A letter in last week's Nature gives evidence that in the British Isles
there was a sharp switch between fish eating and meat eating in about 5 000
BP.  Analysis of C13 content in bones has shown a wide range of values in
remains before this date but a very narrow one (corresponding to the meat
diet) which cuts in afterwards.  The authors speculate that this was the
beginning of herding.

"Marine foods seem for whatever reason to have been comprehensively
abandoned from the beginning of the Neolithic in Britain"

Nature 25 Sept 2003: v425, p366 "Sharp Shift in Diet at Onset of Neolithic"
Richards Shulting & Hedges.

The implication seems to me to be that commercial factors conditioned diet
then, as now!

Dick Bird