The phrase “survival of the fittest” makes it tempting to think of natural selection as an unequivocal engine of progress, one that only makes humans stronger and healthier specimens. But, in reality, the process is more complicated.
“I preach about this in my classes all the time,” Karen Rosenberg, a paleoanthropologist at the University of Delaware, tells mental_floss. “We think of ‘fit’ to mean aerobically fit, or able to run far, but in evolutionary biology, ‘fit’ means being reproductively successful.” In other words, you just need to be able to survive long enough to pass on your genes to the next generation.
To achieve reproductive success, natural selection sometimes makes compromises, and as a result, humans have developed some traits that pose real challenges to our health today. From back injuries to difficult childbirth, here are six downsides of being human that you can blame on evolution.
1. WE HAVE BACK PAIN.
The birth of bipedalism was a high point in human evolution. Standing upright allowed us to travel long distances and freed up our hands to use tools and carry food, but it also came at a cost.
In chimpanzees and our other quadrupedal cousins, the vertebral column acts like a suspension bridge. “But if you take that horizontally stable structure and tilt it vertically, it loses its stability,” Jeremy DeSilva, a paleoanthropologist at Dartmouth College, tells mental_floss.
The most obvious way to make a structurally sound spine in an upright creature would be a straight stack of vertebrae. But this arrangement would block the birth canal, and clearly you need to have babies to ensure the survival of your species. So the human spine had to evolve into the “curved mess” that it is today to make way for our big-brained babies to be born, DeSilva says. The price we pay is back pain—and prevalent injuries like slipped disks and spontaneous compression fractures.
2. WE HAVE WEIRD APEY FEET.
If you look at the most high-tech prosthetic feet available today, their structure is more like an ostrich’s foot. They don’t replicate human anatomy because the anatomically correct human foot is sort of awkward.
“Humans were not designed from scratch,” DeSilva says. “We’ve inherited a lot of the anatomies we have from our ape ancestry, and the foot is a wonderful example.”
When we starting walking on two feet, we no longer needed the flexible feet that our ape ancestors required to climb trees and grab branches. In order to give us more stability and allow us to better push off the ground, evolution took a “paper clips and duct tape” approach, DeSilva says. But because we walk around on modified ape feet that can twist and roll quite easily, we sprain and break our ankles. We get shin splints, plantar fasciitis, and collapsed arches. This isn’t just a modern phenomenon; scientists even see some of these common foot injuries in the fossil record.
"It works well enough, and that’s all you really need in evolution,” DeSilva says. “What we have as a consequence of a just-good-enough foot is a billion-dollar podiatry industry.”
3. CHILDBIRTH IS TRICKY.
Compared with other apes, humans experience very difficult childbirth. That’s largely because the human pelvis is very narrow relative to the big heads and broad shoulders of our babies.
“The pelvis serves two conflicting functions in humans: allowing us to walk on two legs and allowing us to give birth to big-brained babies,” Rosenberg says. The shape of the pelvis is a compromise between those two things.
But humans have come up with an interesting cultural answer to the problem of long and painful birth. While birth is a solitary event for most mammals, Rosenberg pointed out that virtually all human mothers seek delivery assistance from relatives, midwives, or doctors.
In a paper in the British Journal of Obstetrics and Gynaecology, Rosenberg and her colleague Wenda Trevathan wrote that natural selection likely favored the behavior of seeking assistance during birth. This probably wasn’t a conscious decision by expectant mothers. Rather, seeking help might have been driven by fear, anxiety, and pain, but over time, this led to reduced mortality.
4. WE CRAVE JUNK FOOD.
There’s a good reason it’s hard to give up fast food and candy. Sugar is a basic form of energy, and excess sugar is stored as fat to get us through times of hardship. Before the rise of agriculture and industrialization, when food sources were scarce or unreliable, a taste for sugar was necessary for survival. But now that processed sugar is readily available in grocery stores, humans are overdoing it. As a result, we’re facing an obesity epidemic and a rise in conditions like diabetes and high blood pressure.
“The food industry has made a fortune because we retain Stone Age bodies that crave sugar but live in a Space Age world in which sugar is cheap and plentiful,” Harvard evolutionary biologist Daniel Lieberman wrote in an op-ed in The New York Times a few years ago. (He was arguing at the time that New York City’s proposed ban on big sodas might actually help restore the healthy constraints of a hunter-gatherer world.)
5. A LOT OF US HAVE MENTAL ILLNESS.
Natural selection didn’t weed out potentially harmful conditions like schizophrenia and depression, even though many of these disorders are associated with lower birth rates. Some scientists have theorized that the unaffected siblings of the people with mental disorders might be responsible, as they may pass the mutations on to their own children, keeping these disorders in the gene pool. Other scientists have looked at the origins of mental disorders, showing that while devastating, some of these illnesses seem connected to an evolutionary advantage.
For example, while some symptoms of depression can be debilitating, some researchers have argued that the condition can also promote an analytical style of thought that can be very productive at solving problems. Other research has shown that schizophrenia-related genes may have helped humans achieve complex cognition.
6. OUR THIRD MOLARS ARE A PAIN.
After humans started walking upright, we underwent another major transformation: Our brains got much bigger. To accommodate a larger brain, the shape of our faces changed, and our jaws had to become narrower. But for many people, this means that their third molars, or wisdom teeth, once vital for chewing, have no room to erupt through the gums, so they become impacted. If these impacted teeth are not extracted, they can become extremely painful or cause infections.
But natural selection is still at work: A genetic mutation that stops wisdom teeth from forming has been spreading, and more people today are born without third molars.