"Stuff happens but we don’t know why," might be an apt description of assumed evolutionary innovations such as complex flight feathers.
A recent paper in Nature Communications begins with these words:
“Adaptation of feathered dinosaurs [sic] and Mesozoic birds to new ecological niches was potentiated by rapid diversiﬁcation of feather vane shapes. The molecular mechanism driving this spectacular process remains unclear.”
They don’t know why these feathers appeared. Then they attempt to give a jargon-filled explanation:
"Here, through morphology analysis, transcriptome proﬁling, functional perturbations and mathematical simulations, we ﬁnd that mesenchymederived GDF10 and GREM1 are major controllers for the topologies of rachidial and barb generative zones (setting vane boundaries), respectively, by tuning the periodic-branching programme of epithelial progenitors."
This is followed by more jargon, until they dish out an orthodox Darwinian explanation:
“Incremental changes of RA gradient slopes establish a continuum of asymmetric ﬂight feathers along the wing, while switch-like modulation of RA signalling confers distinct vane shapes between feather tracts. Therefore, the co-option of anisotropic signalling modules introduced new dimensions of feather shape diversiﬁcation.”
Next, the birds figured out new uses for their feathers:
“Major novel functions of feathers that evolved include endothermy, communication, aerodynamic flight and so on. These are achieved through stepwise retrofitting of the original feather forms.”
The authors invoke a “self-organizing branching programme,” as if feathers were no more complex than snowflakes.
However, as we can learn from the real world, stuff does not just happen without a good reason.