Origine of life

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When life first evolved on earth some 4 billion of years ago:
Atmosphere was probably reducing (no oxygen present), electrically charged and hot, and water waspresent. Organic molecules such as aa, sugars, nucleotide bases formed spontaneously (Miller Urey experiment supporting the pre-biotic soup theory)
Pre-cells or Protobionts: these early life-like“particles” made of polymers of organic molecules were able to divide by binary fission, maintained homeostasis and exhibited catalytic activity.

: Used RNA as informational molecule instead of DNA.RNA is capable of self-replication and exhibits specific catalytic activities including peptide bond formation.

First cells:
• prokaryotic chemoheterotrophs, used organic molecules fromenvironment (from the pre-biotic soup)
As organic compounds became scarce (rare), cells became autotrophs fixing CO2 into organic compounds.
• (Chemoautotrophs: using energy fromoxidation of molecules such as NH3 to fix CO2)
• Photoautotrophs: using light energy to fix CO2:
Cyanobacterial ancestors were first water-splitting photosynthetic(photoautrophs): O2 was produced as byproduct
• Only when O2 accumulated in environment did the first aerobic heterotrophs evolve:
These obtained the maximum amount of energy from theoxidation of food.

Endosymbiont theory – the origin of eukaryotes
Ancestral anaerobic heterotroph engulfed aerobic prokaryote (
Endosymbiotic relationship established between prokaryote andeukaryotic host cell (
First aerobic eukaryotic heterotroph: prokaryote became mitochondria
A second endosymbiotic relationship established when an aerobic eukaryote engulfed an ancestral cyanobacterium (First plant like photoautotroph (probably ancestral green algae): cyanobacteria became chloroplast

Evidence for Endosymbiont theory
1) chloroplasts and mitochondria have inner and outer...