Norbert: My wife has the luxury of debating which ice cream flavor she wants. I wish I had that problem!

Dave: Me too! My body no longer makes enough lactase to break down the sugar in dairy called lactose.

Norbert: But you know, none of our human ancestors could break down lactose past childhood once they stopped nursing. So why did that change?

The prevailing theory is that when humans moved north and got less sun, their bodies produced less vitamin D, forcing them to find another source. Since milk contains the vitamin - though not much - it was an alternative.

Scientists believed humans that developed a mutation, a variant of the lactase gene, allowed them to digest milk, giving them a selective advantage. But a new study provides another theory.

Researchers took DNA samples from skeletons in northeastern Spain, fully expecting them to carry the mutation. These skeletons are five thousand years old and we know the evolutionary shift came about eight thousand years ago allowing many northern Europeans to digest milk.

To their surprise, none of the samples carried the mutation, even though DNA sequencing confirms they're related to modern-day Spaniards. A computer simulation showed shifting a population from one that could not digest milk five millennia ago to today's where a third of Spaniards can, requires a strong selection.

That means some strong selective pressure favored that trait. Researchers believe early farmers turned to milk when food became scarce. Those with the mutation would have thrived while others would have suffered.

This doesn't debunk the vitamin D theory, but certainly questions it.

Norbert: Now if only I could get my lactase gene working again...