Rob Bartlett wrote:

 > Since fishing didn't evolve until 20,000 years ago, how did man
 >obtain these relatively rare fats needed for brain development if he wasn't
 >eating fish?

I posted this link last summer:

Why Childhood Lasts, and Lasts and Lasts
http://www.nytimes.com/2002/07/02/science/social/02CHIL.html

It includes:

At low tide, young and old alike rush out to the reef. Women hurriedly
gather up shellfish like conchs, clams and cowries, breaking open the
shells to extract the meat and so keep their burdens bearable. Men aim
bamboo spears tipped with iron to lance up snappers, sea perch, cod and
squid; or they toss out baited hand lines to yank in needlefish, perch,
tuna and mackerel. Boys and girls -- some of them barely old enough to walk
-- gather, spear and fish by hand with equal zeal.