Endosymbiosis is the idea that certain organelles found in eukaryotic cells today, like mitochondria and chloroplasts were originally separate organisms which were absorbed into early eukaryotic cells and had a symbiotic relationship with them. Over time, they evolved along with the cells until they were fully part of the cell and no longer self-sufficient organisms.
The best piece of evidence for this theory is that chloroplasts and mitochondria have their own DNA, which is different than the DNA stored in the nucleus. This makes sense since they would have needed their own genetic material when they were independent organisms. Today, though, much of the genetic code controlling mitochondria and chloroplast production and activity has been incorporated
