Aches and pains? Blame the ancestors
It’s the middle of February, and the world is learning that Oscar Pistorius, a sprinter who ran in the Olympics, has been charged with murdering his girlfriend. In the summer of 2012, Pistorius, the first double amputee Olympic athlete, a hero for his personal grit.
His ultra-efficient carbon feet, inevitably, led to the nickname “blade runner.”
At the annual meeting of the American Association for the Advancement of Science, the news about blade runner served as an ideal but sad lead-in for Jeremy DeSilva, an anthropologist at Boston University who studies body structure. Comparing photos of the blades to an X-ray of the foot, DeSilva observed drily that “The [human] foot is not what you would design from scratch.”
Instead of this single bladelike structure, he said, “our foot has 26 bones. Our ape relatives have 26, and that’s the story in the primate line. Many problems we have today are the result of that.”
Our tree-climbing primate ancestors needed a foot able, like their hands, to grab and hold. Now that we are walking on our feet, those moving parts lead to sprained ankles, heel spurs, collapsed arches, Achilles tendinitis, osteoarthritis and plantar fasciitis, says DeSilva. “It all goes back to upright walking.”
Evolution ≠ perfection
Evolution is the organizing principle of biology: a fact of life. Plants and animals that look alike usually share common ancestors. Organisms that have similar genes got them from the same ancestor. And ditto for creatures with similar proteins.
Evolution is driven by selection: The genes of organisms that can survive and produce offspring are represented in the next generation. Genes that fail these twin tests disappear.
But just because evolution has crafted millions of working answers to survival and reproduction does not make the results ideal. “It’s been known for a long time, since Darwin, that evolution does not produce perfection,” says DeSilva. “Evolution works from raw material that was previously created, and it modifies and tinkers with it, producing the biological equivalent of duct tape and paperclips. If the organism survives, passes along its genes, then off we go.”
The genes and structures — like the spine, digestive tract or brain — most conducive to survival and reproduction are repeated, often with modification due to chance mutation.
Instead of inventing from whole cloth, evolution works through a “good-enough” process called descent with modification.
And that brings us to the “scars of evolution,” a theory proposed in 1951 by Wilton Krogman that proposes to explain a number of human shortcomings. DeSilva was one of several scientists at the February meeting who blamed our woes on the ones who came before — the predecessor primates and other mammals whose genes still largely control our fate.
Got the walking blues
Human foot problems are sometimes blamed on shoes, sidewalks, or the life of a couch potato, but DeSilva says nay. “It’s amazing how the fossil record indicates that foot problems go back into the past.”
Some feet have much better design. The ostrich for example, is a biological version of the sprinter’s carbon blades. “Ostriches have a fused bone at the ankle,” DeSilva says, “and big tendons that give it that elastic energy, that kick.”
Blame the ancestors. Birds, DeSilva says, “had a 230-million year head start on us, because they evolved from bipedal dinosaurs,” while people have only been walking on two feet for 4 or 5 million years. “Our ancestors were quadrapedal, spending a good deal of time in the trees, and they evolved this very mobile, grasping foot, made for living in the trees.”
When great-grandma started walking upright, evolution began to remodel her foot, stabilizing it with oodles of ligaments, “But these were Band-Aids,” DeSilva says. “Natural selection did wonders to modify the ape foot so we could walk on two legs, but that led to problems.”
The in-and-out rotation of the ankle helped our ancestors grab limbs, but it also facilitates sprains. And if you stand on one leg, DeSilva says, “You wobble; the ankle is very unstable, and you do this every single time you take a step.”
Through the process of convergent evolution (similar structures arising from different predecessors) foot bones have fused in some mammals that have been walking for millions of years. “In the horse and the antelope, the ankle is reduced in size, the metatarsal bones have fused, and the pedal digits have reduced to one,” says DeSilva. “That’s biomechanically better.”
No sense searching for a collapsed arch in an ostrich or a horse (both sprinters that could outrun blade runner). “You can’t get a collapsed arch if you don’t have an arch,” DeSilva says.
Understanding the evolution of the structure that helps us stand, walk and run shows that evolution is not just “a dusty old science, behind glass in a museum,” says DeSilva. “Our evolutionary history explains why we are the way we are today. Evolution impacts us today.”
Backed into a corner
Worldwide, back problems are humanity’s number-six ailment, says Bruce Latimer, professor of anthropology at Case Western Reserve University. Among musculoskeletal issues, backs rank first. “If you want a place that is really a problem, it’s the back,” he says cheerfully.
Back trouble is rooted in our descent from animals who walked on all fours, with the back in a horizontal position. The transition to the vertical entailed an extra curve to balance the weight over the hips, and while these curves alleviate the shock of walking, they are “a recipe for trouble,” Latimer says.
The spine is built of vertebrae and disks, and “If I gave you 24 cups and saucers, each representing a vertebra or a disk, if you were really careful, you could stack them,” Latimer says. “Now I want you to add in those curves, and if I gave you all the duct tape in the world, you could not possibly do it.”
The pronounced lumbar curve is found in no other mammal, Latimer says. Add in that reverse curve in the thorax, and a third in the neck, “and you wonder why there is a problem?”
On the plus side, our spine is highly flexible, and we are the only mammal that can do a back bend. Twisting and bending have other benefits, Latimer says. “Apes get disk infections because they don’t have a flexible spine, so they have trouble pumping nutrients into their disks.”
