Which organisms are called primitive and how are they different form so-called advanced organisms?

The organisms having characteristics that came into existence earlier are referred to as “primitive” and the organisms having characteristic that have come into existence later on are called, ‘advanced’. For example, unicellular prokaryotic bacteria are primitive and multicellular eukaryotic Mammalia are advanced. Likewise, algae are primitive plants and angiosperms are advanced plants. Fishes are primitive vertebrates and birds are advanced vertebrates.