Evolution: Origin of Life and Evidences — Notes

Abiogenesis (spontaneous generation): living organisms arise from non-living matter (for example, maggots from rotting meat). Louis Pasteur disproved it for present-day conditions with swan-neck flasks: boiled broth in a flask with a curved neck stayed sterile because dust and microbes were trapped in the bend; when the neck was broken, microbes grew.

Biogenesis: life arises only from pre-existing life — true of life today.

Panspermia (cosmozoic theory): spores or other units of life reached Earth from outer space (supported by early thinkers such as Arrhenius). It does not explain how life began in the first place.

Oparin–Haldane theory (chemical evolution). A. I. Oparin (1924) and J. B. S. Haldane (1929) proposed that the first life arose from non-living organic molecules by a long series of chemical reactions. The early atmosphere was reducing: methane (CH₄), ammonia (NH₃), hydrogen (H₂) and water vapour, with no free oxygen. Energy from lightning, UV radiation (no ozone layer) and volcanic heat formed simple organic molecules — sugars, amino acids, nitrogen bases, fatty acids — which collected in the oceans as a “hot dilute soup” (Haldane). These combined into larger molecules (proteins, nucleic acids).

Why a reducing atmosphere was essential: free oxygen is highly reactive and would oxidise and break down organic molecules as soon as they formed; without it, organic molecules could accumulate. The lack of ozone also let UV radiation reach the surface as an energy source.

Protobionts are aggregates of abiotically produced molecules surrounded by a membrane-like boundary, maintaining an internal environment different from the surroundings. They are not living but show some properties of cells (growth, simple division, selective uptake). Two kinds: coacervates (Oparin) — colloidal droplets of proteins, polysaccharides and water that absorb substances from the surroundings; and microspheres (Sidney Fox) — protein-like droplets formed when amino acids are heated and then cooled in water. The first true cells (heterotrophic, anaerobic prokaryotes) probably arose about 3.5 billion years ago.

In 1953, Stanley Miller, working with Harold Urey, tested the Oparin–Haldane idea. A closed glass apparatus contained methane, ammonia, hydrogen and water vapour. Water was boiled in one flask to send vapour round the system; in a larger flask, electrodes produced electric sparks (representing lightning); a condenser cooled the gases (representing rain), and the liquid collected in a U-shaped trap. After about one week, the liquid contained amino acids such as glycine, alanine and aspartic acid. Later similar experiments produced sugars, nitrogen bases, pigments and fats.

Conclusion: the organic molecules of life can be formed abiotically from simple inorganic gases when energy is supplied in a reducing atmosphere, supporting chemical evolution.

Homologous organs have the same basic structure and embryonic origin but may perform different functions. Examples: forelimbs of human (grasping), cheetah (running), whale (swimming) and bat (flying), all with humerus, radius and ulna, carpals, metacarpals and phalanges; the heart and brain of vertebrates; in plants, the thorn of Bougainvillea and the tendril of Cucurbita (both modified stems). Homology shows divergent evolution: related organisms adapting to different needs.

Analogous organs perform the same function but differ in structure and origin. Examples: wings of a butterfly and a bird; eye of an octopus and of a mammal; flippers of a penguin and a dolphin; sweet potato (modified root) and potato (modified stem). Analogy shows convergent evolution: unrelated organisms adapting to similar needs.

Vestigial organs are reduced and functionless (or nearly so) in an organism but functional in its ancestors or relatives. In humans: vermiform appendix, nictitating membrane (plica semilunaris), coccyx, muscles of the ear pinna, wisdom teeth, body hair. They indicate common descent.

Embryology. Embryos of all vertebrates are alike in early stages, with gill slits (pharyngeal pouches), a notochord and a post-anal tail. Ernst Haeckel proposed the recapitulation theory (biogenetic law): “ontogeny recapitulates phylogeny” — the development of an individual repeats the evolutionary history of its group. Karl Ernst von Baer showed that embryos resemble the embryos, not the adults, of other animals, so the theory is not accepted literally; embryonic similarity is still evidence of common ancestry.

Palaeontology. Fossils are remains or impressions of past organisms preserved in sedimentary rocks. Deeper strata are older and contain simpler forms; younger strata contain more complex forms. Connecting links show features of two groups: Archaeopteryx (feathers and wings like birds; teeth, clawed fingers and a long bony tail like reptiles) links reptiles and birds.

Dating fossils. In living organisms the ratio of radioactive carbon-14 to carbon-12 stays constant; after death, carbon-14 decays with a half-life of 5730 years and is not replaced. Age = number of half-lives × 5730 years (for example, one quarter left → 2 half-lives → 11460 years). Radiocarbon dating is useful up to about 50 000 years; older rocks are dated by uranium–lead or potassium–argon methods.

Geological time scale (approximate, million years ago):

• Precambrian (before 541): bacteria, algae, soft-bodied invertebrates.
• Palaeozoic (541–252): invertebrates (Cambrian), fishes (Devonian, “age of fishes”), amphibians (Carboniferous); pteridophytes and seed ferns form coal forests.
• Mesozoic (252–66): “age of reptiles” (dinosaurs); gymnosperms dominant; first mammals and birds (Jurassic); first flowering plants (Cretaceous).
• Cenozoic (66–present): “age of mammals”; angiosperms dominant; humans evolve in the Quaternary.

Biogeography. The distribution of species reflects their history. Oceanic islands (such as the Galapagos) have endemic species, related to but different from mainland species. Australia, separated early, has many marsupials and few native placental mammals.

Adaptive radiation is the evolution of different forms from one ancestral form, radiating into different habitats or niches. Examples:

• Darwin’s finches (Galapagos): from a seed-eating ancestor, finches evolved with beaks for seeds, insects, cactus flowers, grubs in bark and buds and fruits.
• Australian marsupials: from one ancestral stock, marsupial wolf, mole, sugar glider, numbat, kangaroo, koala and others evolved. Many resemble placental mammals on other continents (Tasmanian wolf and wolf; marsupial mole and mole; sugar glider and flying squirrel) — convergent evolution between the two groups.

Molecular evidence. All organisms use DNA (or RNA) as genetic material, a nearly universal genetic code, ATP as energy currency and similar enzymes and pathways. The degree of similarity in DNA sequences and proteins (for example cytochrome c, haemoglobin) reflects relatedness: human and chimpanzee DNA are about 98–99 percent similar.