Here is a link to a basic overview of Brain-tanning (or actually tawing as the hide is not actually tanned).  Brain-tanning is a method of preserving hides (often deer) with the natural emulsified oils found in animal brains and then fixing those oils with smoke.   While the use of animal brains for this method is, I believe, unique to North America, oil tanning using fish oils or eggs has been practiced around the globe.  It produces a leather that is incredibly soft and has a multitude of uses, its also an amazing amount of work, trust me.

http://en.wikipedia.org/wiki/Buckskin_%28leather%29#Wet_scrape_method

And don't even get me started on the merits of wet-scrape versus dry scrape.  Or for you Archaeologists out there about stone scrapers (they probably weren't used  on hides, just saying)