single-celled eukaryotes around 1,500 Ma.  Sexual reproduction  may also have arisen  as early as ????.  Meiosis/mitosis? req's cytoskeleton? All animal phyla are multi-cellular, oxygen-breathing ( aerobic ) eukaryotes. All develop from a blastula . Single-celled eukaryotes are classified as Protoctista, with an uncertain date of origin, probably beginning around 1,750 Ma.