What led to the conclusion that single-celled prokaryotic organisms evolved in the absence of oxygen

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products

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d.  

there was a variety of food available on island x, but there were only large seeds and nuts on island y.