How did Booba Evolve? The Weird Origin of Milk

Milk is kinda weird if you think about it. Mothers excreting liquid food out of an erogenous zone into their baby’s mouth? Ew! Overthinking breastfeeding is almost as gross as overthinking sex.

Why do mammals do this?

• Contents:
• Evolution of Birth
• Evolution of Milk
• Evolution of Boobs
• Conclusion

Evolution of Eggs on Land

To truly understand the evolution of milk we’re going to have to go way back.

Lactation is thought to have evolved back when we still laid eggs. If you look at monotremes (mammal species who still lay eggs and are thought to resemble very early mammals) like the platypus and echidna, you’ll see that they don’t really commit to the whole egg laying thing. 

The problem with being a fish that evolved to walk on land is that it’s very dry most of the time. Real bad for your skin, y’know? But if you’re a fish with legs, instead of getting chapped skin you like, die, so it’s pretty inconvenient.

we also hadn’t really invented moisturizer lotion back then. So, at first, early land lubbers had to stick near damp areas to make sure they stayed nice and moist.

Amphibians, like frogs and salamanders, still do this as they have yet to evolve any significant resistance to drying out. ¹Something which is being exploited by a fungus to kill more frogs than ever before in history!

But even if you are smart enough to stay away from dry areas, what about your eggs? The first animals on land were still laying caviar, which can dry out. Eggs are generally too stupid to stay out of the sun. In fact, I don’t believe they can move at all. So we had to lay them in water to keep them from drying out, like how frogs still do.

This is a bit inconvenient, so different groups evolved different ways of dealing with the problem.

Sauropsid Solution

The first solution was to put a thin layer of bone around the egg to seal it off. This sorta helps, but they couldn’t make the shell hard and thick enough to completely seal off the egg from losing moisture. If the layer of bone is so thick that water can’t get out, how is the fetus inside the egg supposed to breathe? Yes, eggs do breathe. 

The sauropsids, who would eventually evolve into birds, dinosaurs, and reptiles, actually worked out a pretty good solution. They make their eggshells out of crystals of calcium carbonate of specific size and shape. Because if their shape, there are microscopic gaps between the corners of the crystals that some air can flow through. This means the shell can be thick and strong without suffocating the baby inside.

But the best solution to a problem isn’t always worth doing. Do you have any idea how many evolution points sauropsids spent on their eggs? They could have evolved some fuckin’ laser eyes or something, I dunno.

And now they’re stuck with these over engineered eggs. Which, I remind you, still cannot move. Meaning they either need to guard them or make so many it doesn’t matter that most of their defenseless eggs get eaten.

So basically; birds and reptiles are like the Germans who’ve made the best tank in the world, but it’s so expensive and unreliable that it’s more of a liability. (and I guess amphibians are like the Italians²In WWII, the Italians managed the amazing feat of making tanks that were not only not the best in the world, but were also pretty unreliable as well?)

Therapsid Solution

Therapsids, which would evolve into mammals, dealt with this whole evolutionary conundrum by not dealing with it at all.

Instead, they just carried their shitty thin shelled eggs in a little pouch on their belly that’s always nice and moist with sweat. Yes, it was critical to protomammal survival that they keep their eggs moist by sweating on them.

Eventually, we’d evolve to load pouch sweat with nutrients and vitamins and shit that the egg can absorbed. (Pouch sweat isn’t the official term, but I’m using it as I think it’s funny)

This meant the baby didn’t need to depend on the yolk. It also meant that the pouch was probably a paradise for bacteria, but whatever. ³That is the stage where monotremes like platypuses are still at. They lay a little yolkless egg inside a pouch. This is both gross, and kind of defeats the entire point of an egg.

But in this case that’s a good thing. As it turns out; having to guard a nest is really hard. So some other mammals just cut out the middleman and stopped making an egg at all. They just made the baby inside their womb.

But they already spent the evolution points on a pouch and nutritious pouch sweat, so might as well get their money’s worth and let the kid crash there for a while. This is the equivalent of living in your mother’s basement. Marsupials still do this.

Placentals, which are all mammals with a placenta, got rid of the pouch all together but kept the pouch sweat.

Evolution of Mammary Glands

As you might have guessed, milk evolved from pouch sweat. Which is why henceforth, I will be referring to it as milk sweat™. Have fun remembering that next time you drink a latte or have some ice-cream.

Though milk sweat originally evolved from normal sweat, milk sweat glands had already become genetically distinct. You wouldn’t really want to be putting all those nutrients and minerals in normal sweat. That’d probably make your B.O. a lot worse.

But milk sweat glands were structurally similar to normal sweat glands. Which is to say they contain a gland made of clusters of specialized exocrine cells. These produce the ingredients of sweat (water, salt, skin oil, some waste products) and dump them into a small tube which leads up to a pore in the skin. Sweat then leaks out.

Mammary glands are a bit more modified than this, though. For one, they’re a lot bigger thanks to having many more glands and ducts, which is nice. Also the pores are concentrated into one area making a nipple, which is also nice.

Also, there’s an addition of a small holding chamber for produced milk called an ravioli alveoli between the milk making cells and the duct. This lets the mother to hold milk inside the breasts until needed. Otherwise, it’d just leak out constantly like sweat, which would be a little embarrassing.

The exocrine cells themselves are also different. In normal sweat glands, these cells just dump out the sweat. In the milk sweat glands, the cells fill half of their volume with milk ingredients. Then that half of the cell shatters apart!

Imagine that a cell is like a big soap bubble getting pinched into multiple smaller bubbles. It’s pretty much like that. The half of the cell with all the important stuff like DNA survives, while the half with milk in it floats off. This kind of membrane without any organelles is called a micelle.

Yes, milk does contain micelles when you drink it. That’s where the fat comes from. And if these micelles break, from spoiling or acid, the milk curdles. So I guess you could sort of argue that drinking milk is like cannibalism? Like, it’s not just an excretion but also contains parts of the mother’s cells.

Conclusion

For more on the science of boobs, try this other article of mine; Why do Humans have Such Big Boobs? (They’re Not Just for Milk).

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