Why do we have two of some things but only one of others?

Barring injury or some odd birth defect, a person has two kidneys, two lungs, two testicles or ovaries. However, we only have one liver, one heart, one set of intestines, one pancreas.


Most animals evolved a system of two lungs and one heart (along with the rest of their organs) because that’s what was needed to survive and thrive on Earth. Humans didn’t develop two hearts or four kidneys because we didn’t need them for survival.

But the liver has two lobes, the heart has two ventricles and two auricles, and the pancreas has a duct running through its center that divides it fairly neatly.

For the OG take -

The two ‘key’ paragraphs if you don’t want to read the link (or it’s not working for some reason:

In creatures as in Chryslers, external bilateral symmetry — a trait shared by 99 percent of the world’s animal species — just makes sense. Most obviously, it’s good for locomotion: it helps us walk, run, swim, or fly in a straight line, pivot quickly and reliably, etc. But there are plenty of further theories about the adaptive nature of the external body plan, as it’s called: some relate to partner selection (a more symmetrical appearance could imply better genetic well-being), others to self-defense (symmetrical placement of the eyes means that a prey animal doesn’t have a blind side).

Once within the abdominal cavity, however, aerodynamics and attractiveness count for zilch. It makes for a more efficient body to have the inner workings crammed in as compactly as possible, symmetry be damned, and in fact that’s how they evolved. Again, it’s the same as with cars: on the outside it’s about interfacing with the environment; inside, it’s about optimized use of space.

We are bilaterally symmetrical. Which means that almost all things look like one half mirrored into a second half. Some of the things you mentioned having “just one of” are things that used to be along the center line in our evolutionary ancestors. The digestive system was a simple central tube, and the heart a simple central squeezy thing, for example. It was only later that evolution managed to make them develop asymmetrically when it was useful.

There are other things that once were double but now have been reduced to single central organs. For instance, female monotremes have two vaginas and two uterii, while male monotremes have bifurcated wangs. That is probably the ancestoral condition of the earliest mammals that was later lost in theria.

Nitpick: Uteri. “Uterii” would be the plural of “uterius”, if that word existed, just like “radii” is the plural of “radius”.

(and I guess that your choice of “wangs” was because you didn’t want to even try to figure out the Latin plural for “penis”. It’s “penes”, if you were wondering)

Marsupials have two vaginas. (Or 3, depending how you define them.) Monotremes don’t have a vagina or uterus at all, they have cloacas.

That should have been a clue to you that both spellings were tounge-in-cheek, but this board has never been short of people with much pedantry and little perception of humor. This has happened before:

Also, this: is 49 too soon to blame a senior moment?

I’d be as happy as a dog with two penes if you could assist me with the correct pronunciation of this wonderful word. Is it like:

  • Peenies [like peonies]

  • Peens [like beans]

  • Penays [like plural penne pasta - aurally and visually]

  • Peness [like finesse]

  • Pennies [like from heaven]?

I still don’t quite get why we have two kidneys and testicles/ovaries but only one of almost every other internal organ. But oh well.

The liver, especially, seems like a key thing.

Didja hear about the guy with five penes?

His pants fit like a glove

American o’possums have a bifurcated penis, but it closes together when they get it on.

I wish that you would not use technical terms that some of us do not understand.

Again, you need to look back on evolutionary history. Go way back and we are descended from ancestors that developed bilateral symmetry instead of one of the other paths. So we have a right side and a left side that are mostly mirrors of each other. Our bauplan is a torus. All chordates are doughnuts that have been stretched and twisted in different ways. Digestion actually happens outside the body, it is just that the doughnut has been twisted around so much that it is hidden. Imagine a standard doughnut. Drop a marble in the center and it is still outside that doughnut. Our digestive tract is the doughnut hole with digestion taking place on the outside surface of that.

So our simple ancestors are an elongated straight tube running along the midline with the “mouth” opening at one end and the “anus” opening at the other. The nervous system is a branching cord running along that midline path. The circulatory system is a tube running that midline path. It it is during development that there is differential growth that causes asymmetry. Ancestors needed more room to digest their food than a simple straight tube could handle, so the tube got twisted around various ways. Circulation got to the point of needing more than just a squeezing muscle on an open-ended tube, so the circulatory system got complicated. Look at the developmental and evolutionary history of most anything we have “one of” and you are likely to find that it was once something that was a more simple shape on the body’s midline. Look at fish anatomy, or lancet anatomy for ideas for how things used to look.

Well, I was thinking of terms of early hearts. They weren’t complex chambered pumps, but a ring of muscle wrapped around and periodically squeezing an open-ended central tube. Think of the earthworms you probably dissected in middle school.

The “p” and the “n” are more or less as they are in English. The “e” is approximately like an English long A. And “s” in Latin is always pronounced as “s”, never like “z” as it often is at the end of English words. So a close approximation might be “peyneyce”

As to why we have two of some things, for the immediate pathological “why” (rather than the evolutionary why),

It seems that the FOXD1 gene “promotes nephron progenitor differentiation”, which I read as meaning it causes a single proto-kidney tissue to split into two distinguishable proto-kidneys. So whatever causes us to have two kidneys, may be contained in that protein.

Here is a study from 2005 where, in their own words,

Genetic deletion of the forkhead transcription factor, Foxd1, results in striking renal abnormalities, including the loss of these discrete zones and pelvic fused kidneys.

Levinson RS, Batourina E, Choi C, Vorontchikhina M, Kitajewski J, Mendelsohn CL. Foxd1-dependent signals control cellularity in the renal capsule, a structure required for normal renal development. Development. 2005 Feb;132(3):529-39. doi: 10.1242/dev.01604. Epub 2005 Jan 5. PMID: 15634693.


Tops, pops!