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7.4Natural Selection

Origin of Life & Extinction — AP BIOLOGY Revision Notes & Topical Questions

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7.4 Origin of Life & Extinction — AP Biology Notes — AP BIOLOGY notes: Describe hypotheses for the origin of organic molecules and the first cells, and explain how extinction shapes biodiversity.Geological evidence — fossils preserved in rocks and rock composition indicate early …

  • Atmosphere
  • Primitive Earth
  • Polymerisation
  • Protocells
  • Later change
  • Background extinction

Section 1 of 9Evidence for the origin of life

1Geological evidence
  • Geological evidence fossils preserved in rocks and rock composition indicate early environmental conditions
2Timeline
  • Timeline life on Earth originated between 3.9 and 3.5 billion years ago (bya)
  • Earth formed about 4.6 bya.
  • Conditions remained too hostile for life until about 3.9 bya.
  • The earliest fossil evidence for life (such as stromatolites) dates to 3.5 bya.
3Atmosphere
  • Atmosphere early Earth had little free oxygen, so organic molecules were not oxidised

Section 2 of 9Origin of organic molecules

Primitive Earth
  • Primitive Earth it had inorganic precursors, available free energy and time
Models for abiotic synthesis
  • Oparin–Haldane primordial soup, with the Miller–Urey experiment (1953) showing that amino acids form from simple gases and electrical energy.
  • Deep-sea hydrothermal vents providing energy and minerals.
  • Delivery of organic compounds by meteorites.
Polymerisation
  • Polymerisation monomers could polymerise on clay or hot rock surfaces
Protocells
  • Protocells lipids self-assemble into vesicles that can grow and divide, enclosing polymers

Section 3 of 9RNA world hypothesis

Proposal
  • Proposal early life was based primarily on RNA molecules before the evolution of DNA and proteins
Key assumptions
  • RNA functioned as hereditary material and could replicate itself, passing information from one generation to the next.
  • Base pairing allows accurate RNA replication, because RNA strands build complementary copies like DNA.
  • Enzymes were not needed, because RNA could be both genetic material and catalyst.
Support
  • Support ribozymes are RNA molecules that catalyse reactions
Selection
  • Selection faster, more accurate replicators would be favoured
Later change
  • DNA is more stable than RNA because it is double-stranded and lacks the 2′ –OH group, so DNA and proteins later took over most roles

Section 4 of 9From chemistry to cells

1
Simple inorganic molecules react with energy (lightning, UV, vent heat) to form organic monomers.
2
Monomers join into polymers on mineral surfaces.
3
Polymers are enclosed in lipid membranes, forming protocells.
4
RNA acts as a self-replicating genetic material and catalyst, and natural selection begins.
5
DNA later replaces RNA as the more stable information store, and the first cells appear by about 3.5 bya.

Section 5 of 9Endosymbiosis

Theory
  • Theory mitochondria and chloroplasts originated as free-living prokaryotes engulfed by an ancestral host cell
Evidence
  • They have their own circular DNA.
  • They have 70S ribosomes.
  • They have double membranes.
  • They divide by fission.

Section 6 of 9Extinction

Background extinction
Background extinction the normal, continuous, low rate of species loss over geological time
Mass extinction
Mass extinction a geologically brief event in which a large proportion of species die out, and five major ones are recognised in the fossil record
Causes
  • Causes catastrophic changes such as asteroid impact, volcanism and climate change
  • The Chicxulub asteroid impact about 66 million years ago likely caused the mass extinction that ended the age of dinosaurs, shown by iridium deposits and the fossil record.
  • The end-Permian event about 252 million years ago was the largest, with over 90% of marine species lost.
Diversity
  • Diversity when extinction outpaces speciation, diversity falls
Human activity
  • Human activity habitat destruction, invasive species, pollution, overexploitation and climate change raise extinction rates far above background
Vulnerability
  • Vulnerability species with low genetic diversity, small populations or narrow niches are more vulnerable
Recovery
  • Recovery surviving lineages undergo adaptive radiation into vacated niches, as mammals did after the end-Cretaceous event

Section 7 of 9More from the specification

1Universal tree of life phylogenetic trees
  • Universal tree of life phylogenetic trees comparative ribosomal RNA sequencing enables evolutionary biologists to construct universal phylogenetic trees linking all terrestrial life.
2Common ancestor
  • An earlier organism from which different species descended.
3Phylogenetic tree
  • Diagram showing evolutionary relationships between species.
4Selection pressure
  • A factor, such as an antibiotic, that favours survival of some individuals.
5Convergent evolution
  • Similar adaptations arising in different species from shared pressures.
6Allopatric speciation
  • Speciation caused by geographical isolation.
7Genetic drift
  • Change in allele frequency due to chance, most important in small populations.
8Punctuated equilibrium
  • Speciation in rapid bursts between long stable periods.
9Isolation
  • Separation of populations so that they cannot interbreed.
10Resistance
  • The ability of bacteria to survive an antibiotic.
11Cladogram
  • Diagram showing branching pattern of relationships with no timescale.
12Barrier
  • A structure that prevents pathogens from entering.

Section 8 of 9Key terms

TermMeaning
Abiotic synthesisFormation of organic molecules from inorganic precursors without living cells.
Miller–Urey experiment1953 experiment showing amino acids form from simple gases and electrical energy.
ProtocellA membrane-bound droplet of organic molecules, a possible precursor of cells.
RNA world hypothesisThe idea that early life used RNA to store information and catalyse reactions.
RibozymeAn RNA molecule with catalytic activity.
Mass extinctionA brief event in which a large proportion of species die out.
Adaptive radiationRapid evolution of many species from one ancestor as niches open.
Endosymbiotic theoryOrganelles originated as engulfed free-living prokaryotes.

Section 9 of 9Exam technique

1
Saying life began in the first moments after Earth formed — conditions were too hostile until about 3.9 bya.
2
Saying the RNA world hypothesis claims RNA needed proteins as enzymes — it proposes RNA was both genetic material and catalyst.
3
Treating mass extinction as the normal rate of species loss — that is background extinction.
4
Saying extinction only follows natural disasters — human activity is raising current extinction rates.

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