DAVID: And now an exactly opposite viewpoint, that mitochondria did not get engulfed:-http://www.origin-of-mitochondria.net/?page_id=222-As usual, there is no consensus, and there will probably never be one since we have no way of knowing for sure what happened. All the more surprising, then, is the conclusion to this article:-"In order for an evolutionary theory to be considered a scientific fact or a valid scientific theory, there are some basic requirements. First, it is necessary to have a reasonably detailed mechanism that explains the basic steps in the endosymbiotic scenario. Second, this mechanism should be placed in the context of current Darwinian evolutionary theory and should contain no fundamental problems or falsifications. Third, a substantial body of empirical evidence that directly supports this scenario should be present. Fourth, no credible or logically sound alternatives should exist. If these criteria are not met, the endosymbiotic theory cannot be considered to be a scientific fact that has been proven beyond reasonable doubt. Remarkably, the endosymbiotic theory fails all points."-First and third, I can't argue about the mechanism or the evidence, but Lynn Margulis can and does. Fourth, who judges whether a scenario is credible or logically sound, and by what criteria (see Point 2 below)? Fifth, no theory can be considered to be a scientific fact until it has been proven beyond reasonable doubt, and since no evolutionary theory on the subject of origins or innovations has been proved beyond reasonable doubt, none of them is a fact, which brings us to Point 2: why "should" the endosymbiotic mechanism be placed in the context of current Darwinian evolutionary theory? This is tantamount to saying that all aspects of current Darwinian evolutionary theory are scientific fact, but neo-Darwinism has no satisfactory solution to the problem of origins or innovations, and I would have thought this problem was "fundamental". Reliance on the mechanism of random mutations and the possibility that gaps in the fossil record will one day be filled can hardly count as "a substantial body of empirical evidence" proving the theory to be "beyond reasonable doubt". Since it's not a fact, why should all theories be placed in its context?-I remain convinced that much of Darwin's theory is true, that evolution happened, that the non-creative process of natural selection is going on all the time, but innovation remains its Achilles heel, and as yet there are no scientific facts to explain it. Nothing but theories, of which Lynn Margulis's sounds to me at the very least no more improbable than that of Darwin's random mutations.