The idea that multicellular organisms evolved from single celled microorganisms is based on a hypothesis that involves an endosymbiont relationship (a cell that lives within a host cell). The endosymbiont proposal in effect, attempts to explain the existence of multicellular organisms by suggesting that all eukaryotic cells (cells with membrane-enclosed nuclei and membrane-enclosed organelles) evolved from prokaryotic cells (cells without membrane-enclosed nuclei or membrane-enclosed organelles) via symbiotic associations.
Evolutionists will attempt to justify the endosymbiotic origin of eukaryotic cells by pointing to the existence of endosymbiotic relationships in nature today. They also point out that similarities between chloroplasts and mitochondria should be considered as a supporting evolutionary foundation for endosymbiotic theory. However, the existence of endosymbiotic biological relationships in the world today does not demonstrate that eukaryotic cells evolved from prokaryotic cells. Furthermore similarity does not prove common origin. It is actually evidence for an intelligent designer.
While endosymbiotic theory is an interesting illusion, there is no evidence it ever happened to create multicellular organisms. There has also been no lab experiment demonstrating that an endosymbiont relationship can provide the required mechanistic pathway for the evolution of eukaryotic cells from prokaryotic cells. If evolutionists want to spout diatribes about the scientific method, then lets require them to stand by their own musings.