Human Evolution and Theories of Organic Evolution: Hominization, Lamarck and Darwin

1.4 Human Evolution and emergence of Man: Biological and Cultural factors in human evolution. Theories of Organic Evolution (Pre-Darwinian, Darwinian and Post-Darwinian). Synthetic theory of evolution; Brief outline of terms and concepts of evolutionary biology (Doll’s rule, Cope’s rule, Gause’s rule, parallelism, convergence, adaptive radiation, and mosaic evolution).

Official Syllabus: Paper I, Unit 1.4

This article covers the biological and cultural factors in human evolution and the pre-Darwinian, Darwinian and post-Darwinian theories. The synthetic theory and the terms of evolutionary biology are in Unit 1.4b.

1. Human Evolution and the Process of Hominization

At Laetoli in Tanzania, volcanic ash preserved a trail of footprints about 3.66 million years old. The prints show a raised arch, a big toe in line with the others and a heel strike: their makers walked upright like us. Yet those makers, almost certainly Australopithecus afarensis, had a brain the size of a chimpanzee’s (about 400–500 cc). One trail shows the central fact of human evolution: the human body was assembled part by part, at different times.

• Human evolution is the history of the human lineage after it separated from the chimpanzee line, about 6–7 million years ago (mya) on fossil and molecular evidence. The oldest claimed member, Sahelanthropus from Chad (about 7 mya), is still debated.
• Hominization is the set of biological and cultural changes that turned an ape-like ancestor into Homo sapiens: upright walking, a skilful hand, a large reorganised brain, a smaller face and teeth, speech, a long childhood and dependence on culture.
• Why “process” matters: hominization was not a ladder with a goal but many branching species, several living at once; only one survives.
• Evidence: fossils and artefacts, and living and ancient DNA, which dates splits and detects interbreeding.

1.1 Major trends in hominization

The trends did not appear together. The table lists them in the order the fossil record shows them, and that order is itself the main lesson.

Trend	What changed	Earliest clear evidence	Why it mattered
Bipedalism	Upright posture, two-legged walking	Ardipithecus 4.4 mya; Laetoli 3.66 mya	Freed the hands; reshaped spine, pelvis, legs, foot
Tool-making hand	Long, fully opposable thumb; precision grip	Lomekwi tools 3.3 mya; Oldowan 2.6 mya	Made planned tool-making possible
Facial reduction	Smaller canines, no diastema, parabolic arch, flatter face	Australopithecines onward; chin only in H. sapiens	Tools and cooking took over the jaws’ work
Encephalization	Brain grows faster than body; cortex reorganised	Early Homo, about 2 mya	Planning, language, social intelligence
Long life history	Slow growth, long childhood, helpless newborn	Homo erectus onward	Time to learn culture
Speech and symbol	Lowered larynx, language areas, art and ornament	Clear symbols from about 100,000 years ago	Culture becomes cumulative

Read down the third column: feet before brains. Upright walking appears millions of years before the brain expands, and symbolic culture comes last. This uneven pace is mosaic evolution (different parts evolving at different rates; Unit 1.4b). It also corrects the old assumption, behind the acceptance of the faked Piltdown skull (exposed 1953), that a big brain came first. Older textbooks also placed Ramapithecus, jaws from the Siwalik Hills of India and Pakistan (about 12–8 mya), at the base of the human line; fuller finds in the 1980s showed it belongs with Sivapithecus, an ancestor of the orangutan (Unit 1.5).

1.2 Why Africa is central

• Every early stage is African: the earliest hominins (Sahelanthropus, Chad; Orrorin, Kenya; Ardipithecus, Ethiopia), all australopithecines, the earliest Homo (Ledi-Geraru jaw, Ethiopia, about 2.8 mya), the earliest tools (Lomekwi, 3.3 mya) and the earliest Homo sapiens (Jebel Irhoud, Morocco, about 315,000 years ago).
• Genes agree: Africans carry the greatest genetic diversity, and the mitochondrial DNA of all living humans traces to an African ancestor (Rebecca Cann, Mark Stoneking and Allan Wilson, American geneticists, 1987). Diversity is highest where a species has lived longest.
• Darwin predicted it (The Descent of Man, 1871), because chimpanzees and gorillas live in Africa.
• But Asia is not a mere receiver. Hominins and tools reached Dmanisi, Georgia (about 1.8 mya) and Shangchen, China (about 2.1 mya). In India, Attirampakkam (Tamil Nadu) has Acheulean tools about 1.5 million years old (Shanti Pappu and colleagues, 2011). Asia held Denisovans and Homo floresiensis, and modern humans interbred with Neanderthals and Denisovans after leaving Africa.
• Balanced view: Africa is the cradle; Eurasia was a theatre where lineages evolved, met and mixed (fossils in Unit 1.6).

