Free oxygen formed on earth after
(1) photosynthesis occurred
(2) much of the iron in the oceans had oxidized
(3) two phase changes of water
(4) surface temperatures vaporized all the water

(1) photosynthesis occurred

Explanation: The photosynthesis process in cianobacteria changed the composition of earth's atmosphera, rising oxygen levels. This could be possible since photosynthesis used energy from the sun to produce energetic intermediaries to synthesize sugars, and as byproduct due to water photolisis, is oxygen.

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lesson summarykaryotypes a genome is the full set of all the genetic information that an organism carries in its dna. chromosomes arebundles of dna and protein found in the nucleus of a eukaryotic cell.