Vestigial structures<span> are often homologous to </span>structures<span> that are functioning normally in other species</span>
Photosynthesis creates glucose and oxygen. used for humans to inhale and then exhale carbon dioxide to go back into plants and redo this cycle.
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The correct answer to this question is this one: they lacked oxygen. The earliest organisms were completely heterotrophic in nature because of the reason that they lacked oxygen. <span>A heterotroph is an organism that cannot fix carbon and uses organic carbon for growth. </span>
