Is the duality in nature on Earth common throughout the universe?

Why do most animals, birds and fish tend to have two of everything?

Most higher level animals, birds and fish on Earth have two of most everything. They have two eyes, two ears, two forelegs and two hindlegs (with birds the wings are analogous to forelegs), or with humans arms and legs and with fish fore and aft fins. They have brains split in two down the middle. For camouflage, birds and fish have their bodies divided into two, with usually a darker top side and lighter underside.

 This is also reflected to an extent in broader aspects of nature on Earth. We have the two genders, essentially equal periods of night and day, dry land and wet seas, predators and prey, warm and cold blooded creatures and so on. Even our internal organs tend to be divided in two, with two kidneys, two lungs, a large and small intestine etc.

These traits are so fundamental to how nature functions on Earth that we tend to be fairly blind to them and overlook them when we are examining nature. We instead look at distinctions like different colors of plumage for birds, the different sizes of animals, their speed, dexterity in climbing trees, fur, feathers or skin and other distinctive features.

 Yet what we have in common is perhaps of far more importance. Are these traits universal? If we were able to visit alien worlds would we see a bicameral division in the variety of physical life there as well, or is this something that developed that is unique to Earth? There’s no reason why a bird or a hippopotamus couldn’t have three or four eyes or why a fish couldn’t have some sort of extra appendage like a central hand for grasping things.

Evolutionary biologists will say that the features that animals have are evolved over millions of years to serve important functions. There are many functions that natural organisms lack however that would do them a tremendous amount of good if they had them. Birds could definitely make use of an extra grasping appendage instead of just their beaks or feet. Tigers and hyenas could use the ability to see in the wide range of infrared instead of the normal, narrow visible spectrum so that they could sense the body heat of other animals nearby. Human beings could benefit greatly from gills so that we could swim in the ocean for extended periods of time without the need for mechanical equipment.

 If some of these traits existed in the past in certain species, why would they be lost? What would be the evolutionary advantage to humans losing gill structures if we once had them? Another good example is the Tyrannosaurus Rex dinosaur that most everyone is familiar with. Its forelegs are generally considered useless appendages, somewhat like our tonsils or appendix. But if this is the case, how did they become that way? It certainly seems like a beneficial trait to have small grasping forelegs for a predator of that sort to hold its prey once captured.

 Many of the mysteries of Earth’s biological diversity may never be fully understood until we actually make contact with an alien race and are able to examine the flora and fauna of its home world up close for comparison. We can’t even be sure if DNA is a universal genetic trait, or if the division between flying, swimming and crawling creatures is common (why not a creature that can effortlessly do all three?)

The most surprising thing that may be discovered however is that most life forms in the galaxy, if there are any others, are not based on a division of two, or a split of functions down the center of the body that is common here. There may be entire planets where the division is based on the number three, or something random like the number 17, with wildly varying forms of life. Or it may be the case that most life forms don’t have any redundancy at all in their sensory or functional organs and ability to move about, making them smaller and more mobile, but also more vulnerable to attack. In such an environment, would reproduction be more rapid and furious than it is here, to replace the injured and consumed?