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  • Ann is a contributing correspondent for Science
 

Human Evolution: Gain Came With Pain

16 February 2013 1:32 pm
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Ouch. Shin splints are just part of the joy of being human.

BOSTON—Humans are the most successful primates on the planet, but our bodies wouldn’t win many awards for good design. That was the consensus of a panel of anthropologists who described in often-painful (and sometimes personal) detail just how poor a job evolution has done sculpting the human form here Friday at the annual meeting of AAAS (which publishes ScienceNOW). Using props and examples from the fossil record, the scientists showed how the very adaptations that have made humans so successful—such as upright walking and our big, complex brains—have been the result of constant remodeling of an ancient ape body plan that was originally used for life in the trees. “This anatomy isn’t what you’d design from scratch," said anthropologist Jeremy DeSilva of Boston University. "Evolution works with duct tape and paper clips."

Starting with the foot, DeSilva held up a cast with 26 bones and said: "You wouldn’t design it out of 26 moving parts." Our feet have so many bones because our ape-like ancestors needed flexible feet to grasp branches. But as they moved out of the trees and began walking upright on the ground in the past 5 million years or so, the foot had to become more stable, and bit by bit, the big toe, which was no longer opposable, aligned itself with the other toes and our ancestors developed an arch to work as a shock absorber. "The foot was modified to remain rigid," said DeSilva. "A lot of BandAids were stuck on these bones." But the bottom line was that our foot still has a lot of room to twist inwards and outwards, and our arches collapse. This results in: ankle sprains, plantar fasciitis, Achilles tendonitis, shin splints, and broken ankles. These are not modern problems, due to stiletto heels; Fossils show broken ankles that have healed as far back as 3 million years ago.

A better design for upright walking and running, DeSilva said, would be a foot and ankle like an ostrich. An ostrich’s ankle and lower leg bones are fused into a single structure, which puts a kick into their step—and their foot has only two toes that aid in running. "Why can’t I have a foot like that?" asked DeSilva. One reason is that ostriches trace their upright locomotion back 230 million years to the age of dinosaurs, while our ancestors walked upright just 5 million years ago.

Turning up the pain threshold a notch, anatomist and paleoanthropologist Bruce Latimer of Case Western Reserve University in Cleveland, Ohio, limped to the podium, dangling a twisted human backbone as evidence of real pain. "If you want one place cobbled together with duct tape and paper clips it’s the back," said Latimer, a survivor of back surgery.

When humans stood upright, they took a spine that had evolved to be stiff for climbing and moving in trees and rotated it 90 degrees, so it was vertical—a task Latimer compared to stacking 26 cups and saucers on top of each other (vertebrae and discs) and then, balancing a head on top. But so as not to obstruct the birth canal and to get the torso balanced above our feet, the spine has to curve inwards (lordosis), creating the hollow of our backs. That's why our spines are shaped like an "S." All that curving, with the weight of the head and stuff we carry stacked on top, creates pressure that causes back problems—especially if you play football, do gymnastics, or swim the butterfly stroke. In the United States alone, 700,000 people suffer vertebral fractures per year and back problems are the sixth leading human malady in the world. "If you take care of it, your spine will get you through to about 40 or 50," said Latimer. "After that, you’re on your own."

Paleoanthropologist Karen Rosenberg of the University of Delaware, Newark, moved beyond pain. As our bodies had to adapt to upright walking and bigger brains, they had to balance both of those changes with the limitations of the birth canal—and allowing enough mothers and babies to survive that the big-brained, upright walking species didn’t go extinct. "Death in childbirth used to be leading cause of death for women in reproductive years." That’s because compared with other primates, humans give birth to babies with larger bodies and brains—on average, human babies are 6.1% of their mother’s body size compared with chimp babies (3.3%) and gorilla babies (2.7%).

Despite the high risks for death and injury in childbirth, our ancestors’ solution to the problem was to give birth with social support. Today, humans rely on culture, often in the form of modern medicine, to change that outcome, using assisted birth with doctors or midwives, for example. One sign of that is that is that caesarean sections account for about 30% of all births in the United States, Rosenberg said.

The point of citing all these problems? Evolution doesn’t "design" anything, says anthropologist Matt Cartmill of Boston University, a discussant on the panel. It works slowly on the genes and traits it has at hand, to jerry-rig animals’ and humans body plans to changing habitats and demands. “Evolution doesn’t act to yield perfection," he says. "It acts to yield function.”

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