Origin of life
Look at a clear night sky: the light you see left its star millions of years ago. The story of life on Earth also starts far back, in the story of the universe.
The universe is about 13.8 billion years old and Earth formed about 4.5 billion years ago. The chemical evolution theory (Oparin and Haldane) says that the first life forms came from non-living organic molecules that formed in a reducing atmosphere (CH₄, NH₃, H₂ and water vapour). In 1953 S.L. Miller showed in the laboratory that amino acids can form under such conditions.
In the figure : Chemical evolution came first; the first cellular life followed.
In the figure : The experiment supports chemical evolution, not the details of the first cell.
Rule 1: Spontaneous generation versus biogenesis — Earlier people believed that life could arise from decaying matter (spontaneous generation). Louis Pasteur showed with sterilised flasks that life comes only from pre-existing life (biogenesis). Yet the first life must have come from non-living matter, giving the idea of chemical evolution.
- Easy : Pasteur disproved spontaneous generation.
- Medium : Pasteur's flasks stayed free of life until the neck was broken.
- Exam-type : Q: What did Pasteur's experiment prove? A: That life comes from pre-existing life.
Rule 2: Chemical evolution and first cells — Oparin and Haldane proposed that the first life came from molecules formed in the early reducing atmosphere. Giant molecules (RNA, proteins, polysaccharides) formed capsule-like structures; the first cells may have been single-celled and lived in water, using chemical energy.
- Easy : Early atmosphere lacked free oxygen.
- Medium : Miller used an electric discharge to mimic lightning.
- Exam-type : Q: Why is the early atmosphere called reducing? A: It had CH₄, NH₃ and H₂, with no free oxygen.
Chemical evolution came before cellular life. The first cells (about 2000 million years ago) were single-celled and may have released O₂ using light.
Saying that Miller made life in his experiment. He only made amino acids, the building blocks.
Memory clue: O-H-M — Oparin–Haldane proposed it; Miller tested it.
Spot the difference
| Point | Biogenesis | Spontaneous generation |
|---|---|---|
| Idea | Life from pre-existing life | Life from non-living matter |
| Supported by | Pasteur's experiment | Old belief (disproved) |
| Status | Accepted | Rejected |
Practice questions
- 1How old is the universe?
- 2Who proposed the chemical origin of life?
- 3Who performed the experiment on the synthesis of amino acids?
- 4What is biogenesis?
- 5Name two gases used in Miller's experiment.
- 6When did the first cells appear according to the textbook?
- 7Why was an electric discharge used?
- 8Why does Miller's experiment not fully prove the origin of life?
Show answers (try them yourself first)
- 1About 13.8 billion years.
- 2Oparin (Russia) and Haldane (England).
- 3S.L. Miller in 1953.
- 4Origin of life from pre-existing life.
- 5Any two of methane, ammonia, hydrogen.
- 6About 2000 million years ago.
- 7To provide energy like lightning on early Earth.
- 8It shows formation of simple molecules, not the formation of living cells.
Evidences for evolution
A whale's flipper, a bat's wing and your own arm look different, yet the bones inside follow the same plan. This is a clue to a shared ancestor.
Evidence for evolution comes from fossils (paleontological), comparative anatomy and morphology, embryology, biochemistry and from natural selection seen in action. Homologous organs have the same structure and origin but different functions (divergent evolution); analogous organs have different structures but the same function (convergent evolution).
In the figure : Homology shows common descent; analogy shows similar adaptation.
Rule 1: Anatomical and fossil evidence — Fossils in different rock layers show that life forms changed with time, and radioactive dating tells their age. Homology (vertebrate forelimbs, potato tuber and sweet potato tuber) shows adaptive radiation from a common ancestor. Vestigial organs (appendix) show a former use.
- Easy : Fossils show life forms changed over time.
- Medium : Bat wing and human arm are homologous.
- Exam-type : Q: Give one example of analogous organs. A: Wings of a bird and an insect.
Rule 2: Natural selection in action — The industrial melanism of peppered moths: after industrialisation in England, dark moths survived better on soot-covered trees and their number increased. Antibiotic resistance in bacteria, and herbicide and pesticide resistance in pests, also show selection of resistant types.
- Easy : Dark moths were hidden better on dark trees.
- Medium : Overuse of antibiotics selects resistant bacteria.
- Exam-type : Q: Explain industrial melanism in one line. A: Dark moths had a survival advantage on dark, polluted tree trunks, so their proportion went up.
