The foundations of genomic complexity in multicellular animals have deep roots in their unicellular prehistory, both in terms of innovations in gene content, as well as the evolutionary dynamics of genome architecture.
Sponges and ctenophores lack hypoxia-inducible factors, suggesting that the metazoan last common ancestor could have lived aerobically under severe hypoxia and did not need to regulate its transcription in response to oxygen availability.
The transition to the aggregative stage of Capsaspora owczarzaki, a close unicellular relative to Metazoa, is associated with significant upregulation of orthologs of genes that are important for multicellularity in metazoans.
Cellularization in Sphaeroforma arctica generates a self-organized structure that morphologically resembles an epithelium, and is associated with tightly regulated expression of cell adhesion pathways.
Molecular profiling of annelid myocytes reveals that the last common protostome-deuterostome ancestor already possessed a dual musculature, with visceral smooth muscles ensuring digestion and somatic striated muscles ensuring locomotion.