Apparently, if you'd paid attention to the actual science of BSG—specifically blood types and machine diseases—you'd have been able to piece together that humans and Cylons would get it on, interspecies style.
At the New York Comic Con's Science of Battlestar Galactica panel—centered around Patrick Di Justo's book of the same title—Di Justo revealed that the show itself gave two very large hints that, in the end, humans and Cylons would become one species.
In the second-season episode "Epiphanies," Helo and Sharon's unborn daughter, Hera, is examined by Doc Cottle, who explains that the hybrid fetus has no blood type—and that, apparently, is strange news to the Colonial survivors. But 40 percent of humanity today has no blood type—Type O—which means that, somehow, that particular Cylon-specific trait got introduced into humans.
And in the third-season episode "A Measure of Salvation," the Cylons come down with a mysterious disease—lymphocytic encephalitis—which is deadly to the machines but to which the show's humans are immune. Now, lymphocytic encephalitis is an actual disease—one primarily carried by rodents—and humans are most definitely not immune to. So how did humanity lose their immunity? Centuries of breeding.
If only we had been a little more alert—and knew a lot more about medicine—we'd have been the King of the Watercooler the day after the finale aired.