2. Biological Factors in Human Evolution

A human newborn’s head barely passes through its mother’s pelvis; the baby must rotate during birth, and most mothers are helped by others, which is rare among primates. Two defining traits pull in opposite directions: upright walking favours a narrow pelvis, a large brain needs a wide birth canal. The American biological anthropologist Sherwood Washburn called this the obstetrical dilemma (1960). The compromise was a baby born with a brain only about 30 per cent of adult size, so most brain growth happens after birth, in a cultural world. One example ties bipedalism, brain and culture together.

2.1 Erect posture and bipedal locomotion

Bipedalism (habitual walking on two legs) is the earliest hominin trait, and it reshaped almost every bone.

• Skull: the foramen magnum (the hole for the spinal cord) moved forward and down to the centre of the skull base, so the head balances on the spine. Apes have it further back and need heavy neck muscles.
• Spine: a C-shaped curve became an S-shaped curve with a forward bend in the lower back (lumbar lordosis), placing the body’s weight over the hips. Humans usually have five lumbar vertebrae, great apes three or four.
• Pelvis: the ilium (upper blade of the hip bone) became short, broad and bowl-shaped. Two buttock muscles moved to the side, where they stop the pelvis tilting when one foot leaves the ground.
• Leg: the thigh bone (femur) slopes inward to the knee (the valgus or bicondylar angle), bringing the feet under the centre of gravity; legs became longer than arms.
• Foot: a big toe in line with the others, two arches acting as springs, and a large heel.

Why did it evolve? The main hypotheses (treated fully in Unit 1.5) are freeing the hands for carrying and tools (Darwin, 1871), energy efficiency over knuckle-walking (Peter Rodman and Henry McHenry, 1980), keeping cool by exposing less of the body to the midday sun (Peter Wheeler, 1991), provisioning of mates and young (C.O. Lovejoy, 1981) and endurance running (Dennis Bramble and Daniel Lieberman, 2004). They are not exclusive: early bipedalism was probably about efficient walking between woodland patches, while carrying and running mattered later, in Homo erectus.

2.2 The hand and the precision grip

• Hominins added a long, strong thumb that can be fully opposed to every finger tip.
• The British primatologist John Napier distinguished the power grip (whole hand round an object, as with a hammer) from the precision grip (a small object between thumb and finger tips, as with a needle) (1956). Only humans combine both fully.
• Why it mattered: making a stone tool needs both grips at once, one hand holding the core and the other striking at a controlled angle. A precise hand, a brain that plans the blows, and hands freed by bipedalism made the first tools possible.

2.3 Expansion and reorganisation of the brain

Encephalization is brain growth beyond what body size predicts. Brain size roughly tripled in two million years:

Hominin	Approx. brain size (cc)
Chimpanzee (comparison)	350–400
Australopithecus afarensis	400–500
Homo habilis	about 600
Homo erectus	750–1,250
Neanderthals	about 1,450
Homo sapiens	about 1,350

Neanderthals had larger brains than ours, so organisation, not volume alone, explains human abilities.

• Association areas: growth was mainly in the cerebral cortex. Around each sensory area lie association areas, which match new input with memory and make recognition possible. In humans, large fibre bundles link association areas directly (the basis of thinking) and connect them to the areas that control the hand (the basis of skilled tool use).
• Language areas: Broca’s area (frontal lobe; produces speech) and Wernicke’s area (temporal lobe; understands speech), joined by the arcuate fasciculus, plus the angular gyrus, which links sight, hearing and touch to words. Being in the cortex, they let humans speak without passing through the older emotional centres.
• The cost: the brain is about 2 per cent of body weight but uses about 20 per cent of resting energy. The expensive-tissue hypothesis (Leslie Aiello and Peter Wheeler, British anthropologists, 1995) argues that the gut shrank as the brain grew, which needed high-quality food: meat, marrow and later cooked food.
• The social brain hypothesis (Robin Dunbar, British evolutionary psychologist, 1998): primates in larger groups have larger neocortices, so the brain grew mainly to manage complex social life.

2.4 Reduction of the face, jaws and teeth

The craniofacial changes form a clear sequence.

Feature	Earlier hominins	Modern humans
Prognathism (projecting jaws)	Marked	Reduced; face tucked under the brain case
Canines and diastema	Large canines; gap in the opposite jaw	Small canines; no gap
Dental arcade	U-shaped	Parabolic
Brow ridges, forehead	Heavy ridges; low sloping forehead	Small ridges; vertical forehead
Chin	Absent, even in Neanderthals	Present: unique to H. sapiens
Sagittal crest	Present in robust forms	Absent

Three causes worked together. A larger brain under the obstetric limit made the snout shrink to make room. Tools and cooking took over cutting and softening food, so the mouth stopped being a “fifth hand” and big jaws were no longer favoured. A shorter face helped speech.

