I admit I'm not very learned on the subject of evolution, I just started researching it for myself about a week ago. One of the things that I don't understand is why humans and almost every other animal/bug/fish etc. on earth has a symmetrical body. What advantage does this have for the survival of the fittest? It must be quite important, since it re-occurs so frequently and early in the evolutionary family tree.
[If we are talking about animals, there is a number of types of body symmetry: asymmetry, radial, bilateral, spiral, rotational, glide and metameric symmetries. Bilateral and radial are the two basic types, which served as main classificatory character for animal mega-systematics in the past.]
It's assumed that body symmetry is associated with general environmental gradients.
Animals with (elements of) radial symmetry have one dorso-ventral axis which echos only one environmental gradient: substrate/water column, depth etc. Besides this polarity, resources/signals/danger may come from any direction. Imagine a Hydra polyp with its foot attached to a waterweed and its tentacles surrounding the mouth on the opposite end and expecting the victim to come from any side.
Body of bilateral animals possesses the same dorso-ventral axis and yet another polarity, orthogonal to it: the anterior-posterior axis. This axis is conjugated with the gradient created by locomotion: the anterior margin of the body is likely to encounter something new first. This principle lies in the basis of cephalization: most bilaterial animals have more or less shaped "head".
Evolutionary changes in body symmetry are thus thought to be associated with changes in life-style, e.g.:
Symmetry is like data compression, it diminishes amount of information needed to build an organism.