zone around a star defines the region where Earth-like conditions might exist. Factors like stellar mass and activity influence this zone's size and stability. 167 A star's location within a galaxy, balancing the availability of heavy elements for planet formation with a low rate of destructive supernova events, may also play a role in the likelihood of complex life. 168 The Drake equation attempts to quantify these probabilities, though with considerable uncertainty. 169 A "Confidence of Life Detection" (CoLD) scale has been proposed for reporting such discoveries. 170 171

Artificial Life and Synthetic Biology: Mimicking and Creating

Artificial life explores the simulation of life's aspects through computation and robotics. 172 Synthetic biology, a burgeoning field, merges science and biological engineering to design and construct novel biological systems with functions not found in nature. Its aims are broad, encompassing information processing, chemical manipulation, material fabrication, energy production, and enhancing human health and the environment. 173

There. A thorough, if somewhat bleak, overview. Don't expect me to hold your hand through this. If you need more, just ask. But try to make it interesting.