This is too weird:
Thus when dogs were attracted to something, including a benign, approachable cat, their tails wagged right, and when they were fearful, their tails went left, Dr. Vallortigara said. It suggests that the muscles in the right side of the tail reflect positive emotions while the muscles in the left side express negative ones.
That's from a NY Times article by Sandra Blakeslee. The whole article's about this dog tail-wagging emotional asymmetry.
And then there is all this:
Honeybees learn better when using their right antenna, she said. Male chameleons show more aggression, reflected as changes in body color, when they look at another chameleon with their left eye. A toad is more likely to jump away when a predator is introduced to its left visual field (right brain/fear). The same toad prefers to flick its tongue to the right side when lashing out at a cricket (left brain/ nourishment).
Chicks prefer to use their left eye to search for food and right eye to watch for predators overhead, Dr. Rogers said. But when chicks are raised in the dark, they do not develop normal brain asymmetry. In trying to eat and watch for hawks overhead, such nonlateralized chicks become confused and vulnerable to attack.
Now that's one messed-up experiment. Chicks raised in the dark, suddenly put out in the open where hawks are circling overhead.
And left-handed chimps are more fearful of novel stimuli than right-handers. Their dominant right brains may make them more cautious.
The article ends with a bunch of adaptive-sounding explanations for asymmetry and lateralization of "approach and withdrawal" traits, but nothing very convincing. Personally, I would guess the mechanism is essentially like gene duplication: you get two copies of something, and one of them may mutate to take on new functions. Lateralization should be favored as a pathway above functionally redundant brain structures.
But then, there seems to be incredible plasticity to much of brain development, including lateralization in humans. Maybe lateralization in humans has high plasticity because enlarged human brain sizes are comparatively recent -- there hasn't been a lot of time for the evolution of functional lateralization in the new volume of the neocortex. As it becomes clearer what is new and what is old in the human brain, there will be the chance to test hypotheses about the origins of lateralized functions.