Historical Geology Understanding Our Planet\'s Past

Sulfur combined easily with metals such as iron to form sulfates. Since

the atmosphere and ocean lacked oxygen, the bacteria obtained energy by the

reduction of sulfate ions.The growth of primitive bacteria was thus limited by

the amount of organic molecules produced in the ocean. Although this form

of energy was satisfactory at the time,bacteria were allowing a plentiful source

of energy go to waste—namely sunlight.

PHOTOSYNTHESIS

The ratios of carbon isotopes in Archean rocks suggest that photosynthesis was

in progress at an early age.The seas contained an abundance of iron. Oxygen

generated by photosynthesis was lost by oxidation with this element, a fortu-

nate circumstance since oxygen was also poisonous to primitive life-forms.

Abundant sulfur in the early sea provided the nutrients to sustain life without

the need for oxygen. Bacteria obtained energy by the reduction (the opposite

of oxidation) of this important element.

Microorganisms called cyanobacteria began using sunlight as their primary

energy source possibly as early as 3.5 billion years ago. The cells exploited the

Figure 21Tall tube
worms, giant clams, and
large crabs inhabit the
seafloor near hydrothermal
vents.

Historical Geology