Convergent evolution gives analogous organs; divergent evolution gives homologous organs. Biochemical evidence: similarities in proteins and genes among organisms.
Writing that the wings of a bird and a bat are analogous. They are homologous as forelimbs, but the wings of a bird and an insect are analogous.
Memory clue: H → Heritage, A → Alike use — Homologous = same ancestry; Analogous = same function.
Spot the difference
| Point | Homologous organs | Analogous organs |
|---|---|---|
| Origin | Same | Different |
| Function | Different | Same |
| Evolution | Divergent | Convergent |
Practice questions
- 1What are fossils?
- 2Give one example of homologous organs.
- 3What is a vestigial organ?
- 4Name the moth studied in industrial melanism.
- 5Which kind of evolution gives analogous organs?
- 6How can the age of a fossil be found?
- 7Why do the flipper of a dolphin and the fin of a shark show analogy?
- 8How does antibiotic resistance support Darwin's idea?
Show answers (try them yourself first)
- 1Remains or impressions of organisms that lived in the past.
- 2Forelimbs of human, bat and whale.
- 3An organ that is reduced and no longer useful, like the human appendix.
- 4The peppered moth (Biston betularia).
- 5Convergent evolution.
- 6By radioactive dating.
- 7Both are for swimming but they evolved independently in different groups.
- 8Resistant bacteria survive and multiply when antibiotics are used, so natural selection increases their number.
Adaptive radiation and theories of evolution
Birds on an island may have beaks shaped like pliers, scissors or tweezers, depending on what they eat. All are descendants of one ancestor.
Adaptive radiation is the evolution of different species from a common ancestor in a geographical area, each adapting to a different habitat (Darwin's finches in the Galápagos; Australian marsupials). Lamarck proposed that organs strengthen with use and weaken with disuse and that acquired characters are inherited (giraffe's neck). Darwin proposed natural selection; Hugo de Vries proposed the mutation theory.
In the figure : Fitness means the ability to survive and reproduce.
In the figure : Modern theory unites both: mutations give variations and selection sorts them.
Rule 1: Darwin and Wallace — Darwin sailed on the HMS Beagle and saw different finches on the Galápagos islands. His theory is based on observations: stable population size, natural resources are limited, variations exist and are heritable, more offspring are born than survive. Alfred Wallace came to the same conclusion independently.
- Easy : Natural selection chooses the fittest.
- Medium : Darwin's fitness means reproductive success, not strength.
- Exam-type : Q: Who independently proposed the idea of natural selection? A: Alfred Russel Wallace.
Rule 2: Lamarck and de Vries — Lamarck's idea of use and disuse (giraffes stretching their necks) is not accepted because acquired characters are not inherited. De Vries studied the evening primrose and said that evolution is by large, sudden mutations; today mutations are seen as random, directionless sources of variation.
- Easy : Lamarck: use and disuse.
- Medium : De Vries: mutation, saltation.
- Exam-type : Q: What is saltation? A: Single-step large mutation causing speciation.
Darwin's finches: beak shapes adapted to different foods from a single ancestor. Marsupials in Australia are another case of adaptive radiation.
Saying natural selection means survival of the strongest. It means survival of the best adapted (those that leave more offspring).
Memory clue: V-O-V — Variation, Overproduction, Variation survives (selection) — the logic of natural selection.
Spot the difference
| Point | Lamarck | Darwin |
|---|---|---|
| Basis | Use and disuse | Natural selection |
| Acquired characters | Inherited | Not inherited |
| Example | Long neck of giraffe | Galápagos finches |
Practice questions
- 1What is adaptive radiation?
- 2Name two examples of adaptive radiation.
- 3Who proposed the theory of use and disuse?
- 4On which ship did Darwin travel?
- 5Who proposed the mutation theory?
- 6What is meant by fitness?
- 7Why is Lamarck's theory not accepted?
- 8How do the views of Darwin and de Vries differ?
Show answers (try them yourself first)
- 1Evolution of different species from a common ancestor in an area, adapted to different habitats.
- 2Darwin's finches and Australian marsupials.
- 3Lamarck.
- 4HMS Beagle.
- 5Hugo de Vries.
- 6The ability to survive and leave more offspring.
- 7Acquired characters are not passed on to offspring.
- 8Darwin: gradual, small variations; de Vries: sudden, large mutations.
