Lecture 2 Life Origins: Lecture Notes

History of Life on Earth


  1. How did life originate from non-life?
  2. How does one species split into two?
  3. How is the tree of life formed?

Origin of Life (4 theories)

  1. Primordial Soup hypothesis: Earth's atmosphere was strongly reducing, some form of energy (e.g. lightening or UV radiation) and the existing ammonia and methane created simple organic molecules
    • Substantiated by Miller and Urey experiment
  2. Pockets of Reduction: necessary amounts of reducing compounds and energy were found in ocean vents
  3. Chondrites (carbon-rich meteorites): landed on earth, contained both D and L form of organic structure found in life, suggesting extra-terrestrial origin of life
  4. Non-scientific: founded in religious and philosophical debate

Endosymbiotic theory:

  • Eukaryotic cells evolved through the engulfment of prokaryotes
  • Supported by present-day symbiosis within the modern cell
  • Created branches of the Universal Tree

RNA as a pre-cursor to DNA

RNA has the capacity for coding information and catalyzing the synthesis of other molecules. It's thought to have been the pre-cursor to DNA
On TalkOrigins, they suggest that RNA may not have been the first molecule to code information and catalyze the synthesis of other molecules.

Earth's Early History

  • 4.6 bya: earth formed via condensation of rocks and dust around the sun
  • atmosphere was strongly reducing: major components included ammonia, methane, hydrogen cyanide, water vapour
  • there were no oceans
  • the atmosphere had no oxygen
  • 3.9 bya: first evidence of liquid water
  • nitrogen and carbon dioxide become dominant in the atmosphere, this cooled the atmosphere and caused precipitation of water — development of oceans.
  • 3.8 bya: first evidence of life: chemosynthetic bacteria (i.e. bacteria that use hydrogen or other chemicals to fix energy)
  • 3.5 bya: first evidence of prokaryotic cells in the fossil record
  • 3 bya: photosynthesis evolves
  • 2.5 - 2 bya stromatolites: evidence of prokaryotes
    • oxygen starts to enter the atmosphere. First, it precipitates out with Iron (Iron Oxide), then it starts to accumulate. Over time, the concentration of O2 increases from <1% to 21%
  • 1.5 bya: eukaryotic algae (evolved via endosymbiosis)
  • 0.6 bya: Cambrian explosion

Cambrian Explosion

  • 565 mya explosion of life, diversification and movement out into the ocean (still water-based)
  • 480 mya colonization of land
  • Heterotrophs (consuming organic compounds) pre-date autotrophs (producing organic compounds from inorganic material
  • Still larger diversity of phyla in water today as opposed to land
Just Something Interesting

It was mentioned in the lecture that all animal phyla evolved in the ocean. He means all up to Chordata (animals with a backbone), but I wonder if it could also mean evolution of all subgroups in the phyla. Somebody once came up with a theory that humans may have had aquatic ancestors (whale-like mammals) as well as simian—I can't remember who that person was or what was written about that, but I remember reading that in a fiction story: A Bone From a Dry Sea by Peter Dickinson, and it mentions the name of the person who brought up the idea. You can look it up from there if interested.


  • biodiversity is a dynamic system (speciation vs. extinction - input/output model)
  • Causes: Genetic changes, Long-term evolution, Species interactions, Environmental effects
  • mass extinctions in geological time are associated with climate change
  • there seems to be a pattern of crisis or problem —> adaptation in the history of biodiversity
    • e.g. in the History of Plant Diversification: Non-vascular Plants -> Vascular Plants -> Seedless Vascular Plants -> Seed Vascular

Taxonomy and Speciation

Scientific taxonomy in the context of species is the method of dividing and classifying organisms into progressively more finite groups. We're likely all familiar with the mnemonic device Kings play cards on fat green stools to represent the order of taxa from most general to most specific: Kingdom, Phylum, Class, Order, Family, Genus, Species. Organisms are divided into six kingdoms: Fungi, Protista, Animalia, Plantae, Bacteria, and Archaea. Bacteria and archaea are classified as prokaryotes: organisms that lack internal membranes and organization. Fungi, protists, animals, and plants are all eukaryotes: organisms that contain organelles with internal membranes, like chloroplasts and mitochondria, and are more organized (e.g. DNA organized in to chromosomes contained in the nucleus). Because of their advanced internal organization, eykaryotes are more efficient in energy usage and thus able to organize into larger, multi-cellular organisms.

Recently, however, an even higher classification was introduced: the Domain. The Tree of Life consists of three domains, based on common ribosomal RNA sequences: Archaea, Bacteria and Eukarya. This suggests that all organisms are related by common descent from one original organism. Animals, plants, and fungi are all within the domain Eukarya, and represent only a tiny fraction of genetic diversity on Earth. Alternatively, prokaryotes, which only represent two kingdoms, have been divided into the domains Archaea and Bacteria based on their rich genetic diversity.

Classifying species
Professor Allen introduced three forms of taxonomy for classifying species: morphologically, phylogenetically, and biologically.

  • Morphological classifications, notably the Linnaeus Classification scheme, group organisms by similar characteristics. Because it is based on personal judgment, this system has been criticized as arbitrary and indiscriminate with regard to organisms mimicking the appearance of other organisms.
  • Phylogenetic classifications divide organisms based on similarities or differences in genetic code, assuming that organisms with similar genes have evolved from a common ancestor. The tree of life is a phylogenetic taxonomy.
  • Biological classifications define species as reproductively isolated: they cannot reproduce with organisms of a different species. Organisms that can produce successful and fertile offspring with other organisms are considered to be the same species. Organisms almost always achieve reproductive isolation due to geographic isolation: over a long time period, two populations of the same organism placed on separate islands will form different genetic mutations and adaptations, leading to speciation. This system has been criticized because it is impossible to prove reproductive capabilities of extinct organisms, it cannot be used with spatially disconnected populations, and it fails to clearly classify hybrids.

Speciation: how species are formed
* Focused on types of reproductive isolation: pre-zygotic (before mating) and post-zygotic (after mating)
* Reproductive isolation often occurs as a result of geographic isolation. Hence, it's important to understand geological history

  • Geographic isolation: (dispersal, continental drift, climate fluctuations) Example: Glaciation - fragmenting and uniting species populations
  • Allopatric speciation: Allopatric species are those that do not occur in the same geographic area. Allopatric speciation generally occurs with small populations, or at the edges of a parent population's range

Molecular clock

The mutation rate of DNA in some genes seems to be constant through time. If you know the mutation rate of a gene, then you can plot that rate against time and use it to date speciation. For example, it can be seen by rates of change of hemoglobin (etc) based on time

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