where science ends, religion begins.
There are some things about this universe that science cannot explain. Such as love, or any of the other emotions. We can only measure it by contrast to other things, but we cannot truely understand it or predict it. Science still holds onto the "random chance" theory of life coming to be. But life, apparently, is so complex that it wouldn't happen by chance because of a random spark of energy fusing into some carbionized ooze.
But still, keeping on the subject of interbreeding, wouldn't all human beings or pre-human beings progress at the same rate and adapt to their environment? Would there really exist two different species of pre-human beings at the same time? If so, could we have some post-human or rather homosuperior (Mutants, from Marvel Comics?) species walking around that some normal humans may be mating with? Is that why our kids are now getting smarter, getting stronger, and getting bigger? Or at least some of them are, and most of them are not? My three year old is about the size of a six year old, and very strong, and very smart, and if he gets any bigger or smarter, I may have to call in professor Xavier to check out my son for possible mutation? ;)