Mechanism of evolution and the Hardy–Weinberg principle
If you count how many people in a village have a certain blood group, the number may remain the same for generations, unless something disturbs the balance.
The Hardy–Weinberg principle says that allele frequencies in a population are stable and constant from generation to generation if no evolutionary forces act. If the frequency of allele A is p and of allele a is q, then p + q = 1 and the genotype frequencies are p² + 2pq + q² = 1. A change from this equilibrium means evolution.
In the figure : The three frequencies add up to 1, as the expansion of (p + q)².
In the figure : Any of these five can disturb the equilibrium, so evolution happens.
Rule 1: Forces of evolution — Gene flow: movement of alleles between populations. Genetic drift: random change in allele frequency by chance, which may cause the founder effect when a few individuals form a new population. Mutation and recombination add variation; natural selection increases the frequency of favourable alleles.
- Easy : Genetic drift is chance-based.
- Medium : Founder effect: a small group starts a new population.
- Exam-type : Q: What is gene flow? A: Movement of genes between populations through migration.
Rule 2: Types of natural selection — Stabilising selection favours the average and reduces extremes; directional selection shifts the population to one extreme; disruptive selection favours both extremes and reduces the average, which may split the population in two.
- Easy : Directional: one extreme is favoured.
- Medium : Stabilising keeps the mean, the curve gets narrower.
- Exam-type : Q: Which type of selection may split a population in two? A: Disruptive selection.
If the frequency of recessive allele q is 0.3, then p = 0.7, and genotype frequencies: AA 0.49, Aa 0.42, aa 0.09 (sum = 1.00).
Thinking Hardy–Weinberg equilibrium is the normal state in nature. It is an ideal case; real populations are always changing.
Memory clue: p-squared, 2-p-q, q-squared — AA = p², Aa = 2pq, aa = q².
Spot the difference
| Point | Gene flow | Genetic drift |
|---|---|---|
| Cause | Migration | Chance event |
| Effect | Brings new alleles | Random change in frequency |
| Effect size | Any population | Strongest in small populations |
Practice questions
- 1State the Hardy–Weinberg equation for allele frequency.
- 2What does 2pq represent?
- 3What is the founder effect?
- 4Name two factors that can change allele frequency.
- 5What is genetic drift?
- 6What is the sum of p² + 2pq + q²?
- 7If q² = 0.04, find p, q and the carriers (2pq).
- 8Why may disruptive selection lead to speciation?
Show answers (try them yourself first)
- 1p + q = 1.
- 2The frequency of heterozygotes (Aa).
- 3A small group starts a new population, so the new population has different allele frequency.
- 4Any two of gene flow, genetic drift, mutation, recombination, natural selection.
- 5Random change in allele frequency by chance.
- 61.
- 7q = 0.2, p = 0.8, 2pq = 0.32.
- 8It removes the average type, so the two extremes may become separate groups.
A brief account of the evolution of life on Earth
If the whole history of Earth were a one-year calendar, humans would appear in the last minutes of December 31.
Life evolved over millions of years (mya): first cells about 2000 mya; invertebrates by 500 mya; jawless fish about 350 mya; the first amphibians from lobe-finned fishes; amphibians led to reptiles; reptiles ruled for about 200 million years; dinosaurs vanished about 65 mya; mammals then took over.
In the figure : Each stage built on the previous one; the sequence is not a ladder but a branching tree.
Rule 1: Fish to reptiles — Fish with stout fins (lobefins like the Coelacanth, caught alive in South Africa in 1938) could move on land and gave rise to the amphibians, which could live on land and in water. Reptiles evolved from amphibians and lay thick-shelled eggs that do not dry in the sun.
- Easy : Coelacanth is a lobe-finned fish.
- Medium : Reptile eggs have a thick shell.
- Exam-type : Q: Why were reptiles better adapted to land than amphibians? A: Their thick-shelled eggs do not dry up.
Rule 2: Dinosaurs and mammals — Dinosaurs (the biggest, Tyrannosaurus rex, about 20 feet tall) disappeared suddenly about 65 mya; some scientists think climate change killed them, others think they evolved into birds. Early mammals were small, shrew-like and viviparous; they spread when reptiles declined. Continental drift isolated Australian marsupials.
- Easy : Mammals care for their young inside the body.
- Medium : Pouched mammals survive in Australia because of isolation.
- Exam-type : Q: What happened about 65 mya? A: Dinosaurs suddenly disappeared.
