After years of study into the matter, they have drawn a blank. Here are additional statements by scientists, in which they discuss this mystery. For more, you will want to open the file, Scientists Speak about Speciation . This includes the top names in the field. The facts fail to give any information regarding the origin of actual species, not to mention the higher categories.
Taxonomists are the ones who decide which species are related to which.
It comes as rather a surprise to most nontaxonomists how uncertain our understanding of degrees of relationship among organisms still is today. For instance, it is still unknown for most orders of birds which other order is a given order's nearest relative. The same is true for many mammalian families and genera, for instance the Lagomorpha, Tubulidentata, Xenarthra, and Tupaia. Honesty compels us to admit that our ignorance concerning these relationships is still great, not to say overwhelming. But modern biologists have abandoned the notion, for they know that the gradations do not exist.