2.5 Speech and a long childhood

• Vocal tract: the larynx sits low in the throat, so throat and mouth form a two-part sound chamber that produces the full range of vowels (at the cost of a higher risk of choking).
• The hyoid bone of a Neanderthal from Kebara, Israel (about 60,000 years ago) is almost identical to ours; the speech-related gene FOXP2 has a human-specific form that Neanderthals shared. Language has deep roots.
• Slow growth: helpless infancy and a long childhood. Washburn explained the newborn’s helplessness (secondary altriciality) by the obstetrical dilemma; the metabolic hypothesis (Holly Dunsworth and colleagues, 2012) adds that birth occurs when the foetus’s energy needs exceed what the mother can supply.
• Why it matters: a long childhood is the period of learning culture, and it needs helpers. The grandmother hypothesis (Kristen Hawkes, 1998) argues that older women feeding grandchildren favoured long life after menopause.

3. Cultural Factors and the Biocultural Feedback

In north-western India, many people of herding and dairying communities digest milk all their lives, while in much of southern and north-eastern India most adults cannot. This ability, lactase persistence, depends on a variant (−13910*T) that keeps the milk-digesting enzyme working after childhood. Irene Gallego Romero and colleagues (2012) found the variant common where cattle were herded and milked, reaching levels that could make up to about 73 per cent of adults lactase-persistent in some north Indian populations, and rare or absent in many southern and north-eastern groups. Dairying made the gene useful, and the gene spread: culture changed biology.

3.1 Tools: “man makes himself”

• The sequence: the earliest tools are from Lomekwi 3, Kenya (3.3 mya), older than Homo. The Oldowan (simple flakes and choppers) begins about 2.6 mya; the Acheulean (symmetrical hand-axes) about 1.76 mya in Kenya and 1.5 mya at Attirampakkam.
• Tools shaped the body: Washburn argued (1960) that the success of even the simplest tools started the trend of human evolution: once tools were used, selection favoured better hands, larger brains and smaller teeth. This reverses the older order in which the brain came first and then invented tools.
• “Man makes himself”: the archaeologist V. Gordon Childe used this as the title of his book (Man Makes Himself, 1936): by creating their own means of living (tools, farming, cities), humans created the conditions of their own development.
• Tools need foresight: a tool helps only if it is carried until needed, which requires planning and free hands. The British palaeontologist Kenneth Oakley called humans “man the tool-maker” (1949) because making tools to a set pattern for future use requires abstract thought.

3.2 Fire and cooking

• Evidence: burnt bone and ash at Wonderwerk Cave, South Africa (about 1 mya), and repeated hearths at Gesher Benot Ya’aqov, Israel (about 790,000 years ago).
• The cooking hypothesis of the British primatologist Richard Wrangham (Catching Fire, 2009): cooking made food softer and more digestible, giving more energy from the same food. This explains the smaller teeth and gut of H. erectus and helped pay for a larger brain.
• Fire also gave warmth, protection and a hearth for sharing and talking after dark.

3.3 Hunting, sharing, language and social life

• Man the Hunter (Richard Lee and Irven DeVore, eds., 1968) argued that cooperative hunting drove intelligence, cooperation and the sexual division of labour.
• The feminist critique “Woman the Gatherer” (Sally Slocum, 1975) showed that among foragers such as the !Kung San of the Kalahari, gathered plant food supplies most calories, and that sharing and child care mattered as much as hunting. The current view stresses food-sharing at a home base, which needs trust, memory and communication.
• Language made culture cumulative: knowledge is taught and improved across generations instead of being rediscovered. Symbols appear from about 100,000 years ago (engraved ochre and shell beads, Blombos Cave, South Africa); in India, ostrich-eggshell beads come from Patne (Maharashtra), about 25,000 years ago.

3.4 Gene-culture co-evolution

Gene-culture co-evolution (or dual inheritance theory) holds that humans inherit genes and culture, and that each changes the other. Its main formulations came from Luigi Luca Cavalli-Sforza and Marcus Feldman (1981), Robert Boyd and Peter Richerson (Culture and the Evolutionary Process, 1985) and William Durham (Coevolution, 1991). Niche construction (John Odling-Smee, Kevin Laland and Feldman, 2003) adds that organisms change their own environment and so the selection acting on them.

Every case below follows one mechanism: a cultural practice changes the environment, and the new environment changes which genes succeed.

