Why your back, feet hurt: blame evolution

jeudi 28 février 20130 commentaires

03:39 PM ET

Why your back, feet hurt: blame evolution


The fact that many people's backs and feet hurt is news that's millions of years old.
It's because of the way we have evolved, uniquely from other mammals, that we also have a lot of aches and pains that our close relatives do not experience, anthropologists said last weekend at a briefing at the American Association for the Advancement of Science annual meeting.
"We’ve known for a long time, since Darwin’s time, that humans have evolved, and that humans are not perfect, because evolution doesn’t produce perfection," said Jeremy DeSilva, anthropologist at Boston University.

For instance, humans are the only mammals that can have scoliosis, a condition in which the spine has an abnormal curvature, said Bruce Latimer of Case Western Reserve University in Cleveland, Ohio.
Owners of a Homo erectus skeleton from around 1.5 million years ago and the Lucy skeleton, more than 3 million years old, both appear to have had back problems, Latimer said.
Humans evolved from an animal whose back was horizontal, parallel with the ground. But the human spine evolved to be upright, and the body needed to balance over hips and feet. That's why the spine needed curves in it, but it's still not an optimal system.

Over the course of a lifetime, with the stresses and loads of daily activities, we're apt to develop problems such as a herniated disk. Even walking, with the process of moving our arms and legs to carry an upright back, results in a constant twisting and torquing of the spine, Latimer said. No other animal has to deal with a mechanical system in this way.

Evolution is a process that tinkers with existing materials; it can't create perfection, Latimer said. "You use paper clips and duct tape to make these things work. You can’t invent a brand new spine."
Humans are also the only mammals that carry things in front of them. It's recommended that you bend your knees when picking up something heavy to reduce the force on the spine, but you might find yourself in pain one day while bending over to lift a grocery bag from the trunk of your car.
"The design specs on your body are about 45 or 50 (years old)," Latimer said. "If you take care of it, your spine will get you through that. But after that, you’re on your own."

Likewise, the human foot's structure has evolved in a way that also causes problems. There are 26 bones in the foot, and as a whole it's not designed to shield us from pain. In fact, there's ancient evidence of ankle sprains, osteoarthritis and fractured ankles, back to the origins of upright walking, DeSilva said.

The arch is an area of the foot that particularly causes a lot of problems - so why do we have them?
There are two leading theories, DeSilva said. One is that the arch acts as a shock absorber, taking up many of the forces within the foot during walking to prevent them from reaching the joints. The other idea is being developed by scientists such as Daniel Lieberman's group at Harvard; they look at how the arch is made of ligaments that can stretch and recoil in relation to the mechanics of running.

And how about the chronic pain from wisdom teeth? Although these "third molars" may not make you smarter, they are actually connected to the evolution of the human brain, said Alan Mann, anthropologist at Princeton University.

The size of our brains is more than three times that of our ancient ancestors, Mann said. The architecture of the brain case had to change to accommodate that, and so did the way that the facial skeleton fit with the cranial skeleton.

Thousands of years ago, a gene mutation appeared that's associated with an absence of wisdom teeth. This is especially pronounced in some groups; about 45% of Inuits are missing third molars, Mann said. Neanderthal adults, on the other hand, appear to have consistently had wisdom teeth.
Walking upright on two feet also makes childbirth tricky, said Karen Rosenberg of the University of Delaware. Our species has come up with a cultural solution: Have someone there to help, be it a midwife or an obstetrician, to mitigate the risks that come with the birth process.

"Evolution has been functional in ourselves, yet we are suffering these problems," Mann said. "And many of those problems are going to remain as part of our biology."
Post by: Elizabeth Landau - CNN.com Health Writer/Producer
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