"So there's one example of free living ... with bacteria living inside it. It wasn't . It's got a cell wall and it's not a . So they can get inside, but we can say for sure it's rare. What does it do? In a nutshell it changes the topology of the cell. It allows you to internalize respiration and it's not just internalizing the membranes. It's internalizing a genetic control system with in our own case, which by standard selection is whittled down to a kind of minimal unit required to do the job, and that in effect allows the nuclear genome to expand up to anything it wants to be. So... it's a structural change. It's not something which you can find by genetic exploration of the evolutionary space. It's something [in] which you change the topology of a cell. And once you've got that, you've got bacteria living inside another bacterial cell. You've got a fight on your hands! They've got to get along with each another somehow. So the chances of it going wrong is quite high. So I would imagine if we know of one or two examples now, there must have been thousands, millions, billions of cases of this over earth history. The fact that... all this searching across the earth that we've done for life, we find bacteria, we find , we find these candidate phyla. We're not sure what they are, exactly, but they seem to be very simple and probably s, and we see Eukaryotic cells, all the cases that appear to be potentially evolutionary intermediates, something slightly different, have turned out to be highly derived... from more complex ancestors."