Cultural practice	Environmental change	Genetic response	Population and study
Dairying	Milk available to adults	Lactase persistence spreads	North-west India, Europe, East Africa (Gallego Romero, 2012)
Forest clearing for yam farming	Standing water breeds malaria mosquitoes	Sickle-cell gene protects carriers	West Africa (Frank Livingstone, 1958); central Indian tribal belts
Starchy diets	More starch to digest	More copies of the amylase gene	Farming peoples (George Perry, 2007)
Breath-hold diving	Long dives for food	Larger spleens	Bajau sea nomads, Indonesia (Melissa Ilardo, 2018)
Mortuary cannibalism	Spread of the prion disease kuru	Protective prion-gene variant	Fore, Papua New Guinea (Simon Mead, 2009)
Caste and community endogamy	Small closed breeding groups	Founder effects; more recessive disease	Many Indian groups (Nathan Nakatsuka, 2017)

In every row culture comes first and the genetic change follows. The sickle-cell gene protects carriers against malaria but causes disease in those with two copies, which is why it is common in malarial regions and why India’s National Sickle Cell Anaemia Elimination Mission (2023) concentrates on tribal districts. In the endogamy row, a social rule studied by social anthropologists has measurable genetic effects: the two branches need each other.

3.5 Biological and cultural evolution compared

Human evolution has two strands that work differently. The table shows why cultural change now outpaces biological change.

Basis	Biological evolution	Cultural evolution
What is inherited	Genes (DNA)	Learned ideas, skills, values, artefacts
Route	Parents to offspring only	Parents, peers, teachers, media
Novelty from	Random mutation, recombination	Invention, often purposeful
Acquired traits passed on?	No	Yes (“Lamarckian”)
Speed	Slow: many generations	Fast: within a lifetime
Pattern	Branching only	Branches and merges (borrowing)
Reversibility	Practically irreversible	Traits abandoned or revived

Because culture passes on what is learned, it responds to a problem in years, while genes need generations. That is why sociocultural evolution is not parallel to biological evolution: the body has changed little in 40,000 years, while societies moved from foraging bands to industrial states. The American anthropologist Alfred Kroeber called culture “superorganic” (1917), a level with its own laws. The Russian-American geneticist Theodosius Dobzhansky took the middle path (Mankind Evolving, 1962): culture is the most powerful adaptation biology has produced, yet it has not stopped biological evolution (Unit 1.4b shows human genes are still changing).

3.6 The bio-cultural approach in biological anthropology

Biological anthropology treats human biology and culture as one system; this bio-cultural approach is its defining feature.

• Adaptability perspective: the Human Adaptability section of the International Biological Programme (1964–74) studied how populations cope with altitude, cold and heat. The American biological anthropologist Gabriel Lasker (1969) showed humans adapt at several levels: genetic (natural selection), developmental (during growth), short-term acclimatisation, and cultural (clothing, housing, technology) (Unit 9.7).
• Political-economy perspective: Alan Goodman and Thomas Leatherman (Building a New Biocultural Synthesis, 1998) argued that the environment humans adapt to is largely made by society: poverty, unequal land, wage labour, discrimination. Inequality shows up in the body as stunting, anaemia and disease.
• Integration: the newer synthesis keeps the adaptability and ecological tools (variation as a fit to environment) but asks who is exposed to which stress, and why. In India, NFHS-5 (2019–21) recorded stunting in 35.5 per cent of children under five overall, but 40.9 per cent among Scheduled Tribe and 39.2 per cent among Scheduled Caste children. An ecological reading looks for adaptation; the political-economy reading explains the gap by unequal access to land, food, sanitation and care.

4. Organic Evolution: Meaning and Evidence

Every human has a coccyx (tailbone) of fused tail vertebrae, a small appendix, and ear muscles most people cannot use. These parts make no sense in a species designed from scratch, but perfect sense if humans descend from tailed, ear-moving ancestors. Such vestigial organs (reduced remnants of organs that worked in ancestors) are one of several independent lines of evidence for evolution.

• Meaning: the English philosopher Herbert Spencer popularised “evolution” for the development of the universe, life and society (1857). Darwin preferred “descent with modification“: new species descend from ancestral ones and are modified along the way.
• Organic evolution is the change in heritable characteristics of populations over generations, leading to new species from earlier ones. Change in societies is cultural evolution.
• Ontogeny and phylogeny (both coined by the German zoologist Ernst Haeckel, 1866): ontogeny is the development of one individual; phylogeny the evolutionary history of a group of species. Evolution concerns phylogeny.

The evidence comes from independent sciences, which is why the conclusion is secure.

