3.8 billion years of life's journey — from single cells to human civilization
Darwin's core mechanism. Four conditions must hold:
1. Variation: Individuals differ in traits.
2. Heritability: Traits are passed to offspring.
3. Differential fitness: Some variants survive/reproduce better.
4. Time: This process repeats over generations.
Result: Allele frequencies in a population change over time. Adaptive traits spread.
Random changes in allele frequency due to chance, not selection. Most powerful in small populations.
Bottleneck effect: Population crashes → survivors carry only a fraction of original diversity. All cheetahs are nearly genetically identical — bottleneck ~10,000 years ago.
Founder effect: Small group colonizes new habitat → limited starting gene pool. Amish have high rates of rare genetic diseases.
Ultimate source of all genetic variation. Random changes to DNA sequence. Most are neutral; some are deleterious; rarely, beneficial.
Point mutations, insertions, deletions, gene duplications, chromosomal rearrangements.
Gene duplication is a key driver of evolution — one copy maintains function while the other can evolve new roles. Explains large multigene families (opsins, hemoglobins).
Movement of alleles between populations via migration. Homogenizes populations — reduces genetic differences. Prevents speciation when high.
When gene flow stops (geographic isolation), populations diverge independently → speciation.
Neanderthal DNA is present in modern non-African humans (1–4%): evidence of gene flow between Homo sapiens and Neanderthals ~50,000 years ago.
Allopatric: Geographic barrier separates populations → independent evolution → reproductive isolation. Most common. Galapagos finches, Hawaiian honeycreepers.
Sympatric: New species arise in same geographic area. Polyploidy in plants — doubled chromosomes create instant reproductive isolation. ~80% of flowering plants have polyploid ancestors.
Biological species concept: Groups that can interbreed and produce fertile offspring.
Selection based on mating success, not just survival. Two forms:
Intersexual: Mate choice. Peahen preference for peacock tails drives extreme trait elaboration. Runaway selection (Fisher's process).
Intrasexual: Competition between same sex. Deer antlers, elephant seal battles. Arms races can produce costly traits.
Can produce traits that reduce survival but increase reproduction — evolutionary "expenditures."
Billions of fossils show progression from simple to complex life. Transitional forms (Tiktaalik, Archaeopteryx) document key transitions.
Homologous structures (human arm, bat wing, whale flipper — same bones, different function). Vestigial structures (human coccyx, whale leg bones).
DNA similarity matches phylogenetic trees from morphology. Human-chimp: 98.7% DNA similarity. Shared "fossil genes" (broken vitamin C gene in humans and other primates).
Species distributions match continental drift. Marsupials concentrated in Australia and South America. Island species closely related to nearest mainland species, not distant similar environments.
Antibiotic resistance. Pesticide resistance. Industrial melanism (peppered moths). HIV evolution in a patient's body. Darwin's finch beak changes after drought years. Evolution is not a theory — it is observed fact.
All vertebrate embryos have gill slits and tails at early stages — retained ancestral developmental programs. Human embryos briefly have pharyngeal arches that become jaw bones and inner ear bones.
Darwin sailed as ship's naturalist on HMS Beagle's survey voyage of South America and the Pacific. He was 22 years old.
Key stops: Cape Verde, Brazil, Patagonia, Tierra del Fuego, Galapagos Islands, Australia, Cape of Good Hope. He collected thousands of specimens and made detailed observations.
~500 miles off Ecuador. 13 major islands. Each island has slightly different finch species, adapted to different food sources (seeds, insects, cactus). Same species of giant tortoise differs between islands.
Darwin initially didn't realize these were separate species. It was back in England, when ornithologist John Gould identified them, that the significance became clear.
Darwin conceived natural selection in 1838, influenced by Malthus's Essay on Population (competition for limited resources).
He spent 20 years gathering evidence and refining the theory. On the Origin of Species (1859) — 1,250 copies sold out the first day.
Alfred Russel Wallace independently conceived natural selection in 1858, prompting Darwin to publish.
Full title: "On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life."
Key ideas: common descent of all life, branching tree of life, natural selection as the primary mechanism, gradual change accumulating over vast time scales.
Darwin avoided discussing human evolution — addressed only obliquely ("light will be thrown on the origin of man").
Darwin had no knowledge of genes, DNA, or Mendel's work (which was published the same decade but ignored). He didn't know the mechanism of inheritance — the "blending" hypothesis was wrong but Darwin had no alternative.
The Modern Evolutionary Synthesis (1930s-50s) merged Darwin's natural selection with Mendelian genetics and population genetics into the unified theory of evolution we use today.
Theodosius Dobzhansky (1973): "Nothing in biology makes sense except in the light of evolution." — still true today.
Evolution is the unifying theory of all biology. It informs medicine (antibiotic resistance, cancer evolution, pandemic preparedness), agriculture (crop and livestock breeding), ecology, and psychology.
Fossilization requires rapid burial (usually in sediment), oxygen-poor conditions to prevent decomposition, and minerals replacing organic material over millions of years.
