Nothing rings in the new year like a solution of bubbling, neurotoxic ethanol. Humanity's long-standing relationship with alcohol poses an evolutionary puzzle: Surely natural selection would weed out those of our ancestors with a taste for something that clouds judgment, slows reflexes, dulls the senses and impairs balance. Animals in such a state would likely be the first picked off by predators, if they hadn't already fallen out of a tree.

And yet humans all over the world drink ethanol in various concoctions, or they enforce strict rules against it — rules that surely wouldn't exist if there weren't a desire. We've been at it a long time: Archaeologists have found wine and beer stains on 10,000-year-old Stone Age pottery.

Scientists are solving the paradox by studying the enzymes our bodies use to digest alcohol. Lots of animals make these enzymes, called alcohol dehydrogenases, and the way these vary from one species to another tells an evolutionary story. Then there's the related question of whether other species imbibe. Preliminary investigations suggest the answer is yes.