7. Evolutionary Biology

7.1 Life and its Origin

The origin of life on Earth is a fundamental question in biology. Early scientists proposed that life arose from simple inorganic molecules under specific environmental conditions.

Oparin-Haldane Theory

Independently proposed by Alexander Oparin (1924) and J.B.S. Haldane (1929), this theory suggests that life originated from a series of chemical transformations in a reducing atmosphere.

• Atmospheric conditions: No free oxygen; abundant gases such as methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapor (H₂O).
• Sequence of steps:
  1. Simple inorganic molecules → organic monomers (e.g., amino acids, nucleotides).
  2. Organic monomers → polymers (proteins, nucleic acids) via condensation reactions.
  3. Polymers → membranous droplets or protocells capable of basic metabolism.
  4. Protocells → true cells with genetic material and catalytic proteins.

The theory is summarized by the concept of a “primordial soup” where energy sources (UV light, lightning, volcanic heat) drove synthesis.

Miller and Urey's Experiment (1953)

Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis by simulating early Earth conditions in a closed apparatus.

• Apparatus: A flask containing water (to simulate oceans) heated to produce vapor; a second flask with gases CH₄, NH₃, H₂, and H₂O subjected to electrical sparks (simulating lightning).
• Results: After one week, analysis revealed the formation of several organic compounds, most notably amino acids (glycine, alanine) and other biomolecules.
• Significance: Provided experimental support that abiotic synthesis of life’s building blocks is plausible under reducing conditions.

7.2 Evidences of Evolution

Multiple independent lines of evidence corroborate the fact that organisms have changed over time through descent with modification.

Morphological Evidence

Structural similarities despite different functions indicate common ancestry.

• Homologous organs: Same basic structure, different functions.
  • Example: Forelimb of human (grasping), whale flipper (swimming), bat wing (flight). All share humerus, radius, ulna, carpals, metacarpals, phalanges.

Anatomical Evidence

Similar function but different underlying structure suggests convergent evolution.

• Analogous organs: Different structure, similar function.
  • Example: Bird wing (feathers supported by bony forearm) vs. insect wing (cuticle extension). Both enable flight but arise from distinct embryonic origins.

Paleontological Evidence

The fossil record provides direct documentation of extinct forms and transitional stages.

• Transitional forms: Fossils showing intermediate characteristics.
  • Example: Archaeopteryx lithographica (Late Jurassic) possesses avian features (feathers, wishbone) and reptilian traits (teeth, long bony tail).
• Faunal succession: Ordered appearance of fossils in rock strata matches evolutionary predictions.

Embryological Evidence

Early developmental stages reveal shared ancestry among vertebrates.

• All vertebrate embryos exhibit pharyngeal (gill) slits, a post‑anal tail, and a notochord at comparable stages.
• Example: Human, fish, frog, and chicken embryos all show gill slits, though they develop into different structures (e.g., Eustachian tube in mammals).

Biochemical Evidence

Universality of biomolecules points to a common origin.

• DNA and RNA: Same genetic code (triplet codons) across bacteria, plants, animals.
• Proteins: Conserved enzymes such as cytochrome c show high sequence similarity; cytochrome c differs by only a few amino acids between humans and chimpanzees.
• Metabolic pathways: Glycolysis, Krebs cycle, and ATP synthesis are nearly identical in all aerobic organisms.

7.3 Theories of Evolution

Several frameworks have been proposed to explain the mechanisms driving evolutionary change.

Lamarckism (Inheritance of Acquired Characteristics)

Jean-Baptiste Lamarck (1809) suggested that organisms change during their lifetime in response to environmental needs, and these changes are passed to offspring.

• Use and disuse: Organs used frequently become stronger; unused organs deteriorate.
  • Example: Giraffe’s long neck purportedly resulted from generations stretching to reach high leaves.
• Criticism: August Weismann’s experiments (1880s) cutting tails off mice for 22 generations showed no inheritance of the truncated tail, disproving the transmission of acquired traits.

Darwinism (Natural Selection)

Charles Darwin (1859) proposed that evolution occurs via natural selection acting on heritable variation.

• Key principles:
  1. Variation: Individuals within a population differ in traits.
  2. Inheritance: Traits are passed from parents to offspring.
  3. Selection: Environmental pressures cause differential survival and reproduction.
  4. Time: Cumulative changes over many generations lead to speciation.
• Summary: “Survival of the fittest” refers to reproductive success, not merely physical strength.
• Example: Peppered moth (Biston betularia) in industrial England: dark‑colored morph increased due to better camouflage on soot‑covered trees.

Neo‑Darwinism (Modern Synthesis)

Integrates Darwinian natural selection with Mendelian genetics and population genetics.

• Sources of genetic variation:
  • Gene mutations (point mutations, insertions, deletions).
  • Chromosomal changes (duplications, inversions, translocations).
  • Genetic recombination during meiosis (crossing over, independent assortment).
• Role of natural selection: Acts on phenotypic expression of genotypes, altering allele frequencies.
• Mathematical representation: Hardy‑Weinberg equilibrium (ideal, non‑evolving population): p² + 2pq + q² = 1 where p = frequency of dominant allele, q = frequency of recessive allele.
• Example: Antibiotic resistance in bacteria arises via random mutations conferring resistance; selection by antibiotic use increases resistant allele frequency.

7.4 Human Evolution

Humans (Homo sapiens) are placed within the taxonomic hierarchy and have a well‑documented evolutionary lineage.

Position of Man in the Animal Kingdom

Differences Between Old World and New World Monkeys

Apes and Man: Distinguishing Traits

• Bipedalism: Habitual upright walking on two limbs (humans); apes are primarily quadrupedal or use knuckle‑walking.
• Brain Size: Average cranial capacity: ~1300‑1400 cm³ in humans vs. 300‑500 cm³ in chimpanzees.
• Tool Use: Humans manufacture complex, multi‑part tools; apes use simple tools (e.g., sticks for termite fishing) but lack cumulative cultural improvement.
• Language: Humans possess symbolic, generative language with syntax; apes communicate via limited vocalizations and gestures.
• Culture: Humans exhibit cumulative cultural evolution (technology, art, religion); ape cultures are relatively static.

Evolution of Modern Man from Anthropoid Ancestors

The widely accepted linear (though branching) sequence from early ape‑like ancestors to modern humans is outlined below.

1. Dryopithecus (Miocene, ~12‑9 mya) – early ape, arboreal, generalized dentition.
2. Ramapithecus (Miocene, ~8‑6 mya) – considered a possible early hominin; thick‑enameled teeth suggesting adaptation to tougher diets.
3. Australopithecus (Pliocene‑Pleistocene, ~4‑2 mya) – bipedal locomotion evidenced by pelvis and femur morphology (e.g., A. afarensis “Lucy”). Brain size ~400‑500 cm³.
4. Homo habilis (Early Pleistocene, ~2.4‑1.5 mya) – “handy man”; first definite stone tools (Oldowan); brain ~600‑700 cm³.
5. Homo erectus (Early‑Middle Pleistocene, ~1.9‑0.1 mya) – fully bipedal, larger brain (~900‑1100 cm³), controlled fire, Acheulean tools, first to leave Africa.
6. Homo sapiens (Middle‑Late Pleistocene, ~300‑30 kya) – brain ~1300‑1500 cm³, symbolic behavior, complex tools, art.
7. Homo sapiens sapiens (Recent, ~30 kya‑present) – anatomically modern humans; refined toolkits, agriculture, civilization.

Key Evolutionary Changes

These transformations illustrate how anatomical, cognitive, and cultural adaptations interacted to produce the unique characteristics of modern humans.