Line of evidence	What it shows	Example
Fossils	Ordered sequences, transitional forms	Archaeopteryx (reptile-bird); horse and hominin series
Comparative anatomy	Homologous organs: same plan, different use	Human arm, bat wing, whale flipper
Vestigial organs	Remnants of ancestral organs	Coccyx, appendix, ear muscles
Embryology	Related species look alike as embryos	Human embryo has pharyngeal arches and a tail
Biogeography	Species resemble those of the nearest mainland	Galápagos finches; Australia’s marsupials
Molecular biology	Shared code; similarity tracks relationship	Humans and chimpanzees differ at about 1.2 per cent of DNA letters
Direct observation	Evolution happening now	Finch beaks after the 1977 drought; antibiotic resistance

Molecular evidence is strong because it is independent of anatomy: humans share with other monkeys and apes a broken vitamin-C gene (GULO) with the same defects, best explained by a common ancestor in which it first broke.

5. Pre-Darwinian Theories

For centuries, women in many Indian communities have had their ears and noses pierced, and Jewish and Muslim boys have been circumcised. Yet no baby is born with pierced ears or without a foreskin. If changes made during life were inherited, these would long since have become inborn. This everyday fact is the simplest refutation of the best-known pre-Darwinian theory, the inheritance of acquired characters.

5.1 From fixity of species to the idea of change

The period moved from belief in fixed, separately created species towards the idea that species change over a very long time.

Idea	Thinker (date)	Core claim	Significance or weakness
Spontaneous generation	Aristotle; Greek tradition	Life arises from non-living matter	Disproved by Redi (1668) and Pasteur (1859–62)
Great chain of being	Aristotle; Augustine, Aquinas	All beings in a fixed ladder up to God	A ladder, not a tree
Special creation	Biblical tradition; Francisco Suárez	God created each species separately	Fixity of species
Classification	Linnaeus, Systema Naturae (1735)	Fixed species, binomial names	Nested groups later read as common descent
Preformation	Malpighi; Hartsoeker’s homunculus (1694); Bonnet’s encasement	A miniature adult in egg or sperm	Disproved by embryology
Epigenesis	C.F. Wolff (1759)	Organs form gradually	Correct; basis of embryology
Mutability	Buffon, Histoire naturelle (from 1749)	Species change with environment	First serious challenge to fixity
Common origin	Erasmus Darwin, Zoonomia (1794–96)	All life from one source	Charles Darwin’s grandfather; no mechanism
Catastrophism	Cuvier (early 1800s)	Extinction is real; repeated catastrophes	Founded palaeontology; opposed evolution
Uniformitarianism	Hutton (1788); Lyell (1830–33)	Slow present processes over vast time	Gave Darwin deep time
Population pressure	Malthus (1798)	Population outgrows food	Gave Darwin the struggle for existence

The last three rows matter most. Cuvier proved that species go extinct, Lyell that the earth is very old, and Malthus that more are born than can survive. Darwin combined all three. Lamarck, meanwhile, offered the first complete theory of how species change.

5.2 Lamarckism: the inheritance of acquired characters

Jean-Baptiste Lamarck (1744–1829), a French naturalist, set out his theory in Philosophie Zoologique (1809). He was the first to argue that evolution is a general fact covering all life, against the fixity defended by Cuvier.

The four propositions:

1. Internal tendency to growth: organisms and their parts tend to increase in size and complexity.
2. New needs produce new organs: a changed environment creates new needs (besoins), new habits and, through them, new organs.
3. Use and disuse: organs used constantly develop; unused organs weaken and vanish.
4. Inheritance of acquired characters: changes acquired in life are passed to offspring.

His examples: the giraffe, whose ancestors stretched for high leaves, lengthening the neck a little each generation; webbed feet of ducks from stretching the toes; limbless snakes from creeping through narrow places; blind moles and cave animals from disuse. It was simple, matched experience (exercise builds muscle) and could be tested.

Critique, proposition by proposition:

• Growth in size is not universal: giant tree-ferns gave way to smaller flowering plants.
• Need does not create organs: humans have always wished to fly but never grew wings.
• Use and disuse works within a lifetime (a blacksmith’s arm) but proves nothing unless inherited; the heart works constantly yet does not keep growing.
• Inheritance of acquired characters failed every test:
  • August Weismann, a German zoologist, cut off the tails of mice for five generations (901 young) and reported in 1889 that none was born with a shorter tail.
  • Ear and nose piercing in India, circumcision and Chinese foot-binding produced no inherited change.
  • William Castle and John Phillips transplanted the ovaries of a black guinea pig into a white female (1909); mated to a white male, she bore only black young. Offspring followed the genes in the ovary, not the body carrying it.
• Why it fails: changes in body cells do not alter the sex cells that carry heredity (§7.1).

