Especially when we only have one each of our other major organs (except lungs)? I’m sure there’s a good reason but I can’t think it.
The kidneys develop from the same embryological origins as the gonads and genitals which have bilateral symmetry (i.e. two testes, two ovaries, two labia majora, etc).
As a backup if one fails due to disease or injury. Animals with 2 kidneys would have a survival advantage.
Animals also have 2 ovaries or testes.
OK thanks for the explanations.
If nature has created two kidneys for survival reasons then does that mean that kidney failure is more common in the animal kingdom than liver or heart failure, for example?
…or perhaps having two livers or two hearts is not feasible? How would two hearts stay synchronized? Could it be done, and if so why has it not been?
Birds - although animals - generally have only 1 ovary or testes, and just 1 kidney and 1 lung. It’s a weight reduction thing for them.
I think there might be critters with more than one heart - but not among the vertebrates. Maybe starfish and sea stars…? Don’t quote me on that, just not sure.
yep, birds have one kidney i remember that from grade school 
have you considered that maybe THERE IS NO REASON ? humans are not perfect you know, as much as you would like to believe it.
I think that USCDiver hit it on the head. Bilateral symmetry. We have 2 arms, 2 legs, 2 of most other things. But the things that are on the midline (heart, nose, mouth, anus, etc.) are singular. Even the aorta, of which there is usually only one, starts with 2 aortic arches - one of which regresses during development. Usually in humans it’s the left which remains, but in other animals (birds for example) the right is maintained. On very rare occasion a human will have a right aortic arch - only discovered if there is a medical problem requiring surgery, or if they donate their body to science for medical school dissection (where I saw one while teaching).
And most of the departures from bilateral symmetry (in vertebrates anyway) are a matter of weight or space reduction - why bother fitting 2 lungs into a snake’s skinny body when one long one will do just fine.
That simply begs the question: why are they in the midline?
Actually, in one manner of thinking, humans have three livers, and four hearts. Its just that they are smooshed together, so we think of them being one. And in the case of our hearts, they are connected in series rather than parallel, so the failure of one means trouble for all of them (although they do provide some degree of redundancy- a minor fibrillation in one will be corrected by the others, and if one is weakened the others will take up the slack up to a certain point).
Thank you Figimingle, obviously if you’ve been teaching at a medical school then you know this. But some midline, singluar structures are bilaterally symmetrical and develop from two lateral folds meeting in the middle. In particular, the mouth, oropharynx, nose, penis, and scrotum. People with cleft lips and cleft palates have slightly misaligned folds on their face during development.
The GI tract develops as a hole and then a tube that extends from one end of the developing embryo to the other. The lungs, liver, pancreas, and gall bladder all develop as “buddings” from the GI tract and thus are singular (except for the lungs, and actually the pancreas too, but that’s another story). The embryology of the heart is more complex and IIRC develops from 6 arches. The brain and spinal column develop from a singular neural tube that forms very early in embyological life.
Anyone who is currently involved in embryology, please correct me where I am wrong. And feel free to add info on organs that I’ve left out, like the spleen. Where does that thing come from anyway?
Let’s nail this error quickly. Two kidneys are probably a survival advantage but two kidneys were not “created … for survival reasons” by nature. Repeat after me, “Characteristics arise by means of genetics and are then selected by having enough individuals survive until offspring can be generated to assure continuance of the characteritic.”
Meaning, basically, if a mutation or genetic trait is not detrimental to survival, then it most likely remains.
Is that right? I think it is.
Yeah. In fact, a low level anti-survival trait such as sickle-cell disease can survive at a relatively low incidence if it aids the individual to better withstand another anti-survival factor such as malaria. Or at least that’s my current understanding.