If you look at the supposed transition from simple bacterial life to modern life, a key step early on in that process is going to be the transition from what’s called prokaryotic life – simple bacterial life – to eukaryotic life. All complex life has a cell type that’s called eukaryotic cell type where the DNA is packaged in a nucleus, the cells are larger, you have all kinds of organelles, mitochondria, golgi apparatus, endoplasmic reticulum. All of these structures that don’t exist at all in the bacterial world, they exist everywhere else. Complex life has these structures. Fundamentally different ways of doing cellular life separates the bacterial world from the eukaryotic world. So the question is: How did you get from that simpler world to the more complex world? How do you get these new structures within the cell that don’t exist in bacterial life?
It doesn’t seem plausible at all that you could do it simply by mutating a bacterial cell because mutating the DNA is not going to form some structure within the cell. I don’t think anyone believes that’s how it happens. There are theories that have been put forward and one is called the endosymbiotic theory for transitioning in evolution from simple bacterial life to eukaryotic life. And perhaps the most plausible account of this and this has been advanced since the early 20th century, and then it was additional evidence for it was given by Lynn Margulis in the late 1960s. People noticed and Lynn argued this quite convincingly that there’s a similarity between mitochondria which are the powerhouses in eukaryotic cells. So all of our cells have mitochondria, they produce ATP by oxidizing chemicals. If you look at bacteria, they also produce ATP by oxidizing chemicals. The mitochondria have a little wall around them and a membrane and they pump protons. Bacteria have a wall around them and a membrane and they pump protons and those protons are used for ATP biosynthesis. The size of the mitochondria inside a eukaryotic cell is similar to the size of some small bacteria so there’s connections that make one think “Could it be possible that a bacterial cell – one small bacterial cell – got enveloped by a larger bacterial cell and that became a mitochondrion?” It’s somewhat suggestive. It doesn’t work though if you look at the other organelles. There’s no way to account for the endoplasmic reticulum. There’s no way to account for the golgi apparatus. Furthermore I don’t think there’s a feasible demonstration even for the mitochondria. No one has shown that one cell can swallow another and then let it take up residence and become a powerhouse inside the bigger cell.