5.3 Neo-Lamarckism and its modern echoes

• Neo-Lamarckism (term coined by Alpheus Packard, 1885) revived Lamarck in the late 19th century, especially in America (Edward Drinker Cope, Packard, Alpheus Hyatt) and in Herbert Spencer. Its followers held that environment and habit, not selection, drive evolution.
• Failed or false experiments: Paul Kammerer‘s midwife toads, whose inherited “nuptial pads” were shown in 1926 to have been injected with ink; William McDougall‘s rats, said to learn a water maze faster each generation (1920s–30s), not replicated; and Trofim Lysenko, whose state-backed Lamarckism suppressed Soviet genetics (the geneticist Nikolai Vavilov died in prison, 1943), a warning about ideology overriding evidence.
• Epigenetics: chemical tags on DNA (such as methylation) change how genes are expressed without changing the sequence, and the environment can set them. People conceived in the Dutch Hunger Winter (1944–45) still showed altered methylation of the IGF2 gene six decades later (Bastiaan Heijmans and colleagues, 2008). But most tags are erased each generation, and inheritance over several human generations is not established. Epigenetics qualifies Weismann’s rule; it does not restore Lamarckism.
• Baldwin effect (James Mark Baldwin, American psychologist, 1896): a learned behaviour can change which genes are favoured, so learning guides genetic evolution without inheriting acquired traits. Lactase persistence (§3) is a human case.

6. Darwinian Theory: Natural Selection

On Daphne Major, a tiny Galápagos island, the biologists Peter and Rosemary Grant measured every finch for decades. In the drought of 1977, small soft seeds ran out and only large hard seeds remained. Medium ground finches with deeper beaks could crack them; most others starved. Because beak depth is inherited, the next generation had measurably deeper beaks (Peter Boag and Peter Grant, 1981). This is natural selection observed directly, on the birds Darwin collected.

6.1 How the theory was formed

• Charles Darwin (1809–1882), son of a Shrewsbury doctor, sailed as naturalist on HMS Beagle (1831–36).
• What he saw: fossils of giant extinct mammals in Argentina resembling living South American forms; and on the Galápagos, finches, mockingbirds and tortoises that differed from island to island yet resembled mainland species, suggesting descent from mainland colonists with change on each island.
• What he read and learned: Lyell‘s deep time; Malthus (read in 1838); and breeders’ artificial selection of pigeons and cattle.
• Wallace: in 1858 Alfred Russel Wallace, a British naturalist in the Malay Archipelago, sent Darwin an essay with the same theory. Their papers were read at the Linnean Society on 1 July 1858, and On the Origin of Species appeared on 24 November 1859.
• “Survival of the fittest” is Spencer‘s phrase (1864); Darwin adopted it in 1869.

6.2 The logic: five facts, three inferences

The German-American biologist Ernst Mayr (The Growth of Biological Thought, 1982) reduced Darwin’s argument to five facts and three inferences. Each conclusion follows from observation.

Observed facts	Inference
1. Overproduction: populations could grow geometrically (even the slow-breeding elephant, Darwin calculated, would leave nearly nineteen million descendants from one pair in about 750 years). 2. Yet populations stay roughly stable. 3. Resources are limited.	I. Struggle for existence: most offspring die young. The struggle is within species, between species, and against drought or cold.
4. Variation: no two individuals are alike. 5. Much variation is heritable.	II. Natural selection: those with favourable variations survive and reproduce more (differential reproduction).
All five, over many generations	III. Evolution and origin of species: favourable traits accumulate; populations in different conditions diverge into species.

Every step rests on something observable, which is the argument’s strength. “Fittest” means not the strongest but the one that leaves most surviving offspring in that environment; a drought-tolerant plant or a cooperative animal can be the fittest.

6.3 Darwin’s supplementary theories

• Artificial selection: selection under human control, which produced breeds as different as greyhound and bulldog from one wolf stock; Darwin’s model for natural selection.
• Sexual selection (The Descent of Man, 1871): traits that help in winning mates, through male competition (antlers) or female choice (the peacock’s tail). It explains traits that seem costly for survival.
• Pangenesis (1868): every body part sheds particles (gemmules) that travel to the sex organs, so bodily changes could be inherited. His cousin Francis Galton transfused blood between rabbits of different colours (1871) and found no effect on the young; the idea was abandoned.

6.4 Critique of Darwinism

Each objection was real in Darwin’s time, and each was answered by a later discovery.

