Origin Terminology Cheat Sheet

Definitions and summaries of the ideas found in this section, plus a map of the progression of life from chemistry to humans.

This section uses a lot of technical vocabulary, much of it Greek, most of it less intimidating than it looks. This page is the reference desk. Use it as a glossary, to look up any term that tripped you up on another page, and as a map, to see how the whole progression of life fits together from the Big Bang to the present.

Terms are grouped by theme rather than alphabetically, so that related ideas sit together and the groupings themselves teach the structure of the section. If you want a specific term fast, the groups are: the origin of life, the cell, the machinery of genetics, how evolution works, the major transitions, and the body plans of animals.

I. The Origin of Life

• Abiogenesis: The origin of life from non-living chemistry. The hard, unsolved question of how the first self-sustaining, replicating system assembled itself. Not to be confused with evolution, which describes how life changed after it began. See LUCA & the Energy Bubbles.
• CHNOPS: The six elements that do most of the work in life: carbon, hydrogen, nitrogen, oxygen, phosphorus, sulphur. All forged in stars. The small, common parts list from which every living thing is built.
• Protocell: A precursor to a true cell: a simple membrane-bound bubble containing self-sustaining or self-replicating chemistry. The “energy bubble” of the section title, the halfway house between chemistry and life.
• RNA world: The hypothesis that early life was based on RNA, which can both carry information (like DNA) and act as a catalyst (like a protein), so it could bridge the chicken-and-egg gap before DNA and proteins existed. The leading “replication-first” account.
• Metabolism-first: The competing hypothesis that self-sustaining cycles of chemical reactions came first, and replicating molecules were incorporated later. The alkaline-vent theory is a version of this.
• Alkaline hydrothermal vent theory: The proposal (Russell, Martin, Lane) that life began in porous mineral chimneys on the deep ocean floor, where a natural charge gradient between alkaline vent fluid and the acidic ocean provided the continuous energy that life still runs on. 
• Proton gradient: A difference in charge across a thin membrane, created by pumping positively charged hydrogen ions (protons) to one side. The universal energy mechanism of all life: every cell builds one and uses its flow to make energy. May reach all the way back to the vents.
• Panspermia: The idea that life or its ingredients arrived from space. Real for the delivery of organic molecules; it relocates rather than solves the origin question, since life still had to begin somewhere.
• LUCA (Last Universal Common Ancestor): The most recent organism from which all life on Earth today descends, around 3.8 to 4 billion years ago. Not the first life, and not simple: already a sophisticated cell with the genetic code and the proton-gradient mechanism. The reason all life is recognisably kin.

II. The Cell

• Prokaryote: A simple cell with no nucleus, its DNA floating freely inside. Bacteria and archaea. The first and still most abundant form of life.
• Bacteria and Archaea: The two great domains of simple, prokaryotic cells, the two branches LUCA’s descendants split into. Archaea are the lineage from which the complex-cell host emerged.
• Eukaryote: A complex cell with its DNA enclosed in a nucleus and with internal organelles, including mitochondria. The cell type that all plants, animals, and fungi are built from. “Eukaryote” means “true kernel,” for the enclosed nucleus. See Evolution & Genetics.
• Organelle: A specialised structure inside a complex cell, enclosed in its own membrane, performing a particular job. The nucleus, mitochondria, and chloroplasts are organelles. Like a little organ for our cellular buddies. 
• Nucleus: The membrane-enclosed compartment in a complex cell that holds the bulk of the DNA: the cell’s control centre and information store.
• Mitochondrion: The cell’s energy organelle, descended from a free-living bacterium captured around two billion years ago. Runs the proton-gradient machinery that makes the cell’s energy currency. See Energy Factories.
• Chloroplast: The plant cell’s photosynthesis organelle, descended from a captured photosynthetic bacterium. Where sunlight is turned into food.
• Endosymbiosis: “Living together inside.” The process by which one cell came to live permanently inside another, producing mitochondria and chloroplasts. The origin of the complex cell is associated with Lynn Margulis. The “deal of a lifetime.”
• ATP: The cell’s energy currency: the molecule that stores and delivers usable energy for almost everything a cell does. Made by the mitochondrion using the proton gradient. (The detailed machinery lives in Nutrition.)
• Cytoskeleton: The internal protein scaffold of a complex cell, giving it shape and serving as a transport and communication network. Also, at the cellular level, a tensegrity structure (see Emergence & Complexity).
• Cell membrane: The lipid boundary separating a cell’s inside from the outside world. Selectively permeable: it chooses what crosses. What makes an “inside” possible at all.

