No one is quite sure how eukaryotic cells first appeared on Earth.Three hypotheses have been proposed for ongoing study: the eukaryote-first hypothesis, the nucleus-first hypothesis, and _________.(Outcome #4b)

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The two other hypotheses are not widely accepted and lack substantial evidence. The nucleus-first hypothesis proposes that a nucleus formed first and then engulfed bacteria to become eukaryotic, but it does not explain the origin of other eukaryotic organelles such as mitochondria. The eukaryote-first hypothesis suggests that eukaryotes evolved before prokaryotes, which is not supported by fossil evidence. The hypothesis that mitochondria developed in a prokaryotic cell that later acquired a nucleus is called the endosymbiotic theory and is widely accepted as the origin of eukaryotic cells.

No One Is Quite Sure How Eukaryotic Cells First Appeared