The theory of serial endosymbiosis states that eukaryotic cells, of which plants and animals are composed, evolved from the symbiosis of smaller aerobic and autotrophic prokaryotic cells (bacteria) living within larger prokaryotic cells. Unlike plant and animal cells, prokaryotic cells contain no membrane bound organelles or an organized nucleus. Based on this theory, organelles such as the mitochondria in modern eukaryotic cells would be the evolutionary descendents of the aerobic prokaryotic cells engulfed by the larger prokaryotes. Mitochondria have a few minor characteristics that are similar to bacteria, such as their size, lack of introns in the mitochondrial genome, and a bi-layer cell covering that have been used to support the theory of serial endosymbiosis. However, there are also significant differences that make the transition of a bacterium into a eukaryotic mitochondrion impossible. Mitochondria and bacteria both have ribosomes, made of protein and ribonucleic acid (RNA), to catalyze the synthesis of proteins. When the theory of serial endosymbiosis was first proposed, it was assumed that ribosomes occurred in only two forms: a smaller 70S variety found in prokaryotes and a larger 80S ribosome found in the cytosol of plant and animal cells (eukaryotes). According to the theory of serial endosymbiosis, the ribosomes present in mammalian mitochondria were expected to resemble the prokaryote 70S ribosome. However, the structure of mammalian mitochondrial ribosomes and their RNA and amino acid sequences indicate that mammalian mitochondrial ribosomes are completely different from prokaryotic ribosomes. The evolution of mammalian ribosomes from prokaryotes requires major mutation and selection events to change a prokaryote-like ribosome into the mammalian mitochondrial ribosomes observed today. However, computer simulations with yeast and human genomes have shown that natural selection is unable to create new beneficial structures from random mutational events. Experiments introducing minor changes in the RNA and protein sequences of ribosomes have also demonstrated that these changes are deleterious and lead to decreased fitness. It is apparent from the knowledge gained about mitochondria ribosome structure and function since the proposal of the Serial Endosymbiosis Theory that prokaryotes are not the ancestors of eukaryote or mammalian mitochondria.