Complex sentence: The emergence of this metabolic pathway was very important in the pre-oxygenated oceans because adaptations to otherwise inhabitable or "toxic" environments like this may have played a role in the diversification of early eukaryotes and protozoa in the pre-Phanerozoic.
Split into simpler sentences: The emergence of this metabolic pathway was very important in the pre-oxygenated oceans. Adaptations to otherwise inhabitable or "toxic" environments like this may have also played a role in the diversification of early eukaryotes and protozoa in the pre-Phanerozoic.