Are there any interesting theories? I think a lot can be attributed to just randomness. Some time in the evolutionary process we got infected by a bacteria, the mitochondria is now a vital part in our bodys energy system...
One of the nerves that go from the brain to the eye and is hooked by the top part of the aorta artery.
In the original version, it was in some animal like a fish, and the top part of the aorta was in the gills, so the way from the brain to the eye though the hole that is below the aorta was a straightforward path.
A few million years later, when we lost the gills, and got a neck, the top part of the aorta moved inside the chest to avoid a stupid long trip to the neck and back (and probably lost a lot of speed in the blood). The nerve was hooked, so the aorta got shorter, but the nerve has to go down to the chest, pass below the aorta, and then go back to the eye.
In the case of the giraffe, the trip to the chest and back to the eye is stupidly long.
The problem with evolution is that mutation can do only small changes (most big changes are just deadly). The connections of the nerve and the artery form very early in the development of the fetus, when it looks almost like an ugly fish. You can't unentangle the nerve and the artery later.
