Here is a nice explanation of some science history from: Duboule D and Wilkins AS (1998) The evolution of “bricolage.” Trends in Genetics 14: 54–59.
Although there was abundant evidence by the mid-1970s that many of the products of specialized cell differentiation, the hemoglobins, insulin, steroid hormones, and so on, were widely shared in metazoans, little was known about the regulatory machinery in any of the animal phyla. It was clear, however, that to understand the evolution of different forms would require understanding the evolution of the molecular and cellular regulatory machinery. Yet just how similar or how different the regulatory molecules and mechanisms might be between butterflies and bats was anyone’s guess. Most biologists, however, would probably have bet that diversity and difference in the underlying molecular systems was the general rule.
The work of the past two decades and, in particular, that of the past eight years has demonstrated the reverse: the key regulatory molecules and mechanisms across the world of metazoan are remarkably constant. Indeed, this wholesale conservation of molecular machinery has been one of the most striking findings of recent years. The conserved molecules include virtually all the important classes of transcription regulator and all the forms of the various signal transduction systems, from those based on tyrosine kinase receptors to ion channels. Furthermore, not only can identifiable homologues of these genes be found, but it is also known that several of the conserved molecular systems play comparable general roles in pattern formation and organogenesis in animals that have overtly different adult morphologies and developmental systems. (emphasis added)
So once again, it should be clear that even though the Modern Synthesis had been completed by the 1970s and 80s, it failed to guide scientists about the something as crucial as understanding the evolution of different forms. You may as well say it missed the target on the most important question in evolution. Has this point begun to sink in yet?
Most biologists would have bet “that diversity and difference in the underlying molecular systems was the general rule.” If they turned out to be correct, the hypothesis of front-loading would be extremely weak (and I doubt my book or this blog would exist today). If that bet was won, the machinery needed to generate the different forms would have come into existence along with the different forms, long after any putative design event. As it turns out, this machinery not only is the same machinery among the different forms (much like the same hardware running different software), but thanks to what we have learned in the last 10 years since Duboule and Wilkins wrote those words above, we also know it pre-existed the different forms. “The work of the past two decades and, in particular, that of the past eight years,” made front-loading a serious hypothesis.