We've Been Evolving For Millions Of Years, So Why Are Our Bodies So Flawed?
We humans have been evolving for millions of years and — as any good biologist will tell you — in response to pressures in our environment, we are evolving still.
So how come our bodies are so flawed? Why does sharp vision so often elude us, for instance? Why do our backs hurt so frequently?
The theme of Alex Bezzerides' Evolution Gone Wrong: The Curious Reasons Why Our Bodies Work (or Don't) is that we experience these and other embodied challenges — teeth that require braces, feet that acquire bunions, knees that blow out — not despite of evolution, but because of it. We are animals, and animals' early evolution in the ocean, and our primate lineage's transition from the trees to the ground, continue to affect how our bodies function and break down even today.
A biologist at Lewis-Clark State College who specializes in anatomy and evolution, Bezzerides has written a fantastic, informative book, a home run on his first try (he makes a point of noting his first-time author status in the Acknowledgments). Never did I expect to praise prose like, "The human foot... is made up of a whole gob of bones" but Bezzerides makes it work. He never condescends to his readers. Instead he mixes the technical anatomical stuff we need to know with vivid examples and humorous phrases.
We can grasp the main idea Bezzerides wants to get across by focusing on eyes and backs.
Our eyes evolved originally in the ocean, where ancestral vertebrates dwelled and needed to see underwater. Around 375 million years ago, when they ventured to land, their eyes were already 100 million years old. Gradually, eyes in this lineage became land-adapted, but these organs have retained fluids and, as a result, never achieved the type of light refraction that would result in consistent sharpness of image on land. Light travels more slowly through water than it does through air, but to our advantage in modern times, even more slowly through glass. "Many of us take advantage of this fact by placing glass in front of our eyes to compensate for the imperfect job our corneas and lenses do in bending the light."
Bezzerides offers nifty evolutionary explanations too for why we can distinguish more shades of green than any other color, and why our night vision is poor. He clarifies that it's not only our evolution that makes for vision troubles today, but also our current behavior. Most of us spend way too much time in spaces that lack natural light. "Children who spend greater chunks of their day outside have a lesser risk of developing myopia than children who spend their days inside," he writes. Kids don't even have to be doing healthy things out there, it turns out, because it's the light and not the activity that makes the difference.
Back trouble, the leading cause of disability globally, is directly traceable to primates' leaving the trees for open areas more than 4 million years ago, Bezzerides notes. The move to the forest floor was "a pressure cooker" that caused human ancestors' center of gravity to shift. For the first time, a primate could balance on only two feet; the human spine is shaped quite differently from that of our ape cousins', with curves that cause a "precarious" structure. For example, "The inward, or lordotic, lumbar curve needs to be far enough inward to place the position of the spine under the head and to get the center of gravity above the hips," Bezzerides writes. Back pain, and even intervertebral disc pain, happens all too readily with slight misalignments.
Cultural factors come into play with backs just as much as eyes. People whose work requires them to lift heavy objects may be at higher risk, and those who work hard to maintain core-body strength may offset the worst of back pain. But, Bezzerides warns, for almost all of us, back pain is in the cards.
Thanks a lot, evolution.
If I were meeting with the author to hash out evolutionary issues as scientists like to do, I would ask him a few questions. Why cite that old theory suggesting that monogamy evolved early in the human line by way of males provisioning females? Monogamy isn't even that common an arrangement today, and females past or present are unlikely to be quite so helpless. How come it's "slightly uncomfortable" to think of our ancestors mating with Neanderthals? And hey, that slap at sheep in the brain section? They're smarter than you think, an important point for analyzing comparative mammalian intelligence.
More concerning, the chapters on reproduction are uneven. It's jarring to see four questions grouped together, about why we're prone to choking; why infertility is widespread; why so many people need braces; and why females menstruate. Which one of these things is not like the other? Menstruation isn't a risk or medical condition. Bezzerides refers to "significant blood loss, significant iron loss, and a significantly lousy few days every month." Yet not everyone's experience with menstruation is so lousy — just as the process of childbirth, challenging as it is, doesn't always involve "screaming and trauma."
Bezzerides taught me some cool new science when explaining what's called spontaneous decidualization, a change in the uterine lining. Unlike in other animals, that lining in humans changes not in response to pregnancy but instead in preparation for pregnancy. The reason, more complicated than I can explain fully here, has to do with fetal burrowing into the womb, a type of maternal-fetal conflict that is more elaborated in humans.
Yet — another example of that unevenness I mentioned — in continuing to explore reproduction he replicates without question the old myth of sperm making a "perilous trip" so that "only the strongest, fittest sperm" fight their way to an egg. As anthropologist Robert Martin puts it, "Convincing evidence has instead revealed that human sperm are passively transported over considerable distances while travelling through the womb and up the oviducts. So much for Olympic-style racing sperm!"
I still say this book is a home run. Perfection is no more necessary in order to be grateful that a book was written than it is to experience appreciation for the human body with all its flaws. I recommend Evolution Gone Wrong highly to anyone wishing to grasp the mix of biological and cultural forces at work on our anatomy today.
Barbara J. King is a biological anthropologist emerita at William & Mary. Her seventh book, Animals' Best Friends: Putting Compassion to Work for Animals in Captivity and in the Wild, was published in March. Find her on Twitter @bjkingape
Copyright 2021 NPR. To see more, visit https://www.npr.org.