The first autotrophs probably were ______ and produced ______.

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The first autotrophs probably were photosynthetic and produced no oxygen. Explanation: The earliest autotrophic organisms were anaerobic and likely did not produce oxygen initially. These early bacteria-like prokaryotes synthesized carbohydrates using pigments like bacteriochlorophyll that absorbed sunlight but used substances other than water, such as hydrogen sulfide, as electron donors, resulting in no oxygen release. It was only later, with the evolution of cyanobacteria (true photosynthetic prokaryotes), that oxygenic photosynthesis evolved, producing free oxygen and transforming the atmosphere.

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