As breasts are designed to feed babies, why do women have two instead of just one?
In: Biology
Because nature does not “design” everything to be just enough. We have two lungs instead of one. We have two kidneys but only need one. We’re bilaterally split, meaning we often have two of something, or when we have one, it’s often made of symmetrical parts. Two breasts can feed two children at the same time, or a mother can alternate between them for a single child.
Because the human body is all about symmetry. Two eyes, arms, legs, ears, breasts, lungs, etc.
We don’t really know since evolution isn’t a sentient entity that we can ask questions of. We can’t even run double blind trials. In most cases we can’t conduct experiments at all, both because of ethics and the timescales involved.
That said there are a few hypotheses.
The human body develops mostly in a mirror image. Soon after conception the embryo folds in half. The line down the middle becomes our digestive tract and the remains of the seam are visible all along the front from the attachment at the bottom of the tongue down to your taint. After that most things just develop the same way on both sides.
Some mammals have more than 1 pair of mammaries but they tend to have more offspring at a time.
2 breasts provide some redundancy. If an injury keeps you from having offspring that can grow up to reproduce you’ll quickly lose out to gene lines that aren’t as vulnerable to injury.
Twins need two breasts. This one is less likely since, on humans, it’s not really practical for two kids to nurse simultaneously.
Evolutionary psychology has a theory that when humans shifted from primarily rear mounting mating to primarily face-to-face males were still evolved to enjoy the sight of two globes of butt. So human females evolved to have permanently enlarged mammaries (almost unheard of among other mammals) to mimic the butt. This is a kind of silly hypothesis and I haven’t seen any independant support for it but I like it because it lets me talk about boobs and butts at the same time while still sounding somewhat serious.
Average litter size rounded up to the nearest even number because symmetry is easier to code for.
Mammals that have more babies in a litter have more nips
High availability & fault tolerance
Mammals in general have twice as many nipples as the average number of offspring that have at a time. Humans normally have a single child at a time so they have two nipples.
So one nipple doesn’t take all the abuse. The breast needs time to refill with milk, as well.
The answer is ‘because that is how humans evolved’. People can speculate as to why evolution took that path, but for the fundamental answer is just “random mutation lead to humans having 2 breasts and that is the human that survived”.
Because for whatever reason women with 2 breasts were better at surviving than women with 1 breast or more than 2 breasts. Or maybe more accurately, the children of women with 2 breasts were better at surviving than the children of women with 1 or more than 2 breasts.
Also, for some reason almost all animals have symmetry. And at a minimum bilateral symmetry. Which means no animal would have an odd number of breasts. But again this is just a random mutation that proved to be the most likely to survive.
It makes sense that an animals that had more breasts than they did children, as the children would be more likely to survive. If they had less breasts than the number of children then some children would potentially starve. But having more breasts than needed must not have provided more of an advantage.
At a point, the baby drains both boobs. So two boobs doesn’t mean it’s for two babies. Also, constant saliva on the nipple REALLY hurts! So it does give you an alternating ability. Probably also because nature likes to be a bit redundant, so if something happened to one of them, such as say said baby got front teeth and loved to surprise bite causing one to spew blood, you can swap to the other until the blood all drains out and heals. (true story – actually tossed my baby on the floor when it happened. Don’t judge – it was involuntary.)
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