My question is why humans,birds,fish,animals,butterflies are symmetrical?

Question

From an engineering point of view all floating or balanced moving strutures like ships, aero space crafts need its self balancing status to move with out wandering.

A simple liquid filled U tube has equal columns due to gravitational forces.

Can any body say the cells at its developing stage, when it was in liquidous form (during the evaluation stage) had been formed it self on the basis of gravitationl forces so that gemetrical similarity had been a immense help when they started moving on their own on the earth surface.

Answer 1

The symetrical animals have a similar development as an embryo. Cells divide and form a sphere of cells, which eventually becomes hollow. Then a tube grows from one end to the other. THis becomes your intestines, stomach, etc. Head at one end, legs arms and whatnot all grow from this ball that had the ends selected. Symmetry, or at least something close to it, is relatively easy to accomplish from this, and works out well for placement of eyes, ears, and limbs. Asymetrical animals formed this way are less likely to survive, and voila...symetrical animals. Nonsymetrical animals are significantly simpler...starfish, sanddollars, sponges...and except for sponges, these usually show radial symmetry, and develop around an axis point...mouth in the middle, arms radiating out.

Answer 2

Genetically flys, other insects and animals have had DNA for symmetry thruout evolution. Due to natural selection the symmetrical life form must have been more beneficial.

If you think about it: the need to move, to eat food, to discard waste, all work better for a symmetrical creature. A lopsided creature would have a hard time balancing and moving fast.

And since DNA codes for specific form/function rather than random structures, the symmetrical form for all body features like eyes, mouth, nostrils, legs, etc became the "standard" for insects as well as humans.

Interesting article:

The fruit fly Drosophila melanogaster have altered our estimate of the evolutionary relationship between vertebrate and invertebrate organisms.
Key molecular pathways required for the development of a complex animal, such as patterning of the primary body axes, organogenesis, wiring of a complex nervous system, and control of cell proliferation have been highly conserved since the evolutionary divergence of flies and humans. When these pathways are disrupted in either vertebrates or invertebrates, similar defects are often observed.

also

Drosophila and human promoters use different DNA sequences to regulate gene expression, supporting the idea that evolution occurs by the modulation of gene regulation. (rather than ever changing mutations of genes - which are fairly stable)

http://www.genome.org/cgi/content/full/1...

added:
as the organisms evolved from simple cells to multiple organs able to support movement and more complex digestion, it required legs of some kind to get the body off the ground for faster travel without scraping and wearing the body out. This was in direct response to gravitation and also to irregular surface of land.

So the bigger a species evolved it also needed better and stronger legs to move about to eat a bigger quantity of food - or evolution may have actually worked by randomly creating creatures with bigger, better, and faster legs first which then allowed those creatures to grow bigger and have a better chance of survival.

Symmetrical legs, as a pair, form an arch of sorts, with the body often pivoting between the legs in one way or another, offering balance and agility.
Snakes did not develop legs but they are a lower life form, and cannot move their body very fast, but can strike out quickly.

Creatures in water evolved to be better suited for laws of fluid dynamics than for laws of gravitation.

And since evolution could create unlimited variety there are all kinds of exceptions to the common, symmetrical forms of life in the water where gravity was not the ruling force. I cannot think of any asymmetrical creatures living on land but the oceans have several species like starfish and mollusks.

Plants usually have bilateral symmetry, most likely due to gravity and fluid dynamics. Corals and cacti are good examples where symmetry does not exist as the specimen grows. But neither species needs to move, rigidity is needed for them to survive. Some cactus propagates in the desert by breaking off, getting stuck in the fur or feet ot animals, so having asymmetrical form is beneficial.

Answer 3

Development of symmetry in animals has an evolutionary basis. Bilateral symmetry offers advantage in locomotion and in the survival against odds.