Earth's early atmosphere had no oxygen...?

Question

There is evidence that the earth's early atmosphere had no oxygen.

(a) If the earliest organisms were heterotrophs in the oceans, how did they obtain energy from available "food" molecules?

(b) Following the heterotrophs, what group of organisms was formed that produced oxygen found in the atmosphere?

Answer 1

The earliest organisms were autotrophs!

Answer 2

(a) First heterotrophs were anaerobic. In our metabolism, oxygen is used as a final electron acceptor. But a large variety of other acceptors can be used to achieve a partial (= fermentation) or complete (= anaerobic respiration) oxidation of the "food" molecules, like nitrates or sulfates.

There may also have been non photosynthetic autotrophs, that used energy rich mineral compounds instead of light to produce food.

b) Bacteria really close to today's Cyanobacteria.

Answer 3

trees! / plants!

Answer 4

Is is said that earth’s early atmosphere was full of methane. When photosynthesizing organisms were created and started evolving oxygen levels were very high. Oxygen caused an explosion in the earth and all methane disappeared leaving behind oxygen and nitrogen.

Also the first organisms created where in the ocean. Those organisms were created under evolve from a single cell and they were feed from photosynthesizing organisms [plants] .  

Answer 5

the earth's atmosphere was full of methane~