Bipedal: using only two legs to walk.
CAT scan: computerized axial tomography; this process makes detailed pictures of cross-sections of the body that are 100 times clearer than a common x-ray... more
Culprit: someone or something responsible for a crime or for causing a problem.
Osteoporosis: a disease, usually only seen in elderly people, where bones become weak and are easily broken... more
Porous: having tiny holes and spaces throughout the inside of something, much like a sponge.
Vertebra: any of the bones that make up the backbone.
Vertebral fracture: when one of your vertebrae (backbones) becomes weak and cracks slightly.
Although they aren’t licensed detectives, scientists are on a mission to discover the mysterious origins of spontaneous vertebral fractures in the PLOS One article, “Human Evolution and Osteoporosis-Related Spinal Fractures.”
Over 700,000 people in the United States report suffering from something called spontaneous vertebral fracturing every year; scientists think the real number of cases is much higher. This fracturing occurs when the bones in your back (also called vertebrae) suddenly crack; this can happen even without a fall or accident taking place. Scientists and doctors have a few clues to the culprit, however.
Scientists found the second clue to why spontaneous vertebral fracturing happens in humans’ closest living relatives: other apes. As humans, chimpanzees, and other apes continue to age past adulthood, their bones often become lighter and weaker.
However, in all ape species alive today, humans are the only species that suffers from the cracking. Even if the other apes have their bones lose mass and weaken, their bones don’t suddenly break. Researchers realized this fact implies that something special about humans must be increasing our risks of vertebral fractures.
The scientists then did exactly what investigators do in crime movies. They used CAT scans (also called CT scans) to X-ray the bones and create digital 3D versions of the bones. That way, computers could analyze the shape of the bones— both inside and out! They applied digital forces to the computerized bones, pressing and squashing them to estimate the strength of each bone from each species. Using only these bones, they could even reconstruct the weight of each animal when it was alive.
It turns out that for our size, human vertebrae are wider and larger than the vertebrae of chimpanzees, gorillas, and the rest. But then why are we the only ones to develop these dangerous vertebral fractures?
But our detective from earlier wouldn’t be satisfied just yet. We still don’t know WHY humans have larger, but less dense, vertebrae. Luckily, our scientists study human evolution: they look to our past and our close ape relatives to explain situations we find ourselves in today. They noticed that the apes studied all move around in different ways: gibbons swing from branch to branch with their arms, orangutans climb and swing with four limbs, gorillas and chimpanzees walk using their feet and the knuckles of their hands, but only humans are always bipedal. We are the only apes that spend most of our time walking around on two legs.
Previous studies have shown that walking upright puts more stress on your back and your feet: each time you take a step, all your weight and force channels down into the heel of one foot. It’s very high impact and it takes a toll on your bones. Other apes don’t have this problem, because walking on four limbs or climbing trees spreads out that force and divides it among all their arms and legs.
Aha! It looks like we may have found the culprit! Our scientists found that human vertebrae were ALSO larger and more porous than the same bones in apes. Perhaps the same things are happening in our backs and in our legs: to decrease all that high impact stress from walking around on two legs, the bones in our backs and legs become larger, but less dense.
Although this helps make walking safer for most of our lives, it could potentially cause problems in our old age if we get osteoporosis. It’s what scientists call a trade-off between good and bad situations: even though there are problems with large, wide, porous bones, the benefits of having them outweigh the costs for our bodies. Now that we know why people are getting those spontaneous vertebral fractures, perhaps we can put more focus on fixing the problem once and for all.
EvMed Edits are sponsored by ASU's Center for Evolution and Medicine.
Additional images via Wikimedia Commons. Embryo spine by Ed Uthman.
Andrew Burchill. (2017, June 06). Evolution Detective: The Case of the Broken Bones. ASU - Ask A Biologist. Retrieved January 21, 2021 from https://askabiologist.asu.edu/evmed-edit/evolution-osteoporosis
Andrew Burchill. "Evolution Detective: The Case of the Broken Bones". ASU - Ask A Biologist. 06 June, 2017. https://askabiologist.asu.edu/evmed-edit/evolution-osteoporosis
Andrew Burchill. "Evolution Detective: The Case of the Broken Bones". ASU - Ask A Biologist. 06 Jun 2017. ASU - Ask A Biologist, Web. 21 Jan 2021. https://askabiologist.asu.edu/evmed-edit/evolution-osteoporosis
The developing spine of a human embryo.