Biologically, why does a large percentage of nature have symmetry?

There might be a cosmological reason. Symmetry “broke” this way during the Big Bang.

Something to do with cells splitting perfectly in half? Excellent thought….

It has to do a lot with survival. It helps in moving if you’re symmetrical as well as sensory organs. Because of symmetry, you have two eyes rather than one.

It sounds like you’re referring to bilateral symmetry, a feature shared by many animals including all vertebrates and arthropods. There are numerous phyla, however, that have other symmetries (a jellyfish has radial symmetry, for instance) that depart from this rule. Early in the evolution of multicellular animals, during the Cambrian explosion about 550 million years ago, natural selection brought forth many different body plans of various symmetries. Bilateralians evolved fairly early on.

Interestingly, starfish (i.e., sea stars) & their relatives are considered bilateral because of a larval form that embodies that symmetry.

@Oxymoron
Yeah, 2 eyes help. but there is no need for them to follow a horizontal symmetry line. They could be anywhere.

@jackm – Well it does help. Having sensory organs proportioned so each half of the body has the same amount of a certain sense, this helps a lot.

Great question. Reminds be of a similar question I once asked. Not the same but along the same lines.
http://www.fluther.com/disc/43121/has-nature-ever-produced-asymmetrical-animals/

@Oxymoron Why does it help? I don’t really follow that logic.

In defense of my original answer:

Quote:
Symmetry in physics

In mathematics, the language of physics, symmetry has a more precise meaning. Livio defines it as an immunity to change. “Namely you do a certain operation and something does not change, you call that a symmetry,” he told LiveScience.

This definition takes into account bilateral symmetry but it also includes other symmetries as well:

* Time translation symmetry: Physical laws do not change with time.
* Translational symmetry: The laws of physics are the same whether they are acting in our solar system or at the far end of the universe.
* Rotational symmetry: The laws of physics don’t change if we turn around.

These symmetries are crucial for understanding science, especially physics. If the laws of nature were not symmetrical, there would be no hope of ever discovering them. In a universe where the natural laws were not symmetrical, experimental results might change depending on where and when and in what direction an experiment was performed.

Here’s an example of the importance of all this: One way astronomers are able to determine the material composition of stars that are millions of light-years away is to examine the chemical signatures encoded in the light they emit. In order for the astronomer’s conclusions to be of any value, the atoms in those stars must obey the same laws that govern our corner of the universe.

Symmetry is so integral to the way the universe works that Albert Einstein used it as a guiding principle when he devised his General Theory of Relativity.

Einstein firmly believed that the laws of physics should be the same for all observers, regardless of how they were moving. Through various thought experiments, Einstein discovered another fundamental symmetry in nature, called general covariance. Under this symmetry, physical laws act the same regardless of whether an object is accelerating or at rest. In other words, the force of gravity and the force resulting from acceleration are two facets of the same force—that is, they are symmetrical.

Scientists have glimpsed other symmetries in nature as well.

A positron, for example, can be thought of as a mirror image of an electron. And James Clerk Maxwell, a 19th century mathematical physicist, demonstrated symmetry between electric and magnetic fields. Through a series of equations, Maxwell demonstrated that electricity and magnetism are actually two complementary aspects of a more fundamental force, called electromagnetism.

Many scientists suspect that there may be more natural symmetries waiting to be discovered. Some think that the so-far elusive “Theory of Everything,” which physicists have spent decades searching for, will contain some type of universal symmetry that fully explains and knits all the known laws of physics together.

Source

@dpworkin
All of those make sense, but I am specifically speaking of the symmetry in the bodies of animals.

Also, when/why did the name change?

@jackm Well, let’s say your left eye is where it normally is, and your right eye is centered on your right butt cheek. For one, it would be highly inefficient to wire that right eye since it is so far away from your brain. In fact, it would be downright impractical because you’d have to sit on it, your butt would have to be reworked to include a socket in which to safely house the eye….Aside from these obvious drawbacks, and assuming your brain could figure out how to look forward and backward at the same time, you would have zero depth perception. Have fun trying to make a left turn across four lanes of highway traffic during rush hour!

Many others above have pointed out that there are different types of symmetry, and I wouldn’t say most lifeforms exhibit horizontal symmetry.

The answer to why something in nature exhibits symmetry lies in understanding what that something exists to do, and the constraints nature is bound by in accomplishing this goal. In general, when you design something, you look at the goal, the constraints, and then try to figure out the most efficient way to accomplish the goal under the constraints.

Good design is always about efficiency. I think often design problems (in nature and otherwise) are solved most efficiently with symmetry, and that we can understand this by looking at the relationship between structure and function.

We have two legs because we wouldn’t be able to walk with just one. We are not internally symmetrical because, well, for one, we only need one liver to survive. The nautilus is not laterally symmetrical yet many consider it the perfect example of geometrical symmetry as its construction exactly matches the golden ratio.

That’s the point. Symmetry arose in a deeper manner than mere biology. In a sense we had to be symmetrical.

We evolved into symmetrical creatures as it gave us a great advantage evolutionary wise, being able to see, hear and smell in the same direction we walk. Plus having a bipod or quadpod set of legs is easier for location.

@jackm – I’m sure you’d find it a lot more useful if you had one eye on your nose and the other behind your ear then.