In An Ancestor's Tale
, Richard Dawkins dedicates a few pages (pp 29--32) to Belyaev and his tame Sibirian foxes. He suggests that "breeding for tameness" really works out to breeding against the onset of adulthood. In a lot of ways, Belyaev's tame foxes behave like the pups of regular foxes. Similarly, modern dogs behave much like baby wolves.
From this observation, Dawkins goes on to speculate that a similar process of taming might have happened to us. Humans look but much more similar to baby chimpanzees than to adult chimpanzees. The same is true on the level of hormones: Among chimpanzees, only babies can digest milk. Adults, by contrast, are almost universally lactose-intolerant. Human adults, by contrast, usually digest milk just fine---just as you would expect from a species whose members never fully grow up.
Dawkins concludes with a picture that still makes me smile: "Is lactose tolerance just the tip of the iceberg? Are our genomes riddled with evidence of domestication affecting not just our biochemistry but our minds? Like Belyaev's domesticated foxes, and like the domesticated wolves that we call dogs, have we become tamer, more loveable, with the human equivalents of floppy ears, soppy faces, and wagging tails? I leave you with the thought, and move hastily on."