might be various high energy compounds. Primitive cells used some of these
compounds as energy source. After the compounds were exhausted by the living
organisms, other compounds of similar structure might be new energy source for
other living organisms which acquired enzyme for the new compounds by genetic
mutation. The living organisms sequentially acquired new mutated enzyme and
energy metabolic system extended (Fig. 1.4). Therefore, evolution is essentially
progress of energy usage system.
1.1.4.1 Chemoautotroph
Sulfur bacteria are a diverse group of microorganisms capable of metabolizing sulfur
and its compounds. These bacteria use hydrogen sulfide (H 2 S), sulfur, and thiosulfate (S 2 O 3
2- ) as energy source and finally oxidized to sulfate (SO 4
2-) as shown in
following equations.
Mother LO
A
A is available
↓
Mutant (1)
B→A
B is available
↓
Mutant (2)
C→B→A
C is available
↓
Mutant (3)
D→C→B→A
D is available
Fig. 1.4 Evolution of living organism (LO)
Evolution of metabolism caused by change of nutrition source
(1) Mother species that require organic compound A absolutely as nutrient for living
(2) When A is exhausted, mutant (1) that obtains enzyme converting B to A propagate by use of B
(3) Mutant (2) and mutant (3) propagate similarly by use of organic compounds C and D,
respectively. The schematic image is made according to his hypothesis of evolution [Horrowitz,
N.H.: Proc.NAS USA 31 (1945), 153]
1.1 Origin of Life
7