 Endosymbiotic theory explains how scientists believe eukaryotic cells developed. First the nucleolus was surrounded by the unfolding membrane creating the nucleus. Then further unfolding membrane formed the endoplasmic reticulum. Then the cell engulfed in aerobic bacteria which would ultimately become the mitochondria. Afterward a cell with mitochondria engulfed a photosynthetic bacteria. This photosynthetic bacteria would become the organelle known as a chloroplast. Mitochondria and chloroplasts contain their own small supply of DNA which may be remnants of the genome they had when they were independent bacteria. It is important to note that we know that aerobic bacteria were engulfed first because there are animal cells which lack chloroplasts while both plant and animal cells contain mitochondria.