Evolutionary Biology (Zoology): origin of life, evidences, theories, human evolution

(b) Ammonia, Methane, Hydrogen, and water vapour

Miller and Urey sealed these gases with water in a flask and passed electric sparks to simulate lightning; the mixture produced simple amino acids such as glycine and alanine. Oxygen was deliberately excluded, since the primitive atmosphere was reducing, not oxidising.

(c) Analogous

The wings of a dragonfly (insect), Pteranodon (extinct reptile), hawk (bird) and bat (mammal) perform the same function — flight — but differ completely in their basic structure and embryonic origin. Hence they are analogous organs, which are the product of convergent evolution.

(a) Homo sapiens neanderthalensis

Neanderthal man is treated in the textbook sequence as the immediate predecessor of modern man, Homo sapiens sapiens (Cro-Magnon). Option (b) is modern man himself, while (c) and (d) are much older, more primitive forms of Homo erectus.

Darwinism (Theory of Natural Selection) and Organic Evolution — 5 marks

1. The central idea. Darwinism, proposed by Charles Darwin (1859) in The Origin of Species, explains organic evolution as the gradual change of organisms over generations through natural selection. It states that organisms better adapted to their environment survive and reproduce more, so their useful traits become more common in later generations, while harmful traits are eliminated. This is summed up as "survival of the fittest" (a phrase coined by Herbert Spencer).

2. The main principles (postulates).

• Overproduction (prodigality): Every species produces far more offspring than can possibly survive (e.g., a fish lays millions of eggs).
• Struggle for existence: Because of overproduction and limited resources, organisms must compete for food, space, and mates — intraspecific, interspecific, and against the physical environment.
• Variation: No two individuals are exactly alike; small heritable variations arise among members of a population.
• Survival of the fittest: In the struggle, individuals with favourable variations survive and reach maturity, while those with unfavourable variations perish.
• Natural selection: Nature acts as a selective agent, preserving favourable variations and rejecting harmful ones.
• Inheritance of useful variations: The favourable variations are passed on to the next generation.
• Origin of new species: Over many generations, the accumulated favourable variations become so marked that a new species arises — this is organic evolution.

3. How this produces evolution. Suppose a population of beetles shows colour variation. On green leaves, green beetles escape predators while brown ones are eaten. The surviving green beetles reproduce and pass on green colour. After many generations the population becomes predominantly green — the population has changed, i.e., evolved. Repeated over millions of years and across different environments, this process gives rise to new species and to the diversity of life from a common ancestor.

4. Evidences supporting Darwinism.

• Homologous organs (forelimbs of man, whale, bat) show common ancestry with divergent function.
• Analogous organs (wings of bird and butterfly) show convergent evolution.
• Vestigial organs (vermiform appendix, wisdom teeth) are remnants of once-useful structures.
• Fossils (e.g., Archaeopteryx) show links between groups.
• Embryological similarities among vertebrates indicate common descent.

5. Drawbacks (criticism). Darwinism cannot explain the origin of variation itself (Mendel and later genetics supplied this), the inheritance of acquired characters, the cause of sudden large changes (mutation), and it does not explain the role of isolation in speciation. These gaps were later filled by Neo-Darwinism (mutation + recombination + natural selection + isolation).

Conclusion. Darwinism explains organic evolution as descent with modification driven by natural selection acting on heritable variations in a struggle for existence — the mechanism that accounts for the adaptation and diversity of living organisms.

(b) Jean-Baptiste Lamarck

Lamarck proposed in Philosophie Zoologique (1809) that characters acquired during an organism's life by use and disuse are inherited by offspring. Weismann's germplasm theory later disproved this.

(b) Limulus (King crab)

Limulus has remained almost unchanged for over 400 million years, so it is a living fossil. Archaeopteryx is an extinct fossil, not a living one.

(d) All of the above

Vestigial organs are reduced structures that were functional in ancestors but have little or no function now. Wisdom teeth, the vermiform appendix and the nictitating membrane (plica semilunaris at the inner eye corner) are all vestigial in humans, so all three are correct.

(b) Cycas

A living fossil is a living species that has survived almost unchanged from ancient geological times. Cycas is a gymnosperm that has persisted since the Mesozoic era with primitive features such as motile sperms and a fern-like leaf, so it is called a living fossil.

Miller-Urey experiment (5 marks)

In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane idea that life originated from non-living matter through chemical evolution on the primitive Earth.

Apparatus: a closed, sterile glass system with a flask representing the ocean (boiling water), connected to a chamber containing a mixture of gases thought to make up the early atmosphere: methane ($\text{CH}_4$), ammonia ($\text{NH}_3$), hydrogen ($\text{H}_2$) and water vapour, but no free oxygen. Two electrodes gave electric sparks (lightning). A condenser cooled the vapour, and the liquid collected in a trap.

Procedure: the water was boiled, and its vapour mixed with the gases; sparks were passed continuously at about 800 °C for about a week; the vapour was condensed and the liquid was circulated back.

