"[A]cquiring mitochondria gives you a headache that can go wrong very easily, but here's an interesting problem in a nutshell. You look at a plant cell under a microscope, or an animal cell, or a fungal cell, or an or something, and you'll recognize the same structure in all of them. They've all got a nucleus. They've all got the s as straight chromosomes. They've all got s. They've all got s. They've all got complexes. They all do as a division mechanism. They all do as two steps where you first double everything and then half it twice. They all go through the same rigmarole. They've all got mitochondria. They've all got the same system, endoplasmic reticulum, things like that. ...[Y]ou could list page after page after page in a text book and it would be exactly the same for a plant, or a fungal cell, or an animal cell. Now they have really different ways of life. If you were to simply think, "Well, there's some inevitability that bacteria will give rise to complex life." ...You would imagine that a photosynthetic bacteria, a would give rise directly to photosynthetic , eukaryotic algae, but they didn't. It was by the intermediary of acquisition of a . There was a common ancestor of eukaryotes that was nothing like a cyanobacterium and nothing... quite like an algae except without the chloroplasts. So... why is it that we all have the same machinery inside, but we have such different lifestyles? Why don't we see multiple origins of complex life where cyanobacteria give rise to photosynthetic trees? Why don't we see predatory bacteria?"