Invertebrates 500 mya → jawless fish 350 mya → amphibians → reptiles → dinosaurs (disappeared 65 mya) → mammals.
Thinking humans evolved from chimpanzees. Humans and apes share a common ancestor.
Memory clue: F-A-R-M — Fish, Amphibians, Reptiles, Mammals.
Spot the difference
| Point | Amphibians | Reptiles |
|---|---|---|
| Eggs | Soft, laid in water | Thick-shelled, laid on land |
| Habitat | Land and water | Mostly land |
| Example | Frog | Turtle, crocodile |
Practice questions
- 1Which were the first organisms to invade land?
- 2Name a lobe-finned fish caught in 1938.
- 3When did dinosaurs disappear?
- 4Which animals evolved from amphibians?
- 5What kind of mammals live in Australia because of isolation?
- 6Name one mammal that lives wholly in water.
- 7Why did mammals take over after the decline of reptiles?
- 8Why is the geological time scale useful?
Show answers (try them yourself first)
- 1Plants.
- 2Coelacanth.
- 3About 65 million years ago.
- 4Reptiles.
- 5Pouched mammals (marsupials).
- 6Whale (or dolphin, seal, sea cow).
- 7Reptile competition was reduced and mammals were able to sense danger and protect their young.
- 8It shows which life forms existed in which period.
Origin and evolution of man
Human fossils from Africa and Asia show how upright walking, bigger brains and tool use developed step by step.
About 15 mya primates like Dryopithecus and Ramapithecus existed (the latter more man-like). Later came Australopithecus (2 mya, East African grasslands), Homo habilis (brain 650–800 cc), Homo erectus (about 1.5 mya, brain about 900 cc), Neanderthal man (brain 1400 cc) and Homo sapiens (arose in Africa; modern form 75,000–10,000 years ago).
In the figure : Brain size increased steadily (habilis: 650–800 cc, shown at about 700 cc).
In the figure : Human evolution is a branching bush; several forms lived at the same time.
Rule 1: Stages — Homo habilis was the first human-like being (hominid) and probably did not eat meat. Fossils of Homo erectus were found in Java in 1891; it probably ate meat. Neanderthals lived in the Near East and central Asia 100,000–40,000 years ago, used hides for clothing and buried their dead.
- Easy : Habilis: 650–800 cc.
- Medium : Erectus: about 900 cc.
- Exam-type : Q: Which early human buried the dead? A: Neanderthal man.
Rule 2: Modern man — Homo sapiens arose in Africa, spread across the continents and developed into distinct races. Cave art appeared about 18,000 years ago (for example at Bhimbetka, Madhya Pradesh), and agriculture began about 10,000 years ago, which led to human settlements.
- Easy : Bhimbetka rock shelter has cave paintings.
- Medium : Agriculture began about 10,000 years ago.
- Exam-type : Q: What was the major change brought by agriculture? A: Humans began living in settlements.
Brain size: H. habilis 650–800 cc; H. erectus about 900 cc; Neanderthal 1400 cc. Dryopithecus and Ramapithecus were primates of about 15 mya.
Mixing up the order of brain sizes. Habilis is the smallest (650–800 cc), then erectus (about 900 cc), then Neanderthal (1400 cc).
Memory clue: D-A-H-E-N-S — Dryopithecus, Australopithecus, Habilis, Erectus, Neanderthal, Sapiens.
Spot the difference
| Point | Homo habilis | Homo erectus |
|---|---|---|
| Brain size | 650–800 cc | About 900 cc |
| Diet | Probably no meat | Probably ate meat |
| Fossil site | East Africa | Java (1891) |
Practice questions
- 1Which primate was more man-like: Dryopithecus or Ramapithecus?
- 2What was the brain size of Homo erectus?
- 3Which ancestor is called the first human-like being?
- 4Where were Homo erectus fossils found in 1891?
- 5How big was the brain of Neanderthal man?
- 6Where is a famous prehistoric rock shelter with cave paintings in India?
- 7Why is the use of fire and tools important in human evolution?
- 8How did agriculture change human life?
Show answers (try them yourself first)
- 1Ramapithecus.
- 2About 900 cc.
- 3Homo habilis.
- 4Java.
- 5About 1400 cc.
- 6Bhimbetka (Raisen district, Madhya Pradesh).
- 7They helped early humans to survive, get food and protect themselves.
- 8It led to settled living and the growth of villages and civilisations.