Breasts by Florence Williams Read Free Book Online Page A

there’s nothing else like mammary glands. To have a skin gland producing a large flow of liquid rich in nutrients is very strange.”
    Darwin did not have the advantage of being able to date fossils, so he couldn’t know how far back lactation went. It’s very far indeed. Lactation even predates mammals, which now sounds bizarre since it is the defining characteristic of them. Here’s how Oftedal thinks it happened. Once upon a time, long before there were mammals, there were mammal-like reptiles called synapsids. These split off from other reptiles and proto-dinosaurs around 310 million years ago. Instead of scaly skin, synapsids had leathery, glandular skin containing hair follicles. They developed specialized teeth and a unique jaw that would someday evolve into a mammalian palate, nose, and ear bones. Early synapsids looked like giant terrestrial lizards. After thriving for tens of millions of years, the synapsiddescended therapsids were nearly wiped out—along with 70 percent of all creatures—by the Permian-Triassic mass extinction event 250 million years ago. Fortunately for us, a few survived, evolving into small, mammal-like creatures called cynodonts, but they would soon be eclipsed by emerging dinosaurs in the late Triassic and the Jurassic period. Somewhere in here, lactation proper began.
    The proto-mammals had kangaroo-like pouches that transported eggs, then hatchlings. The eggs, said Oftedal, were made of leathery shells with porous, parchment-like coverings. Because they were porous, they lost moisture easily and were susceptible to harmful microbes. But the mother could help solve these glitches. Maternal skin glands in the pouch began to secrete fluids and fight germs. The first fluid was a sort of natural Lysol. It wasn’t much of a leap for nutrients to eventually find their way into the mix. Eventually the happy hatchlings had constant, enriched fast food: Lysol on a burger. It’s worth mentioning again that mammary glands likely first evolved for immune support (stay tuned for more on this in chapter 9).
    Out of the hostile, climate-addled Cretaceous period, only a few small, straggler mammals, about eighteen genera in all, survived. They adjusted their skeletons and airways for better running, and they became nocturnal. They also invested more time and energy in their young and internally regulated their temperatures in ways the reptiles could not. Lactation enabled most of these changes. Endothermy, or body-temperature regulation, for example, wouldn’t be possible in tiny offspring without the fast metabolism offered by specialized, high-fat milk and intensive, snuggling parental care.
    “Little creatures that had become warm-blooded and active and lactating when the world went to hell were in a better position to survive,” Oftedal explained. “The consequence of lactation was that ultimately you could defer becoming an adult, when you have to kill or find your own food.” Consequently, mammals could become much more specialized because they weren’t forced to stay in a habitat that provided kid-friendly food. They could transform adult food into milk. Ruminants, for example, evolved eating stuff that their babies could never handle. Another example Oftedal offered are whales, which spend part of the year getting fat by feeding in the rich polar regions, then migrate to the warm but food-scarce tropics for birthing and nursing. “They do this because they can lactate!” he said, getting more excited now that we were talking about marine mammals. Crocodiles, by contrast, are stuck by the riverbank all day long so the baby crocs can go fishing. Another benefit of lactation is that mammalian babies’ heads can start smaller (because they don’t need teeth) and later grow larger to accommodate more specialized teeth and bigger brains. Being born with a small head and body is also very helpful for the mother’s mobility.
    The key concept here is flexibility across habitats and niches.