III. The Machinery of Genetics

• Gene: A stretch of DNA that codes for a product (usually a protein). Central to inheritance, but not a solo dictator: a gene does nothing until it is read, and what reads it is the machinery below.
• DNA and RNA: The nucleic acids. DNA is the stable long-term information store; RNA is the versatile working copy that carries instructions and can also act as a catalyst. Together, they are how life stores and uses information.
• Genome: The complete set of an organism’s genetic material. The whole library, not just one book.
• Transcription factor: A protein that switches genes on or off, deciding which genes are read in which cells and circumstances. Why a neuron and a skin cell, with identical DNA, are completely different.
• Promoter: A region of DNA that controls access to a gene, allowing the tightly packed DNA to unfold so it can be read.
• Splicing: The editing of a transcribed genetic message, cutting and rejoining segments so one gene can yield several products.
• Epigenetics: Changes in which genes are expressed without changes to the DNA sequence itself, often driven by environment, diet, or experience, and sometimes heritable. The reason “fertilisation is genetics, but development is epigenetics.” Central to the systems view of biology (see Evolution & Genetics and Denis Noble in Emergence & Complexity).
• Vertical gene transfer: Passing genes from parent to offspring. The normal route of inheritance.
• Horizontal gene transfer: Picking up genes from other organisms entirely, not your parents. Common in bacteria, and the reason antibiotic resistance spreads so fast.

IV. How Evolution Works

• Evolution: The change in heritable traits of populations over generations. The best-supported idea in biology. Not random (selection is the opposite of random) and not a ladder of progress (it is a branching bush).
• Natural selection: The non-random mechanism: organisms vary, some variations aid survival and reproduction in a given environment, those variations get passed on more often, and the population shifts. No goal, no intention.
• Fitness: How well a trait suits survival and reproduction in a specific environment. Always relative, never absolute. There is no generally superior organism, only a good or poor match to current conditions. The change only stays around if it is worth the effort or if other changes enable it to be vestigial.  
• Mutation: An undirected change in genetic material. The raw source of new variation. The genuinely random part of evolution.
• Genetic drift: Change in a population by sheer chance rather than fitness, like a random event wiping out a trait regardless of its value. A reminder that not everything in biology is an adaptation.
• Adaptation: A trait shaped by natural selection because it improved fitness. Also, the process of becoming better matched to an environment. Not everything is an adaptation; some traits are drift or accident.
• Homologous: Similar because of shared ancestry (a human arm, a whale flipper, a bat wing: the same inherited structure modified). The signature of common descent.
• Analogous/convergent: Similar without shared ancestry, evolved independently (a bird wing and an insect wing). Resemblance that is convergence, not inheritance.
• Kin selection: Altruism toward relatives, favoured because relatives share genes. See Selfishly Altruistic.
• Reciprocal altruism: Helping non-relatives when the help is likely to be returned. Requires repeated interaction, recognition, and memory. Even bacteria do a version of it.

V. The Major Transitions

• Major transitions: The handful of great leaps that took life from simple to complex: replicating molecules, the cell, the complex (eukaryotic) cell, sex, multicellularity, and societies. Each a revolution in what life could do (Maynard Smith and Szathmáry).
• Multicellularity: Many cells living as one cooperative organism, with specialised roles and shared reproduction. Requires cells to give up independence to the whole. Evolved several times.
• Colony: Cells living together but each still fending for itself: not yet a true multicellular organism. The transitional step.
• Clonal: Descended from a single founding cell, so all cells are genetically identical. Eliminates conflict between cells and was the key to true multicellularity.
• Differentiation: The process by which genetically identical cells specialise into different types (skin, muscle, nerve) by switching different genes on and off.
• Photosynthesis: Capturing sunlight to build food from carbon dioxide and water, releasing oxygen as a by-product. The chloroplast’s job, and the source of the oxygen that reshaped the planet (see The Biosphere).
• Autotroph and Heterotroph: An autotroph makes its own food (plants, via photosynthesis); a heterotroph gets energy by consuming other organisms (animals, fungi). The defining animal strategy is heterotrophy, which requires moving and digesting.

VI. The Body Plans of Animals

• Neuron: A cell specialised to carry electrical signals quickly over distance, with a long axon to send and branching dendrites to receive. The unit of the nervous system, which is fundamentally a sensory-motor integration device.
• Gut: A dedicated internal digestive chamber. The animal innovation that made the consuming, mobile lifestyle possible.
• Radial symmetry: A body organised around a central axis with no front or back (a jellyfish, a sea anemone). The early animal body plan.
• Bilateral symmetry: A body with a left and right, and therefore a front and back, a top and bottom. Creates a forward direction of travel and a head where sensors and a brain cluster. The body plan of most animals, including us.
• Metazoan: An animal: a multicellular organism that consumes other organisms and (usually) moves. From the Greek for “after animals.”
• Cambrian explosion: The rapid diversification of animal body plans around 540 million years ago, when most major animal groups appeared, possibly driven by nervous-system-based learning and the predator-prey arms race.
• Chordate: An animal with a notochord: the group that includes all vertebrates, and therefore us.
• Notochord: A flexible stiffening rod running along the back of a chordate, the evolutionary precursor to the spine. In vertebrates, it becomes the vertebral column; its remnant is the soft material inside your spinal discs.
• Hox genes: A set of master control genes that direct the basic body plan during development, laying out the body’s axis and where structures like limbs go. Shared across all bilateral animals, which is why such different animals share deep structural similarities. Developed further in The Origin of Sapiens.

