So we have two ears that allow us determine which direction a sound came from, and two eyes to see with depth.
We also two of most other body parts but why/how then only one heart, one liver but two kidneys etc?
DanielB
I think a better question would be why we have one of some things, specifically heart and liver. the liver is just reasonably resilient by itself, and perhaps a mammal with two livers be too bulky and have no real advantage, and the logistics of having two hearts in a mammal are quite complicated. Everything else evolved in pairs because they are relatively cheap features that make the chances of survival that much better.
It all comes down to offering a reproductive advantage, and having two kidneys does - kidneys are relatively easy to injure and having two increases your chances of survival.
My personal theory is that it’s easier to tell how genetically sound a bilateral organism is. Symmetry is a good indicator, I’d say.
In other words, with a bilaterally symmetrical creature, it’s easier to tell if it isn’t prime breedin’ material, since the body parts won’t match the ones on the opposite side. Can’t really do that with a non-symmetrical organism- is that arm supposed to look like that, or what? But if it’s got a matching arm on the other side, it’s fairly easy to tell if one of them is misshapen.
Notice that all of the visible, external body parts are either bilaterally symmetrical, or are in the center line of the body, while the inside isn’t quite as neatly organized.
This means that it’s easier to selectively breed out defects with a symmetrical critter than it is with one that’s not symmetrical.
Just a theory, but it’s mine, and I like it. 
Quite simply we have two of most organs because we are bilaterally symmetrical. It really is that obvious.
As the embryo develops the development of tissues and later organs is achieved by the diffusion of chemical messengers relative to the poles of the zygote and later the notochord. That means that all organs and tissues initially develop at a specific location along the ‘length’ of the embryo but necessarily also on both sides of the midline. It would be very difficult to arrange for cells one just side of the embryo and not the other to respond to a hormone or chemical and the evolutionary advantage is so small that not surprisingly it has almost never evolved.
Organs that are unpaired or appear to be unpaired come about by a number of means. Commonly, as in the case of the spinal cord the organ is effectively paired but since it’s located directly on either side of the midline the pairing isn’t obvious. But take a look at a section of spinal cord and you will see an obvious symmetry with two equal halves either side.
The heart also starts out development on the midline but later goes through some migration, contortions and folding that leads to an uneven distribution of tissue in the adult. That’s also the case with the gut which stars as a basic tube down the midline and then throws itself into loops. As a result we have one apparently unpaired stomach on one side of the body but only because the stomach starts on the midline. The same is also true of the liver IIRC. Hence these organs are not paired simply because they start out in the dead centre of the body and can’t be paired.
Lightnin’ that’s an interesting idea, and the use of symmetry as a sexual selection factor is almost certainly correct. However such a selection pressure could only have evolved after bilateral symmetry already existed as the best functional form. If a creature could function just as well or better without being perfectly symmetrical then mates that selected for symmetry would have been selecting against there own genes and the trait would have died out. IOW the sexual selection is a result of symmetry working, not the cause of it.
And of course it ignores the fact that the majority of animals are bilaterally or radially symmetrical and yet practice what is essentially free spawning where no mate selection at all is possible. That is the primitive state and so once again it’s obvious that bilateral symmetry and paired organs must have an inherent advantage that led to sexual selection but could not possibly have been caused by it.
It’s redundancy, of course.
If you loose one of something the is still a spare! {In most cases.}
See also this thread. As noted by Blake, those internal organs which are singular are those which are associated with the gut, which starts out as little more than a tube running through the body. Ultimately, those organs which are derived from endoderm (as opposed to mesoderm or ectoderm) are singular, everything else tends to be paired.
Makes you wonder why we all don’t have a couple dozen genitals salted away here and there just in case. Talk about a reproductive advantage!
Actually, it doesn’t. It’s a common fallacy that every trait must bestow some advantage to an individual; in actuality, many traits are selectively neutral, and others are subject to constraint, limiting the degree of viable variation allowed. Having two kidneys is more a function of the way development works than because it was selectively advantageous to have two instead of one at some distant point in our history.
Cool! First time a thread of mine has been linked to!