>Many people around the world do not have adult onset lactose
>intolerance (myself included).  I've seen studies that examined cultures
>that started keeping cows (which is the only way to get dairy in your diet,
>realistically) and found that populations whose ancestors kept cattle for a
>certain amount of time (I think it was around 1500 years, but I'm not sure)
>had relatively little lactose intolerance, while those whose ancestors
>didn't have cattle or had them for less time had a lot of lactose
>intolerance.

I too, being lactose tolerant, have wondered about the same thing. There are populations in the world who are 99%+ lactose intolerant (most Amerindians and many Asians), yet some populations swing the exact opposite. I think it's fascinating.

After doing a bit of reasearch I discovered that there is a gene that actually turns off an adult's lactose "factory". So, those of us who can tolerate lactose as adults are, in reality, "mutants". We have a defective gene. It all really boils down, once again, to natural selection - we are around because our mutation was not filtered out of the gene pool. It gave someone an advantage. Perhaps it allowed some small group of late paleolithic people to survive a cataclysm. Or, perhaps it was just perceived as a beneficial, or even superior, trait in some cultures. At any rate - it stuck.

At the beyondveg.org site (which some members here seem to despise for whatever reason), there is a lengthy interview with Ward Nicholson. In it there is a section that explores how long it takes for a given genetic trait to stick. If I recall, the numbers ranged from a few thousand years to tens of thousands depending on how drastic the change was. Which is why a few cultures, most notably in the Middle East, seem to have a greater tolerance for Neolithic foods than others - obviously because they were exposed earlier. Since I am of Northern European stock, I suck at eating grains - and especially beans.