Olive branches have long been considered a symbol of peace throughout the Mediterranean region (and cultures at least partially derived from same, such as Christian Europe). But why specifically olive branches, and not olives or entire olive trees?
There may be a scientific reason for that, and it has to do with the early domestication of olives.
Before domestication, olives were, well, mostly too bitter for humans to eat. But, somewhere around 4000 BC or so (maybe earlier, the exact time is a bit hard to pin down), a mutant olive arose that just so happened to be much less bitter. That was very useful–it meant that olives, instead of just being a source of fuel (both olive wood and olive oil) or animal fodder, could also be a source of food. They were still bitter, but enough less so that they could be easily processed (eg by brining) into edibility.
But, like a lot of fruits, olives don’t necessarily “breed true”. If you plant a seed from your nice (comparatively) sweet olive tree, the odds are pretty good that you won’t get another nice “sweet” olive tree, but instead a bitter one. So, how can you share the wealth, if you want to give your neighbor a “sweet” olive tree as a present?
The answer is grafting. Or, well, just sticking a branch in the ground (though I imagine grafting works better). Basically, you take part of the tree you want to propagate, and attach it (in an appropriate way) to an adequately related tree. There are, of course, advanced grafting techniques that involve tools that they didn’t necessarily have thousands of years ago, but sometimes grafting basically just involves cutting a hole in your rootstock plant, sticking in some of your scion, and hoping for the best.
So, a good “Hi, neighbor, I want to be your friend” gift, if you had a “sweet” olive tree and lived in the Mediterranean would be, well, a branch from your tree. Once everyone in your village had “sweet” olives, well, when you went to the next (friendly) village over to trade, what could be more natural than bringing along some olive branches? Scale that up to towns, then city-states, and it’s easy to see how olive branches became a near-universal symbol, at least in that general geographical area, for “I want to be your friend”.
Once “sweet” olives spread widely enough that the gesture wasn’t really needed for purely *practical* purposes, it’s likely that the symbolic meaning was retained, while the purely practical reasoning behind it was mostly lost.
Science is Really Weird 