Fish didn't evolve lungs for breathing air; they repurposed an existing organ

Shubin's opening example sets the tone for the whole book: when ancient fish began venturing onto land, they did not evolve a wholly new organ purpose-built for breathing air. Instead, an organ some fish already used for buoyancy control was gradually repurposed to extract oxygen from air, a function it hadn't originally served. The same underlying genetic toolkit responsible for building buoyancy organs in modern fish overlaps substantially with the genetic instructions that build lungs in humans and other land vertebrates.

This matters because it undercuts a common, intuitive picture of evolution as inventing solutions tailored precisely to new problems. Instead, Shubin shows a messier, more opportunistic process: whatever anatomical and genetic material happens to already exist gets nudged, through selection, toward a new use, rather than being designed from scratch for that use.

The same pattern recurs with feathers, likely evolved initially for insulation or display rather than flight, only later becoming useful for generating lift once their structure happened to allow it.

Takeaway: look for the earlier, unrelated function before assuming any biological feature evolved specifically for its current, most obvious use.