Evolutionary Origins

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Age of the earth

  • Data suggests earth formed with rest of solar system 4.5 billion years ago.
  • First part of earth history - earth was bombarded by comets and small planets which steralized it. 
  • Data for the age of the earth derived from looking at the ratios of radioactive elements - typically using uranium-lead dating.
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Key moments in Evolution

1. The origin of Life.

2. The oxygenation of the atmosphere.

3. The origin of Eukaryotic cells.

4. The origin of multicellular organisms. 

5.  The Cambrian Explosion.

6. The colonisation of the land by complex organisms.

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Stanley Millers experiment

  • Used electricity to provide energy to a closed system.
  • After incubation, amino acids formed - the building blocks of life.
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Where does life arise from?

Hypothesis 1 - The Primordial soup 

The sea acts as a trap, collecting amino acids, nucleotides and other organic molecules as they form.

Were then thought to make up macromolecules (protein and DNA)

One problem - soup unlikely to be concentrated enough 

Hypothesis 2 - Hydrothermal Vents in deep oceans

Far more concentrated solution of organic molecules

Also give off energy as heat - essential for life to develop.

These black smokers found on the ocean floor now and have ecosystems that are independent of the sun. 

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The first cells

  • May have been little more than bags enabling higher concentrations of nutrients to build up and construction of biologically useful molecules.

Other possibility - 

  • first cells were lipid vesicles formed becuase fatty acids have 1 hydrophillic end and 1 hydrophobic end meaning fatty acids will aggregate into a sphere in aqueous solution.

Early life dominated by prokaryotes 

  • lipid plasma membrane
  • DNA
  • Ribosomes
  • Cytoplasm
  • Size - 1 - 10 micrometres
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  • Oldest fossils - Apex chart - consist of filaments of cells 'strung' together and date to 3.5 billion years ago.
  • Stromatolites - consist of thousands of layers of individual bacteria.
  • Earliest organisms - cynobacteria - photosynthetic.
  • Many of the first organisms would have originated in an atmosphere that was lacing in O2, and in some cases O2 toxic. Some of these bacteria exsist to this day - anaerobes.
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Oxygenating the Earth

  • Even though PHS began 3.5 bil years ago, it took until 0.5 bil years ago to have significant levels of O2 in the atmosphere.
  • Most of the first O2 produced reacted with dissolved iron in the oceans and was deposited as sediment on the ocean floor.
  • Banded - iron formations are charactersistic of rock formed from that period.
  • Now PHS by cynobacteria results in free O2 being released into the atmosphere or dissolve in water. 
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  • Animals, plants and protists - 5 kingdom model suggests progression.
  • Based on examination of DNA sequences - now clear 3 domains of life.
  • Both archaea and bacteria originally classed as prokaryotes but exhibit some fundamental differences.
  • Eukaryotic cells - similar basic structure to prokaryote cells but more complex with organelles
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Single celled Eukaryotes

  • Giardia was once thought to resemble the ancestral eukaryote as it lacks mitochondria - however this may have been lost in the species. 
  • Amoeba species quite large, 1mm in diameter, visible to naked eye.
  • Paramecium - found in fresh water & feed by ingesting bacterium.
  • Photosynthetic ability of single celled eukaryotes derives from endosymbiosis of chloroplasts.
  • Multicellular plants though to have evolved from single celled eukaryotes (algae).
  • Non photosynthesising organisms similar to amoeba give rise to multicellular animals.
  • Multicellularity evolved many times - plants, fungi, animals.
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Multicellular Organisms

  • Size increased -> compete more successfully for food.
  • In some sense multicellularity from colonies (derive from same original cell).
  • Dichtyostelium - response to starvation - single celled organisms aggregate to form a 'slug' which migrates to a better food source. Some differentiation occurs during formation of the fruiting body.
  • Volvox - evolved by colonial method - resembling chlamydamonas cells. 
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The Ediacaran Fauna

  • Dominated the sea 600 mill years ago.
  • Fossils are rare - most organisms soft bodied.
  • Some may have been related to cnidarians but most have no living descendents that we recognise. 
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The Cambrian Explosion

  • A period of experimentation in body plan - some survive and some do not.
  • Oxygenation of the atmosphere reaches significant levels.
  • Animals with skeletons appear - arthropods, chordates, molluscs - fossils more abundant. 
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