CHAPTER 25 – ORIGIN OF LIFE

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 was present. 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 from environment (from the pre-biotic soup) As organic compounds became scarce (rare), cells became autotrophs fixing CO2 into organic compounds. • (Chemoautotrophs: using energy from oxidation 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 the oxidation of food.

Endosymbiont theory – the origin of eukaryotes
Ancestral anaerobic heterotroph engulfed aerobic prokaryote (
Endosymbiotic relationship established between prokaryote and eukaryotic 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