Objection	Why it held	How it was answered
No source of variation: it explains the survival of the fittest, not their arrival	Heredity unknown	Mutation and recombination
Blending inheritance would dilute new variants (Fleeming Jenkin, 1867)	Offspring thought a blend of parents	Mendel: genes do not blend
Useless beginnings: half a wing cannot fly (St George Mivart, 1871)	Early stages seem valueless	Early stages had other uses (feathers for warmth): exaptation (Gould and Vrba, 1982)
Overspecialisation: giant antlers of the Irish elk	Selection should not create burdens	Sexual selection and growth scaling (Gould, 1974)
Pangenesis was Lamarckian	Darwin needed a theory of heredity	Rejected by Galton and Weismann
Not all change is adaptive	Darwin stressed selection almost alone	Genetic drift and the neutral theory (Unit 1.4b)
“Tooth and claw”	Struggle read as fighting	Struggle includes drought; cooperation aids survival (Peter Kropotkin, Mutual Aid, 1902)

Darwin’s core idea, selection acting on heritable variation, survived every objection; what was missing was a theory of heredity, which genetics supplied. The theory was also misused: Social Darwinism used “survival of the fittest” to justify inequality and empire, and eugenics (Galton’s word, 1883) led to forced sterilisation. Both are rejected as scientifically wrong and morally indefensible.

6.5 Lamarckism and Darwinism compared

Both accept evolution and both explain adaptation. They differ in the order of events: for Lamarck, the change comes first and is then inherited; for Darwin, variation comes first and is then selected.

Basis	Lamarckism	Darwinism
Source of change	Need and effort of the organism	Chance variation already present
Role of environment	Directly causes change	Selects among variants
What changes	The individual, in its life	The population, over generations
Inheritance	Acquired characters inherited	Inborn variation inherited
Direction	Towards perfection	No goal
Status today	Rejected for genes; echoes in epigenetics and cultural inheritance	Core of modern theory, completed by genetics

Contribution to the synthetic theory: Darwin supplied its mechanism (natural selection) and its population thinking. Lamarck’s lasting contribution was that evolution is a general fact and that adaptation to the environment is what needs explaining. Debates on epigenetics, the Baldwin effect and niche construction revisit his questions while rejecting his mechanism.

7. Post-Darwinian Theories

In 1886, in a field near Hilversum in the Netherlands, the Dutch botanist Hugo de Vries found evening primroses (Oenothera lamarckiana) with new forms that bred true at once: giants, dwarfs, plants with odd leaves. He concluded that new species arise suddenly, in one step. The example captures the period: critics now looked for the source of variation, and some believed they had found a rival to natural selection.

7.1 Weismann’s germ-plasm theory

• August Weismann (Das Keimplasma, 1892) divided the body into the germ plasm (hereditary material of the sex cells) and the somatoplasm (all other cells).
• Continuity of the germ plasm: the germ plasm passes unbroken between generations; the body is its temporary carrier, and bodily changes cannot reach it (the Weismann barrier).
• Why it mattered: it ruled out Lamarckism and pangenesis, leaving selection of inborn variation. George Romanes called this purified theory neo-Darwinism (1890s).
• Limits: in plants and many simple animals the germ line separates late, and epigenetics shows some effects can cross (§5.3).

7.2 Mendel and the rediscovery of heredity

• Gregor Mendel, an Austrian monk, showed with garden peas (1866) that traits are carried by separate factors that segregate in forming sex cells and assort independently.
• Ignored for decades, his work was rediscovered in 1900 by de Vries, Carl Correns and Erich von Tschermak.
• Why it mattered: inheritance is particulate, not blending, so a rare new variant is not diluted and can be selected, answering Jenkin (§6.4).
• New vocabulary: Bateson named genetics (1905); Johannsen coined gene, genotype and phenotype (1909); Morgan‘s fruit flies (from 1910) placed genes on chromosomes.

7.3 Mutation theory

• Hugo de Vries (Die Mutationstheorie, 1901–03): new species arise by large, sudden, heritable changes (mutations); evolution proceeds by saltation (jumps), and selection only removes the unfit.
• Strength: he made mutation the source of new variation, which Darwin lacked.
• Weaknesses: most of his Oenothera “mutations” were later shown to be chromosomal irregularities (doubled chromosome sets, unusual chromosome rings), not ordinary gene changes. Real mutations are mostly small and often harmful or neutral, and new species rarely arise in one jump. Richard Goldschmidt’s later “hopeful monsters” (1940) found little support.

7.4 Isolation, orthogenesis and recapitulation

• Isolation theory: Moritz Wagner (1868) argued that geographical isolation is needed for new species, since interbreeding would otherwise erase local differences; Mayr later made it central (Unit 1.4b).
• Orthogenesis (“straight-line evolution”; term by Wilhelm Haacke, 1893, popularised by Theodor Eimer and later H.F. Osborn): evolution moves in fixed directions driven by an inner force, as in a supposed straight-line growth of the horse or the human brain. Critique: George Gaylord Simpson (Horses, 1951) showed horse evolution was a branching bush with reversals; long trends reflect consistent selection, not an inner drive.
• Recapitulation (Haeckel’s biogenetic law, 1866): “ontogeny recapitulates phylogeny“, an embryo passing through its ancestors’ adult stages. Critique: embryos resemble the embryos of related species, not their adults, as Karl Ernst von Baer had observed (1828); Haeckel’s drawings exaggerated. Only a modest version survives, in evolutionary developmental biology (“evo-devo”).