Result: the liquid turned brownish, and analysis showed organic compounds, including amino acids (glycine, alanine, aspartic acid), along with some simple organic acids.

Conclusion: simple organic molecules can form from inorganic gases under early-Earth conditions, supporting chemical evolution as the first step in the origin of life.

Lamarck's theory of organic evolution (5 marks)

Statement: Jean-Baptiste Lamarck (1809, Philosophie Zoologique) proposed that organisms change during their lifetime in response to the environment, and that these acquired characters are inherited by offspring, causing gradual evolution. It is called the theory of inheritance of acquired characters.

Main postulates

1. Internal tendency to increase in size: living organisms and their parts tend to increase in size.
2. Effect of the environment: a changed environment creates new needs in organisms.
3. Use and disuse of organs: organs constantly used become well developed, while those not used degenerate.
4. Inheritance of acquired characters: modifications acquired during life are passed to the next generation.

Example: the giraffe's long neck. Ancestors with short necks stretched to reach leaves on tall trees; the neck lengthened with continued use and this was inherited.

Criticism

1. Acquired (somatic) changes do not affect the germ cells, so they are not inherited. August Weismann cut off the tails of mice for 22 generations and the young were still born with normal tails.
2. Examples such as the giraffe have better explanations by natural selection.
3. Other criticisms: ear piercing and circumcision are not inherited.

Neo-Lamarckism later re-examined some ideas, but the theory is not accepted in its original form.

Evidences of evolution from comparative anatomy and embryology (5 marks)

A. Comparative anatomy

1. Homologous organs: organs with the same basic structure and origin but different functions, showing divergent evolution. Example: the forelimbs of human (grasping), whale (swimming), bat (flying) and horse (running) all have the same bones (humerus, radius-ulna, carpals, metacarpals, phalanges).
2. Analogous organs: organs with different structure and origin but the same function, showing convergent evolution. Example: wings of a bird and an insect.
3. Vestigial organs: reduced, functionless organs inherited from ancestors, e.g. the vermiform appendix, ear muscles and coccyx in humans.

B. Embryology

1. Similarities in early embryos: the embryos of fish, tortoise, chick and human look alike in early stages, with gill slits and a tail, suggesting common ancestry.
2. Von Baer's law: general characters appear before special ones during development.
3. Haeckel's biogenetic law: "ontogeny repeats phylogeny", i.e. the developing individual passes through stages resembling its ancestors (this is now regarded as only partly true).

Evidences of evolution (8 marks)

Evolution is the gradual change in organisms over generations. The evidences supporting it come from many branches of biology.

1. Morphological and anatomical evidences

• Homologous organs: structures having similar basic plan and embryonic origin but different functions, which show divergent evolution from a common ancestor. Example: the pentadactyl forelimbs of human, bat, whale, horse and cat.
• Analogous organs: structures with similar function but different origin and structure, which show convergent evolution. Example: wings of insects and of birds; eyes of octopus and of mammals.
• Vestigial organs: reduced and functionless organs that were functional in ancestors, e.g. vermiform appendix, wisdom teeth, nictitating membrane, ear muscles and coccyx in humans; pelvic girdle in whales.
• Atavism: reappearance of ancestral features (e.g. tail-like growth, extra mammae in humans).

2. Palaeontological evidences (fossils)

Fossils are preserved remains or impressions of past organisms in rocks. They show:

• Succession of life: simple forms in older rocks and more complex forms in newer rocks.
• Connecting links: Archaeopteryx (reptile and bird), Seymouria (amphibian and reptile), Cynognathus (reptile and mammal).
• Phylogeny of the horse: Eohippus $\to$ Mesohippus $\to$ Merychippus $\to$ Pliohippus $\to$ Equus, showing increase in size, reduction of toes (4 to 1) and increase in tooth complexity.
• Fossils also help to date the geological time scale.

Together these show that organisms share common ancestry and have changed gradually over time.

Archaeopteryx as a connecting link between reptiles and birds (5 marks)

Archaeopteryx lithographica is a fossil found in the Jurassic limestone of Germany (about 150 million years old). It is a connecting link because it shows characters of both groups.

Reptilian characters

1. Long tail with about 20 free caudal vertebrae.
2. Jaws with teeth set in sockets (no horny beak).
3. Three clawed, free fingers on the forelimb (wing).
4. Abdominal ribs and unfused skeletal bones, with solid bones lacking air spaces.
5. Weak sternum without a keel.

Avian characters

1. Body covered by feathers, with flight feathers on the wings and tail.
2. Forelimbs modified into wings.
3. Hind limbs adapted for perching, with four toes (three forward, one backward).
4. A furcula (wishbone) and a bird-like skull with an enlarged brain case.
5. Probably warm-blooded.

Thus, with reptile-like features in a bird-like body, Archaeopteryx is a connecting link that supports the evolution of birds from reptiles (dinosaurs).