What does this equation mean?

The logic of that adjudication reduces to one ratio. For sequence data compared under a universal-common-ancestry model against an independent-origins model, $K = \frac{P(D \mid M_{\text{UCA}})}{P(D \mid M_{\text{IO}})}$. and K , the Bayes factor, states exactly how many times more probable the observed data D are under one model than the other - not a correlation coefficient, not a subjective sense of resemblance, but a single number with a precise probabilistic meaning that can in principle come out favouring either model. For the proteins Theobald examined, it was not close. Universal common ancestry came out favoured over the closest competing multiple-ancestry hypothesis by odds reported on the order of ten to the power of two thousand eight hundred and sixty to one, and over the specific alternative that humans arose independently of the rest of life by something on the order of ten to the power of six thousand to one - numbers large enough that Theobald himself, discussing the paper around its publication, called it beside the point to state them as ordinary betting odds. The result held, notably, even when the alternative models were allowed to include horizontal gene transfer and symbiotic fusion events rather than only a strictly bifurcating rival tree [ 5 ] .