Metazoan diversification occurred during a time when atmospheric oxygen levels fluctuated between 15 and 30%. The hypoxia-inducible factor (HIF) is a primary regulator of the adaptive transcriptional response to hypoxia. Although the HIF pathway is highly conserved, its complexity increased during periods when atmospheric oxygen concentrations were increasing. Thus atmospheric oxygen levels may have provided a selection force on the development of cellular oxygen-sensing pathways.
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