Those disks, however, can get crunched by gravity and stresses due to our upright gait, Latimer says. “As we walk, we throw our upper body 180° out of synch with the lower body, as the right arm goes forward, so does the opposite leg. We are constantly twisting, torqueing the spine, and ultimately after millions of repetitions, we wear through the fibrous exterior of the disk.”
Twisting and compression can translate into a disk that is “slipped” or “herniated,” as the soft center bulges and compresses the nerves, creating excruciating back and leg pain known to millions. “If you live long enough, you will have a herniated disk,” says Latimer.
Evolution tried to compensate with larger vertebrae, which reduce bulging and popping in the disks, but these big vertebra are built of porous — and therefore weaker — bone. “When we have the endocrine shift at menopause, women lose bone, and get osteoporosis,” says Latimer.
The great apes do not lose bone mass as they age, but neither do they get the spine-bending disability scoliosis.
After Charles Darwin first published his theory of evolution in 1859, scientists looked to it for explanations of the structures visible in plants and animals. Now, they realize that evolution has present-day significance. “Evolution tinkers with what it has, can’t create perfection; it can’t invent a brand new spine,” says Latimer. “If you think [the spine] is an intelligently designed structure, I suggest you get a new engineer.”
Teeth: Scars being solved!
Describing what he called the “final drop in the bucket of how you are going to fall apart,” Alan Mann, professor of anthropology at Princeton University, focused on the “wisdom tooth,” or third molar, which often jams instead of emerging in young adulthood.
This is supposedly the time when teenagers get “wisdom,” but instead a pain gnaws their jaw!
Teeth are the opening gambit of the digestive system. The front teeth are shaped to tear away food and the back teeth shaped for grinding and shearing food before swallowing and digestion.
The emergence of human teeth is tightly choreographed: The baby teeth start appearing at roughly age one, and later are replaced by the adult teeth as the mouth enlarges. The tough diets of our ancestors quickly wore the teeth, so it made sense for a brand-new molar — the third — to emerge after puberty.
But once the human brain expanded, the brain moved from behind the face to above it, leaving less room for the teeth, and the result, often, was an impacted wisdom tooth.
And the big brain (it’s three times as big as our ancestors’) Mann says, is the ultimate reason why wisdom teeth get stuck.
That’s the evolutionary bad news. The good news is that a random mutation thousands of years ago that prevents the third molar from hardening is spreading as evolution selects against wisdom teeth. Among some populations, 40 percent of people lack at least one third molar.
Impacted wisdom teeth are seldom fatal, so why would they shape our evolution? To answer, Mann offers this scenario: “One evening, a person who had chronic pain from an impacted tooth is asked, ‘How about a bout of reproduction, dear?” and responds, ‘Not tonight, my jaw is killing me.’” The result, he says, could be fewer offspring for the unlucky adults with impacted third molars.
So evolution is not just a source of scars, but also of solace, Mann says. “In the course of human evolution, as the amount of space [in the jaw] became smaller, a random mutation had selective value, and its frequency increased over time. But the third molar remains a scar of human evolution when dental technology [surgery] is not available.”
Birth: the most perilous escape
“If you want an example of something that is not intelligently designed, think about the crazy, tricky, complicated, uncomfortable way we have babies,” says Karen Rosenberg, chair of anthropology at the University of Delaware. “Childbirth is a time in the life cycle where women and babies are under increased risk of injury or death.”
These difficulties “go way back in evolutionary history, possibly back to the beginning of bipedalism,” she says. Our ancestors, after all, lived in trees and were much better at climbing than walking, which they did on all fours.
Efficient upright walking requires narrow hips, which shrinks the birth canal in the pelvis. But the birth canal must still accommodate that pesky-big human head.
Several evolutionary adaptations permit this awkward birth, Rosenberg says:
While apes are born with a fused skull, ours remain flexible until after birth. (The ability to change shape during birth explains the “cone-head” seen after a difficult vaginal birth)
Human infants are born earlier in development than other primates, so our brain is further from full size
Our head rotates during birth to fit the birth canal
Our mothers and infants get support during birth (and for years afterward)
Culture is critical during birth, Rosenberg says. After the head rotates, “the baby usually turns and emerges facing back. And so there is a benefit to having someone there to catch the baby.”
Attended birth is essentially unknown among our relatives, Rosenberg says. “Other primates give birth without assistance, in isolation. Humans give birth socially, and midwives around the world know all kinds of different things that ameliorate the risk. We are able to mitigate the risks because we are cultural animals; we have someone there to help when in labor.”
The result is a positive feedback among brain size, intelligence and culture, Rosenberg says. “The fact that we give birth in such a risky way speaks to the benefit accrued by having such a large brain. We are able to have culture because we have a large brain, and are able to have a large brain because we ameliorate the risks of childbirth culturally.” Even though childbirth remains hazardous, “This tells us how beneficial it is to have large brain, and the complex behavior we have as humans.”
The benefits of culture continue past birth, Rosenberg says. “It’s because we are cultural animals that we can take care of a baby that is born earlier, wrap it in a blanket, use a cradle board to prevent injury to the neck.”
All story long, we have been hearing complaints about the jury-rigged equipment we inherited from our ancestors — feet with too many bones, unstable spines, and teeth that get stuck instead of emerging on schedule. But “childbirth is not a scar of evolution,” Rosenberg insists. “The evolution of a rotating head during birth allowed this continual increase in brain size; we came to a point where the brain could not get bigger” unless we had culture to support the mother and infant. “Childbirth is an ancient and fundamental part of what makes us human.”
– David J. Tenenbaum