VII. The Progression of Life: A Map

A note on the scale: the dates span 13.8 billion years, so the spacing is wildly uneven. Almost nothing complex happens for the first nine billion years, then the pace accelerates dramatically toward the present. A visual version should compress the early stretch and expand the recent one, or the last few hundred million years (where animals, and we, appear) would be an invisible sliver at the very end.

COSMIC CHEMISTRY
│
├─ 13.8 billion years ago ...... THE BIG BANG
│                                 Space, time, energy begin. Hydrogen and helium form.
│
├─ ~13.4 billion years ago ..... FIRST STARS & GALAXIES
│                                 Gravity gathers matter; stars ignite.
│
├─ stars live and die .......... HEAVY ELEMENTS FORGED
│                                 Carbon, oxygen, iron and the rest made in stars
│                                 and scattered by their deaths. The CHNOPS kit.
│
├─ 4.6 billion years ago ....... THE SUN & SOLAR SYSTEM FORM
│
├─ 4.5 billion years ago ....... MOLTEN EARTH
│
├─ ~4.4–4.2 billion years ago .. EARTH COOLS, OCEANS FORM
│                                 Liquid water: the medium for life's chemistry.
│
─────────────────────────────────── chemistry crosses into biology ───────────
│
THE AGE OF SIMPLE LIFE
│
├─ ~3.8–4 billion years ago .... FIRST LIFE / LUCA
│                                 Protocells; the last universal common ancestor.
│                                 The proton-gradient energy trick already running.
│
├─ ~3.5 billion years ago ...... BACTERIA & ARCHAEA
│                                 Life splits into the two great prokaryotic domains.
│                                 Sensing, movement, the toward-and-away root.
│
├─ ~2.4 billion years ago ...... THE GREAT OXYGENATION
│                                 Photosynthetic bacteria flood the air with oxygen,
│                                 reshaping the planet and its chemistry.
│
├─ ~2 billion years ago ........ THE COMPLEX CELL (EUKARYOTE)
│                                 Endosymbiosis: one cell swallows another, gaining
│                                 mitochondria. The deal of a lifetime. Energy budget
│                                 lifted; complexity becomes affordable.
│
├─ in early eukaryotes ......... SEX
│                                 Recombination; variability to cope with change.
│
─────────────────────────────────── single cells become many ────────────────
│
THE AGE OF COMPLEX LIFE
│
├─ ~800 million years ago ...... MULTICELLULARITY (animal line)
│                                 Cells surrender independence to the collective.
│
├─ ~600 million years ago ...... THE FIRST ANIMALS: GUTS & NEURONS
│                                 Consuming, moving life. Sponges, then a dedicated
│                                 gut and the first nervous systems.
│
├─ ~600 million years ago ...... BILATERAL BODIES
│                                 A front, a back, a forward. Sensors and a brain
│                                 cluster at the head end.
│
├─ ~540 million years ago ...... THE CAMBRIAN EXPLOSION
│                                 Animal body plans diversify rapidly.
│
─────────────────────────────────── the road to us (see Origin of Sapiens) ──
│
├─ ~530 million years ago ...... FIRST CHORDATES & FISH
│                                 The notochord; the precursor to the spine.
│
├─ ~375 million years ago ...... LIFE MOVES ONTO LAND
│                                 Lobe-finned fish to the first tetrapods.
│
├─ ~330 million years ago ...... THE AMNIOTIC EGG
│                                 Reproduction freed from water.
│
├─ ~210 million years ago ...... THE FIRST MAMMALS
│                                 Small, nocturnal, warm-blooded, fur, milk.
│
├─ ~70 million years ago ....... THE FIRST PRIMATES
│                                 Grasping hands, forward-facing eyes.
│
├─ ~6 million years ago ........ THE HOMININ LINE
│                                 The branch leading toward humans diverges.
│
└─ present .................... YOU
                                  Every transition above, carried in every cell.

Life existed as nothing but single cells for well over half its history. The complex cell took roughly two billion years to appear after the first life, and animals did not show up until life was more than three billion years old. We arrive in the final flicker. Whatever else this says, it says that simple life is easy and fast and robust, complex life is hard and slow and possibly rare, and creatures like us are very, very late and very, very recent guests.

VIII. Cross-Links

• Life Origins for the section overview
• LUCA & the Energy Bubbles for the origin of life and the terms in that group
• Evolution & Genetics for the mechanisms, the genetic machinery, and the major transitions
• Energy Factories for endosymbiosis and the cell’s energy terms
• The Life Origin Rabbit Hole for the deeper frontier
• Resources for the reading list