7.5 The “eclipse of Darwinism” and the road to synthesis

• From about 1880 to 1920 natural selection lost support: Julian Huxley’s “eclipse of Darwinism“.
• Mutationists (Bateson, de Vries) held that evolution proceeds by discrete jumps; biometricians (Karl Pearson, W.F.R. Weldon, followers of Galton) measured continuous variation such as height and defended gradual selection.
• Resolution: the British statistician R.A. Fisher showed (1918) that many genes of small effect produce smooth, continuous variation, so Mendel and Darwin were compatible. Population genetics, and then the synthetic theory of the 1930s–40s, followed (Unit 1.4b).

Key Terms

Term	Meaning in simple words
Hominization	All the biological and cultural changes that turned an ape-like ancestor into modern humans
Hominin	A member of the human line after the split from chimpanzees (older books: “hominid”)
Bipedalism	Habitually walking upright on two legs
Encephalization	Brain growth beyond what body size predicts
Obstetrical dilemma	Conflict between a narrow pelvis for walking and a big-headed baby
Gene-culture co-evolution	A cultural practice changes which genes are favoured
Organic evolution	Change in inherited traits of populations over generations, producing new species
Homologous organs	Same basic structure and origin, different uses
Vestigial organ	A reduced remnant of an organ that worked in ancestors
Inheritance of acquired characters	Lamarck’s idea that changes made in life pass to offspring
Natural selection	Individuals with favourable inherited variations leave more offspring
Pangenesis	Darwin’s disproved idea that body particles carry acquired traits to sex cells
Weismann barrier	Bodily changes cannot reach the hereditary material of the sex cells
Orthogenesis	The rejected idea of straight-line evolution driven by an inner force

Contemporary Developments

• Selection is still at work (2026): nearly 16,000 ancient genomes from West Eurasia showed 479 gene variants strongly selected over the last 10,000 years, with selection accelerating after farming (Ali Akbari, David Reich and colleagues, Nature, April 2026): a cultural change intensified biological evolution.
• Denisovans get a face (June 2025): proteins and mitochondrial DNA from dental plaque identified the Harbin cranium (north-east China, at least 146,000 years old) as Denisovan (Science, Cell).
• Interbreeding dated (December 2024): early modern human genomes showed Neanderthal gene flow lasting about 7,000 years, centred on about 47,000 years ago (Nature, Science).

Diagrams to Master

1. Topic map: biological and cultural factors around human evolution; below, the three phases of theory.
2. Hominization sequence: bipedalism (Laetoli 3.66 mya) → tools (Lomekwi 3.3 mya) → brain expansion (2 mya) → long childhood → symbols; label “feet before brains”.
3. Biocultural loop: free hands → tools → better diet → larger brain → culture → new selection; mark lactase (north-west India) and sickle cell (malaria).
4. Lamarck vs Darwin giraffe: stretch → longer neck → inherited against variation → selection → more long-necked young.
5. Timeline: Linnaeus 1735 → Lamarck 1809 → Lyell 1830 → Darwin and Wallace 1858–59 → Weismann 1892 → Mendel rediscovered 1900 → de Vries 1901 → Fisher 1918 → synthesis.

Revision Grid

Scholar	Work (year)	Key idea
Lamarck	Philosophie Zoologique (1809)	Use and disuse; acquired characters inherited
Darwin	Origin of Species (1859); Descent of Man (1871)	Natural selection; sexual selection; African origin
Galton	Transfusions (1871)	Disproved pangenesis
Weismann	Mouse tails (1889); Das Keimplasma (1892)	Germ plasm; Weismann barrier
Mendel	1866; rediscovered 1900	Particulate inheritance
De Vries	Die Mutationstheorie (1901–03)	Mutation; saltation
Fisher	1918	Many small genes reconcile Mendel and Darwin
Washburn	“Tools and Human Evolution” (1960)	Tools shaped the body; obstetrical dilemma
Childe	Man Makes Himself (1936)	Humans create their own conditions
Wrangham	Catching Fire (2009)	Cooking hypothesis
Boyd and Richerson	1985	Dual inheritance
Livingstone	1958	Farming, malaria, sickle cell
Lasker	1969	Levels of adaptability
Goodman and Leatherman	1998	Political economy in human biology