I've got a silly question, sorry for that. I know, that we probably have no the right answer and the current answer could be "that's evolution, external conditions". I'd like to speculate, why most of mammals (for ex.) and people especially are symmetric? I know, that we are not completely symmetrical, but anyway.
Thanks a lot.
There's really two answers to the question. The first is overall symmetry: mammals, like all tetrapods, are bilaterally symmetric. This comes from a distant common origin with other bilaterally symmetric organisms. Organisms which evolved from this common ancestor often have organs in pairs, probably evolving as a re-use of regulatory genes.
The other answer is the high degree of symmetry in external appearance. While internal organs are roughly symmetric, there is some asymmetry. However the external appearance of most bilaterally symmetric organisms is highly symmetric. This suggests a different force at work. In fact, there is some evidence for sexual selection for bilateral symmetry. Mate choice is driven by indicators of health, and bilateral symmetry is a common indicator of health.
It is, of course, not just mammals, but nearly all animal life is symmetric. Even plants are usually symmetric in some degree. There are exceptions here and there. For example, one interesting exception is that of the fiddler crab which has one claw larger than another. In general, when a single appendage is present on an animal it is nearly always on the center line of the animal. For example, the horn of a rhinocerous or the central horn on a triceratops, or a human's nose.
The symmetry is generally enforced genetically, not environmentally. For example, if you hold a paper up to your face you will find that the left side of the face is different than the right side, or at least that is the way it is for most people. Thus, once conceived, the two sides of the face grow differently, a phenomena that geneticists call genetic "expression". For example, your fingerprints are "expressed" traits, not genetic traits, and for this reason the fingerprint on your left thumb is different than the fingerprint on your right thumb.
Even genetically, however, many parts of the body are not symmetric. For example, the heart is on the left side of the body, but the liver is on the right.
The evolutionary logic for symmetry probably is because it makes locomotion easier and more effective. If an animal is not symmetric, they may have difficulty moving efficiently.
Although not a biologist, Steven Pinker notes that bilateral symmetry evolved in organisms to allow them to move in a straight line. For a bilaterally symmetrical animal to move, they simply alternate movements between one half of their body and the other. This is true for fish, snakes, insects, mammals, etc. There are exceptions, for instance, flying and hopping, but those creatures evolved from ancestors that first locomoted by alternating limbs.
If you look at jellyfish, for example, they are radially symmetric, and their manner of locomotion is to swim in a sort of spiraling shape.
Probably after it arose, bilateral symmetry became a sexual selection criteria, because it is costly to perform during growth, as Pinker goes on to explain.
