Except, inexplicably, the last

Nearly all animals produce their own vitamin C, but we can’t.
We undertake every part of the process except, inexplicably,
the last step, the production of a single enzyme. —Bill Bryson

          After however
many steps to manufacture
         our own vitamin

          C, we
stop. I could imagine
         in that some

          diligent divinity
who, having mostly finished
         the delicate design

          and straightened
out unexpected kinks, starts
         from the workbench

          and thinks,
I bet they’d like
         oranges, and arranges

          our one
missing enzyme, a shortfall
         that means eons

          later some
ballyhooer will stand blossom-besotted
         and belly-holler blithely

          (if also
somewhat ironically) By Jove!
         during a jaunt

          through those
sweet-scented acres that compose
         his citrus grove.