To the extent that science proves anything, it answered the question for two eukaryotic organelles a long time ago. Mitochondria and chloroplasts evolved from endosymbiotic associations between an ancestral host cell and smaller prokaryotic partners. In the case of chloroplasts, the symbiont was a photosynthetic cyanobacterium; for mitochondria, most likely it was ana-proteobacterium.
The cytoplasm of eukaryotic cells is like chicken soup-it's chock full of organelles suspended like chunks of assorted vegetables and noodles in cytosolic broth. The broth also contains filaments of various dimensions that collectively comprise the cell's cytoskeleton. Like the bones of a large animal, the cytoskeleton provides a structural framework lending shape to cells and against which enzymatic 'muscles' work to elicit movement. That's how amoebae migrate, algae swim, stem cells divide, and cytoplasm streams relentlessly up, down, and across plant cells.
While the cytoskeleton is as much a hallmark of eukaryoticity as any mitochondrion or ...
