Origins of Life

Origins of Life

Important Dates:

·        Universe Created - "Big Bang" - ~12 billion years ago

·        Earth forms: ~4.6 billion years ago

·        Life begins on earth: ~3.5 billion years ago.

 

·        Three major theories of development of life on earth:

1.      Extratarrestrial origin: Life was brought to Earth, either unintentionally, or intentionally.

2.     Special Creation: Life was brought into being by a spiritual "Being".  Most cultures have individual beliefs regarding this, and is the basis of most religeons.

3.     Evolution: Holds that complex organic molecules developed over time through a process of selection.  Here, selection favored molecules which were more able to remain intact in the primitive terrestrial environment.

 

·        Early terrestrial atmosphere was "reducing" - high amounts of Hydrogen.

·        Probably bound as H₂S, NH₃ & CH₄.

·        Little or no free O₂

·        Free O₂ would have probably prevented life from forming, since it tends to break-down organic compounds.

 

·        Early terrestrial environment had large amounts of free energy:

1.      Volcanic

2.     UV Light (Note there was little O₃ as a protective layer

3.     Cosmic rays & other high energy radiation

4.     Earthquake (& meteor impact) shockwaves

5.     Mineral radioactivity

6.     Lightning

7.     Heat

 

 

·        Combination of simple atmospheric compounds & free energy allowed the formation of progressively more complex organic compounds.

 

·        Experimental Evidence:

·        1953: Stanley Miller & Harold Urey

·        Created apparatus to simulate primordial conditions on earth, and applied energy in the form of electricity (lightning).

·        Found that in 1 week 15% of the simple compounds had reformed into complex organic compounds including 2 amino acids glycine & alanine - building blocks of life

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Origin of First Cells

 

Life probably began through the development of Coacervates

·        Small spheres which form SPONTANEOUSLY from lipids in an aqueous environment

·        Form spontaneously into bilayers

·        Can grow and divide

·        Can sequester and promote reactions common to living systems.

·        Evolutionary processes probably led them to form modern cells.  Those coacervates which were more chemically stable, would exist longer and have a greater chance to protect bio-molecules contained within them (esp. nucleic acids).

·        RNA molecules could have been produced first however.

·        Nucleic acids were produced in the Miller-Urey experiment

·        RNA has been found to be able to act as an enzyme, and be able to replicate itself on occasion.

 

First Cells were Eukaryotic:

·        Lacked nucleus

·        No membrane bound organelles

·        Simple circular DNA

 

·        First cells were anaerobic - do not use O₂ for respiration.

·        Were strictly heterotrophic  No photosynthesis

·        2.5 byears - First photosynthetic cyanobacteria appear.

·        Engaged in photosynthesis and produced free O2

·        Caused mass extinction since O₂ was highly toxic to primitive bacteria

·        1.5 byears - Aerobic respiration appears as adaptation to increased O2 levels in atmosphere.

·        1.2 byears - First eukaryotes appear.

·        Endosymbiotic Theory - Dr. Lynn Margolis

·        Large prokaryotic cell ingested the following:

·        Ingested prokaryotic aerobic bacteria Þ Mitochondria

·        Ingested photosynthetic bacteria Þ Chloroplasts

·        Evidence.  Both mitochondria and chloroplasts can replicate themselves and have own circular DNA.

·        600 Myears Cambrian explosion - Multicellular organisms appear and proliferate