<0.1% of organisms that ever lived have been fossilized. Soft-bodied organisms are rarely preserved. The fossil record is incomplete but enormously informative.
| Fossil | Transition | Age |
|---|---|---|
| Tiktaalik | Fish → tetrapod | 375 Ma |
| Archaeopteryx | Dinosaur → bird | 150 Ma |
| Pakicetus | Land mammal → whale | 50 Ma |
| Australopithecus | Ape → Homo | 3.9 Ma |
| Acanthostega | Lobe-fin → tetrapod | 360 Ma |
| Eon | Time | Major Events |
|---|---|---|
| Hadean | 4.6–4.0 Ga | Earth formation, moon impact |
| Archean | 4.0–2.5 Ga | First life, stromatolites |
| Proterozoic | 2.5–0.54 Ga | O₂ atmosphere, eukaryotes |
| Paleozoic | 541–252 Ma | Cambrian explosion, land life, P-T extinction |
| Mesozoic | 252–66 Ma | Dinosaurs, first mammals, K-Pg extinction |
| Cenozoic | 66 Ma–now | Mammals dominate, primates, Homo |
We are currently in a Sixth Mass Extinction — driven by humans. Current extinction rate is 1,000–10,000× the background rate. Estimated 1 million species threatened with extinction.
| Species | Age | Key Feature |
|---|---|---|
| Sahelanthropus tchadensis | 7 Ma | Earliest possible hominin; upright posture? |
| Ardipithecus ramidus | 4.4 Ma | Bipedal but with opposable big toe |
| Australopithecus afarensis | 3.9–2.9 Ma | "Lucy" — clear biped; ape-sized brain |
| Homo habilis | 2.8–1.5 Ma | First Homo; stone tools; larger brain |
| Homo erectus | 1.9 Ma–143 ka | Fire, hand axes; left Africa; longest-lived Homo |
| Homo heidelbergensis | 700–200 ka | Large brain; ancestor of Neanderthals and sapiens |
| Homo neanderthalensis | 400–40 ka | Europe/Asia; tools, art, burials; 1-4% of our DNA |
| Homo sapiens | 300 ka–present | Symbolic thought, language, technology |
Bipedalism (6–7 Ma): Frees hands for tool use. But slower, more energy demanding than knuckle-walking. Why? Carrying food? Thermoregulation?
Brain expansion: Homo habilis 650 cc → Homo erectus 900 cc → Homo sapiens 1,350 cc. Metabolically expensive (20% of our energy). Enabled by cooked food (Wrangham hypothesis) and social complexity.
Language: FOXP2 gene. Recursion — sentences within sentences. Allows sharing of past, future, hypothetical, and abstract concepts.
Modern humans evolved in Africa ~300,000 years ago. Earliest migrations to Asia ~70,000 years ago. Europe ~45,000 years ago. Americas ~20,000 years ago (across Bering land bridge). Australia colonized ~65,000 years ago.
Mitochondrial "Eve" — all living humans trace maternal lineage to one African woman ~200,000 years ago. Y-chromosome "Adam" ~270,000 years ago. Not the only humans alive then — just ancestors of all who survived.
Lived in Europe and Western Asia 400,000–40,000 years ago. Brain as large as ours. Made tools, art, ornaments. Buried their dead. Used medicinal plants. Cared for injured individuals.
We interbred with them. Non-African humans carry 1–4% Neanderthal DNA. Some of their genes affect our immune system and are beneficial — others increase risk of depression, blood clotting.
Denisovans — another archaic human, known from Siberian DNA — contributed ~4% of Melanesian and Aboriginal Australian genomes.
Cumulative culture: Knowledge builds across generations. Each generation starts where the last left off. No other animal does this to our degree.
Cooperative breeding: Alloparenting — multiple adults care for children. Enables longer childhood and brain development.
Theory of mind: Understanding that others have beliefs and intentions. Foundation of deception, empathy, teaching, and complex social life.
Fire: Unique human technology. Cooking predigests food — more calories from same food, smaller gut, more energy for brain.
Natural selection is ongoing but weakened in developed nations (modern medicine reduces differential mortality). Sexual selection continues.
Cultural evolution now far outpaces genetic evolution. The genome changes over generations; culture changes in years or decades.
CRISPR and gene editing give humans intentional control over their own genome — a profound threshold in evolutionary history: directed evolution.
All life on Earth shares a single common ancestor (LUCA — Last Universal Common Ancestor) ~3.8 billion years ago. Evidence: universal genetic code, ATP as energy currency, DNA-based information storage, same 20 amino acids.
The study of evolutionary relationships. Cladograms show branching points (nodes = common ancestors). Clades are groups including an ancestor and all descendants.
Molecular phylogenetics uses DNA sequences. The more similar the DNA, the more recently they shared a common ancestor. Has revolutionized classification — overturned many morphology-based trees.
| Split | Approx. Time |
|---|---|
| Bacteria / Archaea+Eukaryota | ~3.8 Ga |
| Archaea / Eukaryota | ~2.1 Ga |
| Animals / Fungi | ~1.5 Ga |
| Vertebrates / Invertebrates | ~525 Ma |
| Tetrapods / Fish | ~375 Ma |
| Mammals / Reptiles | ~320 Ma |
| Primates / Other mammals | ~85 Ma |
| Apes / Monkeys | ~30 Ma |
| Humans / Chimps | ~6 Ma |
Bacteria and archaea exchange genes directly — not just parent to offspring. This "web of life" supplements the tree metaphor for prokaryotes.
~8% of the human genome is derived from ancient viral infections (endogenous retroviruses). Some of these viral genes are now essential — syncytin genes (from retroviruses) are required for placenta formation in mammals.
Similar traits evolving independently in unrelated lineages, driven by similar selection pressures.
Eyes: evolved independently at least 40 times. Camera-like eyes in vertebrates and octopuses (different embryological origins, different optical architectures).
Wings: insects, pterosaurs, birds, bats — all independent. Echolocation: bats, dolphins, some shrews — independent. Flight, sabre teeth, streamlined aquatic body — all convergent.
Species evolve in response to each other. Predator-prey arms races: cheetah speed vs gazelle speed. Parasite-host: malaria and sickle cell anemia (heterozygote advantage in malaria-endemic areas).
Mutualistic co-evolution: Flowers and pollinators. Orchid mantises mimic flowers. Fig trees and fig wasps — each depends entirely on the other, locked in 80 million years of mutual evolution.