I've mentioned this before, but since it's come up again, I figured I'd throw my 2c in. What if they gave their bonus in a one off burst, a la great people? Having the burst at an amount that is relevant early game but scales out by the time we're in the ancient era seems doable. Alternatively, let us sacrifice animal units to discover a tech "myth of _____" which speeds up other techs. These techs wouldn't be otherwise researchable. So myth of horses would speed up any horse related techs (animal riding, etc.) whereas myth of whales would speed up fishing/sailing techs. They'd only speed up early game techs, so would become irrelevant, but you wouldn't lose the bonus when you researched a certain tech, you'd just have used it up when you've researched all the relevent techs. This would mean that people with myth of wolves wouldn't avoid learning how to domesticate dogs to keep their +1 in their cities, but would rush it earlier, since they can get it faster. Thusly, people exposed to the forces that encouraged the domestication of dogs are more likely to get it sooner. Additionally trade, etc. would spread the myths, as people hear about animals from far off lands. . .